Definition and Etiology

Because of their size, behavior, and relatively thin calvarium, horses are more susceptible to head trauma than other livestock. Traumatic injuries of horses most often result from kicks, sharp blows, or falling over backward.⁷⁰⁰ Blows to the poll of the horse, particularly associated with falling over backward, are very common and result in fracture or displacement of the basisphenoid, occipital, and petrosal bones and the basioccipital and basisphenoid sutures.^701-703 Young horses are particularly prone to this type of injury, probably because of their more fractious nature and tendency to react strongly to restraint, as well as to lesser strength of the immature skull.⁷⁰³ The basisphenoid and basioccipital bones form a part of the foramen lacerum and the jugular foramen. Fractures around these foramina may result in dysfunction of CNs IX, X, and XII.⁷⁰⁴ Hematomas form at the fracture site and extend into the membranous labyrinths and basilar areas of the brain, where they cause vestibular and occipital cortex dysfunction. Basilar region fractures carry a much more guarded prognosis than trauma at other sites.⁷⁰³ Blows to the forehead result in depression fractures of the dome of the calvarium and trauma to the underlying cerebrum.

Skull fractures occur in cattle from blows to the top of the calvarium. Most skull fractures are located in the center of the frontal bones, where the internal and external plates of the frontal sinus are fused into a single-layer dorsal wall of the cranial vault. This position can be located on the skull as the imaginary cross found by intersecting lines drawn between the medial canthus of the eye and the horn of the opposite side. Injuries in this area compress the frontal and parietal lobes of the cerebral cortex. The pressure changes result in loss of sensorium, sensory deficits, blindness contralaterally, or convulsions.

In young goats and horned sheep under 4 to 6 months of age, the calvarium can be inadvertently opened by removal of excessive bone during disbudding or dehorning. In goats, cerebrocortical burns can occur from overapplication of a hot iron or caustic dehorning paste. Cortical necrosis caused by bacterial infections after dehorning of calves has also been described.⁷⁰⁵

Clinical Signs

The clinical presentation of cerebral trauma depends on the area of the brain damaged, the extent of the lesion, and the duration of the injury.⁷⁰⁶Lesions of the cerebral cortex and thalamus are characterized by variably altered mentation, circling, head pressing, pacing, aimless wandering, and cortical blindness (blindness with normal eyes, pupillary light reflexes, and oculomotor reflexes). Seizures also may result; however, occurrence of a seizure does not necessarily indicate a poor prognosis.⁷⁰³ Compression of the midbrain results in decerebrate rigidity caused by loss of the reticulospinal tracts. In more severe cases of midbrain compression, abnormal breathing patterns may be observed, together with hyperreflexia, tetraplegia, and absence of pupillary reflexes. Compressive lesions of the mesencephalon in the region of the oculomotor nucleus result in mydriatic pupils on the ipsilateral side of unilateral brainstem lesions. Medulla oblongata compression is characterized by serial dysfunction of cranial nerves, severe disturbance of consciousness, and abnormal respiratory rhythm. Involvement of the long motor and sensory pathways to the limbs can occur with brain injuries at any level and results in ataxia and paresis, which may be worse on the side contralateral to the injury in the case of cerebral cortex, thalamic, and midbrain injury, or on the ipsilateral side in the case of traumatic damage to the medulla or cerebellum.

Basioccipital fractures of horses result in asymmetric signs of vestibular disturbance, including horizontal or rotary nystagmus, ipsilateral ventrolateral strabismus, contralateral dorsomedial strabismus, head tilt, and contralateral blindness. Horses that remain ambulatory lean or circle toward the side of the lesion. Additional signs of this syndrome include dysphagia, facial paralysis, conscious proprioceptive deficits, recumbency, depression, and coma. Horses that are recumbent struggle violently. Fracture of the petrous temporal bone may cause profuse bleeding from the ipsilateral nares, external ear canal, and guttural pouch.

Brain trauma caused by overaggressive dehorning in goats results in depressed sensorium, loss of menace response, increased extensor tonus on the contralateral side, ipsilateral mydriasis, sluggish pupillary reflex, and loss of conscious proprioceptive responses. The clinical syndrome may be delayed by several days in cases of cortical burns or trauma caused by caustic paste and may be complicated by brain abscess or bacterial meningitis.

Pathology and Pathogenesis

The pathogenetic events leading to cerebral edema and increased intracranial pressure (ICP) are complex. Trauma to the head results in a variety of abnormal physical forces exerted on brain tissue, including acceleration-deceleration, shearing, compressive, tearing, and rotational forces.^706-708 The consequence of direct physical insult to brain tissue that occurs immediately on impact is considered primary traumatic brain damage.⁷⁰⁷ Such physical insult results in axonal injuries that may be immediate (primary axotomy), such as axonal tearing, or occur many hours after the initiating event (secondary axotomy).⁷⁰⁸ Trauma activates neuronal mechanoreceptors, causing cellular depolarization that spreads outward from the site of impact. Together with direct axonal injury, this may underlie initial signs of concussion, including loss of consciousness.^706,708 Processes that follow the initial mechanical trauma and further exacerbate injury are considered secondary brain damage.⁷⁰⁷ Intracranial hemorrhage or loss of vascular integrity and cerebral edema occur after concussive blows to the head. Hemorrhage after head trauma may be epidural, subdural, subarachnoid, or intraparenchymal.⁷⁰⁷ Displacement of the neural tissue is caused by cerebral swelling or hematoma formation. The increased pressure is transmitted to the CSF and interferes with normal vascular flow, resulting in cerebral hypoxia and interneuronal and intraneuronal edema. Diminution of cerebral perfusion results from a combination of increased ICP, disruption of the vascular architecture, and decreased systemic blood pressure. The net result is reduced oxygen delivery to the brain, a tissue that relies on aerobic glycolysis for the production of energy. Interruption of energy production within the brain results in failure of tissue homeostasis. Breakdown of the blood-brain barrier further contributes to brain swelling and loss of intracranial homeostasis.

In addition to gross damage to tissue, brain trauma results in a complex series of biochemical events that disrupt cellular integrity.^706,709 One of the most important is the depletion of adenosine triphosphate (ATP), the main energy store within neurons. This results in dysfunction of cell membrane ionic pumps, permitting influx of sodium (Na⁺) and calcium (Ca⁺⁺) into the cell. Influx of these ions activates a number of secondary pathways within the cell, including the kinin, arachidonic, complement, and xanthine-oxidase pathways. Activation of these pathways results in the production of a variety of substances that are deleterious to cellular function, including oxygen free radicals, vasoactive mediators, cytokines, nitric oxide, excitatory neurotransmitters, and enzymes. Together, these contribute to a destructive cascade of events that further damages cell integrity.⁷⁰⁹

When brain swelling becomes severe, the respiratory centers are depressed, resulting in hypoxemia and acidosis. The extra carbon dioxide diffuses into the brain, and water follows, which further swells the CNS. Acidosis and hypoxemia also worsen the vascular leakage and hypoxemia. Extreme swelling of the cerebral cortex results in herniation through one or more anatomic sites of the calvarium. Four forms of brain herniation have been described in large animals.⁷¹⁰ These include cingulate gyrus herniation ventral to the falx cerebri, herniation of parts of the temporal cortex ventral to the tentorium cerebelli (caudal tentorial herniation), caudal cerebellar vermis herniation through the foramen magnum, and herniation of the rostral cerebellar vermis ventral to the tentorium cerebelli (rostral tentorial herniation). Compressed tissue becomes hypoxemic and edematous. Compression of the CNS causes more hypoxia, prompting a dramatic and rapid deterioration.

Clinical Pathology

Clinical pathologic variables noted after head trauma include nonspecific changes consistent with a stress response, such as mild neutrophilia, lymphopenia, and hyperglycemia, as well as those resulting from systemic trauma, such as elevated serum CK.⁷⁰³ Hyperglycemia has been associated with more guarded prognosis in people with head injuries, possibly from deleterious effects on cerebral vasculature.⁷¹¹ In one study of horses that had head trauma, only elevated packed cell volume (PCV) was shown to be associated with a more guarded prognosis.⁷⁰³

Collection of CSF from the atlantooccipital cistern is generally contraindicated in head trauma, especially if signs of increased CNS pressure, uncontrolled hemorrhage from the ears or the nose, or dorsal sagittal sinus fractures are observed. Clinical signs that may suggest the presence of increased ICP include dull mentation (especially if this is worsening over time), mydriasis, blindness, or papilledema. The CSF changes that occur from traumatic injuries are characteristic. For the first 24 hours, blood is admixed evenly in the CSF. Iatrogenic hemorrhage from the tapping procedure can be differentiated from that caused by trauma because in the former case, the CSF is irregularly streaked with blood; CNS hemorrhage usually results in an even admixture of blood through the CSF. During the first 24 hours, the protein concentration and WBC count of CSF are elevated and are in the approximate ratio as that of peripheral blood. By 48 hours after the traumatic episode, the amount of blood in the CSF decreases, and when centrifuged, the cell-free CSF appears xanthochromic. WBC counts of the CSF are only marginally increased by 24 hours after hemorrhage, and the protein concentration may range from 500 to 1000 mg/mL (albuminocytologic dissociation). Thereafter, the protein concentration gradually decreases, and the xanthochromia disappears by 14 days after the acute hemorrhage. The number of mononuclear inflammatory cells gradually increases as parts of the CNS degenerate. The CK level of the CSF is elevated (10 to 100 IU/dL) for approximately 1 to 2 days after the acute traumatic episode.

Diagnostic Imaging

Radiography or more advanced imaging studies (CT, MRI) are the primary modalities for definitive diagnosis of skull fractures. It should be remembered, however, that significant brain trauma can occur in the absence of skull fracture and that this is a common situation. Radiography has the advantages of being widely available and relatively simple to perform, requiring restraint or only mild sedation in most situations. However, false-negative findings are common, especially in cases where the bony lesion may be particularly difficult to identify, as with basilar bone fractures in horses.⁷⁰³ While basilar fractures may be difficult to identify, other radiographic changes such as soft tissue densities in the tympanic bullae due to hemorrhage, or gas opacities adjacent to the basilar region, may support this diagnosis.^711aCT is the technique of choice for the diagnosis of bony lesions and for acute intracranial hemorrhages, whereas MRI facilitates diagnosis of a variety of pathologic changes within the brain parenchyma. These advanced imaging techniques have limited availability, are costly, and usually require general anesthesia. Clinical findings and historical information may form the sole bases for diagnosis in many cases.

Treatment

The treatment of brain trauma remains one of the most controversial areas of clinical neurology in all species. Opinions differ widely and evidence in the scientific literature is often contradictory. Treatments that show promise in rodent models of head injury often are disappointing in clinical trials.^711b Successful treatment depends largely on early recognition and initiation of therapies that maintain cerebral and whole body homeostasis. General medical principles for treating CNS trauma include (1) establishment of proper respiratory function, (2) support of blood pressure and maintenance of cerebral perfusion and oxygenation, (3) control of seizures, (4) nutritional and fluid support, and (5) protection from decubitus and self-inflicted damage. Many treatment modalities recommended for head-injured large animals are based on anecdotal reports and are not supported by rigorous scientific studies. It is reasonable, however, to apply the principles of head trauma management established in other species, including humans, to the management of larger mammals that sustain similar injuries. Immediate treatment involves establishment of a patent airway and administration of oxygen via mask, endotracheal tube, or nasal catheterization. In cases of severe upper airway obstruction, emergency tracheostomy may be indicated.^711c Aggressive intravenous crystalloid therapy is indicated to establish and maintain a normal systemic blood pressure and to ensure that the brain receives adequate blood supply, with use of colloids or blood products as necessary. Principles of fluid therapy and treatment of shock are described in Chapter 34.

Administration of dexamethasone, methylprednisolone, mannitol, or dimethyl sulfoxide (DMSO) has been recommended for controlling CNS pressure caused by edema. The clinician should bear in mind that little or no scientific data exist to support the use of corticosteroids or DMSO in the treatment of CNS trauma, and that these drugs may have deleterious effects that outweigh any potential benefits. Doses listed below are largely anecdotal and are included because of the continued widespread tendency to use these agents, despite lack of evidence that they are effective. Studies of the use of corticosteroids in people with head injuries are ongoing, and the pendulum of opinion continues to swing between positive and negative.^711d An empiric recommendation for the treatment of horses is administration of dexamethasone at 0.1 to 0.25 mg/kg by slow IV injection every 4 hours for 1 to 4 days or, for mature animals, 100 to 1000 mg of methylprednisolone by slow IV injection.⁷¹⁶ Similar dosages could be used in ruminants.

Data supporting the use of methylprednisolone sodium succinate for treatment of acute spinal cord injuries in people comes from the National Acute Spinal Cord Injury Study (NASCIS). A dose of 30mg/kg bodyweight given by intravenous infusion within 8 hours of injury has been recommended, followed by a second and third dose (15 mg/kg each) given intravenously 2 and 6 hours later and a subsequent infusion of 2.5 mg/kg/hr for the next 48 hours.⁷¹⁷ The use of a similar treatment for head injuries does not yet have good scientific support. In addition, the methodology and results of the NASCIS study have been called into question. The potential deleterious effects of such high doses of corticosteroids in large animal species include enhanced susceptibility to infection, muscular weakness, renal potassium and calcium loss, abortion in ruminants, and laminitis in horses. These adverse consequences of corticosteroid use probably outweigh its uncertain benefits. Table 35-8 presents a list of antiedema drugs often administered to large animals with traumatic brain disease.

Table 35-8 Recommended Drug Dosages for Treatment of Cerebrocortical Edema

The use of the osmotic diuretic mannitol has regained favor in the treatment of head trauma in humans.^711d Intravenous administration of a 20% solution of mannitol (1 g/kg) or oral administration of glycerol (20 mL/kg) has been used for the treatment of increased ICP in large animal species. The physiologic activity of mannitol for lowering the CSF pressure may be related more to its vasoconstrictive effects than its activity as an osmotic diuretic. Response to the treatment may occur as early as 1 hour after administration. Mannitol is expensive and usually only economically justifiable for use in neonates. If response to the initial mannitol dosage is noted, additional treatments should be given every 4 to 6 hours for the first day. Mannitol should be administered through blood administration filter sets to minimize the occurrence of microcrystalline emboli. The drug should not be given to animals with active CNS hemorrhage, because diffusion of mannitol into the center of a newly forming hematoma exerts an osmotic effect, enlarges the size of the lesion, and further attenuates the nervous system tissues. Active CNS hemorrhage can be recognized by the presence of unclotted blood in the nose or ears, or parietal bone fractures that lacerate the dorsal sagittal sinus. Despite this provision, administration of mannitol is probably justified in an animal with rapidly worsening and potentially fatal deterioration in neurologic status, even in the likely presence of intracranial hemorrhage.

Intravenous use of DMSO has been recommended for the reduction of increased CSF pressure in large animals. The drug is administered IV at 0.5 to 4 g/kg twice daily.^{701,712,718,719} DMSO is diluted fivefold to tenfold in saline (10% to 20% solution) to minimize the hemolytic and hyperthermic effects. In horses, administration of 5L of 10% DMSO in a balanced electrolyte solution has been shown to have minimal deleterious clinical or clinicopathologic effects.^719aHigher doses (solutions >20%) have been reported to have a number of adverse side effects such as intravascular hemolysis, colic, diarrhea, muscle tremors, and collapse.^719b The use of DMSO for cerebral trauma is controversial, and benefits may be species specific. For example, anecdotal reports of benefit have been shown for horses, but controlled experiments in dogs have shown limited clinical benefit when treating experimental CNS trauma.⁷¹⁴

DMSO has several beneficial pharmacologic actions, including free-radical scavenging, interference with neutrophil chemotaxis, prevention of microthrombi, increased penetration of corticosteroids and antibiotics into the brain, and vasodilation.⁷²⁰ A major effect of the drug is probably caused by its diuretic action, which is greater than that of furosemide. In experimental situations, administration of DMSO to animals with experimentally induced CNS lesions resulted in more rapid neurologic recovery than treatment with urea, corticosteroids, or mannitol.⁷²⁰ The adverse effects of DMSO include muscular fasciculations, intravascular hemolysis, hemoglobinuria, and sweating.⁷¹⁹ Deaths have been reported in laboratory animals after intraperitoneal injections of 10 mg/kg and in dogs after IV dosages of 2.5 mg/kg.^721-723 The median lethal dosage of DMSO in large animals is unknown. The drug is teratogenic when administered to pregnant laboratory animals.^722,723 When the drug is administered IV, approximately 70% of the dosage is excreted through the respiratory tract.⁷²⁴ These data indicate that DMSO should be administered only in well-ventilated areas. Exposure of pregnant women and animals should be avoided. In cattle, DMSO is excreted rapidly and is essentially completely cleared from the plasma by 5 days.⁷²⁴ A low-level residue of DMSO may persist in the fat tissues for at least 20 days. When administered IV to horses at 1.0 and 0.1 g/kg, the biologic half-life of DMSO is 8.6 and 9.8 hours, respectively.⁷¹⁹

Depression fractures of the frontal and parietal bones may be reduced surgically. Lacerations of cerebral tissue can be treated surgically with gentle cleaning and debridement of contaminated and devitalized tissue.⁷²⁵ Such procedures require general anesthesia. Prognosis in animals with exposed and contaminated brain tissue is very guarded, so expectations should be realistic before such intervention is undertaken.Surgical repair of skull fractures in horses using intrafragmentary wires or bone plates may be indicated to improve functional and cosmetic outcomes.^725a,725b

Convulsions may be controlled initially by IV administration of diazepam (Valium), phenobarbital, or pentobarbital. The recommended dosages and mode of administration of these drugs are presented in Table 35-7. Good nursing care is essential. These drugs are usually highly protein bound in plasma and can be displaced or functionally altered by other drugs. All anticonvulsant treatments should begin at the lowest possible dosage, which can be increased daily or every second or third day until the seizures have been controlled. If seizures cannot be controlled without causing depression or ataxia, a second anticonvulsant is added. The dosage of the second drug is increased gradually until the seizures stop. This combination treatment is continued for 2 to 4 weeks. Thereafter, the dosage of the first anticonvulsant is tapered until it is discontinued. If seizures reappear, the dosage of this drug is increased until the seizures disappear again. The trough blood concentration of all anticonvulsants is checked monthly. The suggested therapeutic trough concentration of phenobarbital ranges from 15 to 40 μg/mL of plasma and that of diphenylhydantoin from 5 to 20 μg/mL. Any attempt to withdraw anticonvulsant therapy should be done gradually over a 4-week period.

Horses with recurrent convulsions should not be ridden or used for sporting purposes. Infrequent seizures generally do not justify anticonvulsant treatment, and economic considerations often limit the amount of drug therapy that is possible. Status epilepticus can be treated with IV diazepam in 5-mg doses until seizures are controlled or by titrated doses of phenobarbital or pentobarbital. Mares with estral-related seizures may be treated with an ovariectomy.

Definition and Etiology

Nervous coccidiosis is a neurologic syndrome of calves and yearling cattle, sheep, and goats that is associated with enteric infections by Eimeria species. The condition is most often seen in western Canada and the northwestern United States and is especially prevalent in feedlots. The incidence of nervous coccidiosis is highest in the winter months. In contrast to enteric coccidiosis, mortality from nervous coccidiosis can be as high as 72%.⁷²⁷ The pathogenesis of the encephalopathy may be related to the elaboration of a labile neurotoxin by the parasite.⁷²⁸ The clinical signs and history of nervous coccidiosis are similar to those of other neurologic diseases that affect the function of the cerebral cortex.

Clinical Signs

The onset of the nervous system signs is usually preceded by diarrhea, tenesmus, and hematochezia. Some calves with severe diarrhea develop prolapsed rectums. Initial signs of CNS dysfunction include depression, incoordination, twitching, and hyperesthesia. As the clinical signs worsen, the animal becomes recumbent and develops numerous cerebrocortical signs, including opisthotonos, periodic tremors, horizontal nystagmus, frothing at the mouth, bellowing, snapping eyelids, and muscular fasciculations.^729-732 Blindness is rarely seen. Stimulation of the patient may precipitate a tonic-clonic seizure.^729-732 The animal may die after 1 to 5 days of encephalopathy. Convulsive calves may regain consciousness but relapse a week later.⁷³³

Clinical Pathology

Fecal flotations from the patient and herdmates show a large burden of coccidial oocysts. Fecal egg counts of affected animals may range from 5000 to 4 million/g. To exclude the possibility of other neurologic diseases, blood should be collected for measurement of electrolytes (calcium, magnesium, potassium). The acid-base status, plasma glucose, and blood lead concentrations should be measured. Acute meningitis and salt poisoning may be ruled out by CSF analysis. The plasma vitamin A concentration should be measured in any animal that has not had exposure to green forage. Polioencephalomalacia, ethylene glycol poisoning, lead poisoning, rabies, petroleum distillate poisoning, and clostridial enterotoxemia should be considered as possible differential diagnoses.

Pathophysiology

The pathogenesis of the encephalopathy is unknown. The nervous form of coccidiosis cannot be transmitted to mice by injection of CSF from infected calves; however, a heat-labile neurotoxin has been identified in the serum of calves with nervous coccidiosis.⁷²⁸ The encephalotoxic activity is precipitable with 30% ammonium sulfate and may have an apparent molecular weight of 300,000 kD.⁷³⁴ The coccidia do not directly invade the CNS.

Epidemiology

Nervous coccidiosis occurs most frequently in feeder cattle, but dairy and pastured beef calves, lambs, and kids also may be affected occasionally. In one epidemiologic survey the prevalence of nervous coccidiosis was 0.3% of the calves that were affected with the intestinal form of the disease. Nevertheless, outbreaks with a large percentage of calves developing CNS disease have been reported.⁷²⁹ In western Canada, nervous system signs have been reported in 21% of herd outbreaks of intestinal coccidiosis.⁷³⁵ Approximately 90% of all cases of nervous coccidiosis occur in January, February, and March.

Necropsy Findings

No macroscopic lesions are seen in the CNS of calves with nervous coccidiosis. The microscopic lesions of the brain are mild and nonspecific and include edema, congestion, and occasional shrunken neurons. Parasitic invasion of the ileum, cecum, and colon results in lesions in these organs.

Treatment and Prevention

Treatment should include 2 to 4 mL/kg of a commercially available calcium gluconate solution that contains magnesium, given subcutaneously. The coccidial infection should be treated with sulfamethazine (110 mg/kg PO for 5 days, or 1 pound/100 gallons of drinking water), or amprolium (50 mg/kg/day PO for 7 days). Diazepam, sodium pentobarbital, or phenobarbital may be used to control tonic-clonic convulsions (see Table 35-3). Slow IV administration of 50 to 100 mL of a 10% magnesium sulfate solution may also be useful as a sedative. The response to treatment is poor and the case-fatality rate is high (∼90%) in calves that develop tonic-clonic seizures. Specific chemotherapeutic regimens and methods of preventing intestinal coccidiosis are described in Chapter 49.

Definition and Etiology

The three recognized species of Sarcocystis that infect cattle—S. cruzi, S. hominis, and S. hirsuta—are sporozoan parasites with definitive hosts of dogs, primates, and cats, respectively.^736,737 Three other Sarcocystis species—S. capricanus, S. ovicanus, and S. tenella—have definitive hosts in dogs and secondary hosts in goats and sheep.⁷³⁸

When a carnivore ingests flesh from an infected cow, Sarcocystis cysts in muscle are broken down by digestive enzymes, and motile bradyzoites are released. The bradyzoites infect the intestinal mucosal cell and differentiate into sexual stages called microgametes (male) and macrogamonts (female). The gametes fuse to form an oocyst, which is shed onto pastures as sporocysts. When eaten by a ruminant, the sporocysts hatch in the proximal small bowel and penetrate into the medium-sized mesenteric arteries, where they enter endothelial cells and form sporozoites. The sporozoites then mature in three successive waves. Each wave of development spreads downstream. The third-generation merozoites finally enter the soft tissues and encyst as sarcocysts. The total period of development in the ruminant requires 10 weeks. The life cycle is completed whenever a carnivore ingests uncooked meat containing viable sarcocysts. Chronic illness in the cow occurs during the maturation of the cyst in the muscles, at approximately 9 weeks after infection.

Clinical Signs

Most cases of Sarcocystis infestation are asymptomatic in both definitive and secondary hosts. However, if a large number of sporocysts are ingested by a nonimmune ruminant, clinical illness may develop. Clinical signs in cattle usually begin between 9 and 11 weeks after ingestion of infectious sporocysts. These signs include fever (>39.5°C [103.5°F]), anorexia, weight loss, symmetric lameness, and diarrhea. Neurologic signs include ataxia, muscular weakness, tremors, hyperexcitability, hypersalivation, recumbency, tonic-clonic seizures, leg biting, blindness, opisthotonos, and nystagmus.^739,740 Cattle may lose the hair of the tail switch (“rat tail”). Sheep may show a wool break.^736-749 Animals with chronic infections may develop edema of the limbs, poor weight gain, muscular atrophy, and pallor.^736-752 Second-trimester abortions may occur in cattle and small ruminants beginning 28 days after ingestion of infectious sporocysts.⁷⁴³ The fetuses may appear either normal or autolyzed. Lactating cows may have reduced milk production.⁷⁵²

Clinical Pathology and Pathogenesis

Prolonged prothrombin times may be observed in some infected animals⁷³⁶; however, the activated clotting times and bleeding times are normal. The concentrations of plasma lactate dehydrogenase, alanine transaminase, sorbitol dehydrogenase, and blood urea nitrogen are increased. The packed cell volume (PCV) and the serum protein concentration are decreased. During early infection there is a marked normocytic, normochromic anemia that is characterized by 75% reduction of the blood hemoglobin concentration and reduced PCV.^753-755 The anemia is thought to be caused by extravascular hemolysis.⁷⁵⁶

Antibodies to solubilized freeze-dried Sarcocystis antigens have been detected by indirect hemagglutination, enzyme-linked immunosorbent assay (ELISA), and an agar gel immunodiffusion (AGID) test.^757-759 Immunoglobulin M (IgM) responses first occur by 3 to 4 weeks after infection and peak by 11 to 15 weeks.^757-759 The concentrations of Sarcocystis-specific IgG begin to rise by 5 to 6 weeks and peak after 11 weeks postinfection, with peak seroreactivity occurring by 39 days after infection. Background titers of normal cattle range from 1:54 to 1:486, and titers of infected cattle often exceed 1:10,000. There is no serologic cross-reactivity between Sarcocystis and Toxoplasma gondii, despite their physical similarities.⁷⁵¹

Pathophysiology

The pathogenesis of Sarcocystis is poorly understood. Pathologic changes in the skin and muscle and in serum chemistry probably are related to a combination of parasite-directed immunologic responses and diffuse vasculitis. Although toxins have not been identified, rabbits die acutely after parenteral administration of purified bradyzoites. The clinical signs exhibited by the inoculated rabbits resemble endotoxic shock. Other studies have indicated that chronic infections result in increased concentrations of somatostatin and decreased concentrations of somatomedin.⁷⁶⁰ Abortions probably occur because of luteolysis that results from the increased concentrations of prostaglandin F_2α, caused by vascular infection by the parasite.⁷⁶¹

Epidemiology

Estimates of infection rates range from 70% to 98% in cattle in the United States.⁷³⁶ When infected flesh is eaten by carnivores, the encysted sporozoites complete their life cycle⁷³⁶ (Fig. 35-7). The prepatent period of the parasite in the carnivore (primary host) ranges from 9 to 45 days. The primary host may shed the sporulated oocysts in the stool for as long as 2 months after a single infection. The oocysts withstand freezing but are rapidly killed by sunlight and drying.⁷³⁶ Reexposure of previously infected canids results in a large fecal output of sporocysts. Ingestion of approximately 250 g of infected meat by a dog can result in an output of 100 to 6000 sporocysts per gram of feces. Wild canids are even more susceptible than domestic dogs and may serve as a major mechanism for propagation of Sarcocystis in range cattle in the western United States.

Fig. 35-7 Life cycle of Sarcocystis parasite.

Most Sarcocystis infestations of cattle are asymptomatic; however, the disease may become clinically apparent with sudden, overwhelming exposure to the parasite in a nonimmune animal. Such conditions occur whenever there is an opportunity for extensive scavenging of ruminant carcasses by carnivores and contamination of feed bunks or pastures with infected carnivore feces.

The economic burden of Sarcocystis infection is unknown. One author has estimated an annual loss of $95 million in the United States alone.⁷³⁶

Pathologic Lesions

The pathologic lesions of the CNS are similar for all species of sporozoans; they include granulomatous meningoencephalomyelitis, focal malacia, perivascular cuffing, neuronal degeneration, and gliosis. The changes are generally most severe in the cerebellum and midbrain but can occur anywhere in the CNS, including the spinal cord. The pathologic diagnosis is based on finding meronts and merozoites in the affected sections of neural tissue.^762,763

Pathologic lesions elsewhere include hemorrhages on the sclera, serous surfaces, and muscles; fluid in the body cavities; and lymphadenopathy. The muscles have alternating light and dark stripes. Macroscopic changes may not be evident in animals with chronic sarcocystosis.^741,742 If changes are not evident, postmortem diagnosis is based on the finding of intravascular schizonts or intramuscular hemorrhages without significant inflammation. Ultrastructural examination of affected areas of CNS shows an intracellular colony with rosette orientation of agents in the cytoplasm of infected astrocytes.

Treatment

Feeding monensin (100 mg/kg daily for 30 days) during the incubation period is prophylactic; however, the efficacy of the drug in symptomatic cattle is unknown. For maximum effectiveness, monensin should be administered continuously for 2 to 5 weeks after exposure. Treatment of infected sheep with salinomycin (1 to 2 mg/kg) also has been recommended.⁷⁶⁴ Administration of amprolium (100 mg/kg once daily for 30 days) may reduce the severity of Sarcocystis infection⁷⁶⁵ but may not completely eliminate the clinical disease.

Control

The best method of controlling Sarcocystis infection is to protect the food supply of ruminants. Scavenging of carcasses by carnivores should also be prevented by deep burial or incineration. Feed bunks should be kept clean and raised approximately 1 to 3 feet (30 to 90 cm) off the ground. All carnivorous pets that have access to the feed or pastures should be fed cooked meat or processed dry food. In range pasture situations, prophylactic feeding of monensin or elimination of predatory or scavenging carnivores may be necessary.

Definition and Etiology

A cyst-forming protozoal parasite that closely resembles Neospora caninum has been identified in aborted fetuses from cattle in California. The condition is predominantly a disease of dairy cattle; however, sporadic abortions can occur in beef cows.^766-768 The predominant clinical sign of a Neospora-like agent is a midterm to late-term abortion (3 to 8 months of gestation). Fetal lesions consist of a focal nonsuppurative necrotizing encephalitis, nonsuppurative myocarditis and myositis, and mononuclear cell infiltrates disseminated in other tissues. Occasionally, calves are mummified. The agent appears to have been responsible for as many as 24% of all abortions in northern California dairy cattle.

Occasionally, however, a nonfatal infection may occur in the fetus. In this case the fetus is born with neurologic dysfunction (Fig. 35-8). The clinical signs of neurologic disease vary because of the randomly widespread distribution of the parasite within the CNS. Affected calves are often unable to stand and suckle and have abnormal spinal reflexes. Flexural contractions of the forelimbs, domed skull, and torticollis have also been reported in spontaneously occurring cases.^769,770 The calves are usually born with the CNS signs, which initially are mild but then progress after birth. Pathologic lesions associated with the fetal infection include focal areas of brain discoloration, focal cavitation with cyst formation, and reduction of gray matter. Microscopic changes in the CNS of affected calves include nonsuppurative inflammation of the gray and white matter, demyelination, perivascular cuffing, focal lymphocytic meningitis, and neuronal necrosis. Changes in other tissues include nonsuppurative myocarditis, myositis, and hepatitis. Protozoa can be seen in microscopic sections of the stained tissues.

Fig. 35-8 Calf with congenital Neospora infection of the central nervous system (left) and a tissue cyst containing Neospora tachyzoites (right).

Neospora organisms have been isolated in pure form using cultured cells.⁷⁷¹ Antibodies have been produced by intubation of laboratory animals, and the agent can be identified microscopically using immunoperoxidase staining on the fixed tissues. The CSF changes in affected calves range from normal to mild pleocytosis.⁷⁷² Similar conditions have been described in sheep^773,774 and goats.^775,776

Definition and Etiology

Equine protozoal myeloencephalitis (EPM) is a multifocal, progressive disease of the central nervous system (CNS) that is primarily caused by infection with Sarcocystis neurona.⁷⁷⁷ Recently, another protozoan parasite, Neospora caninum/N. hughesi, has been implicated as a cause of EPM in six cases.^778-783 The condition has mostly been reported from many U.S. states, Canada, Panama, Brazil, and Argentina.^784-790 Several reports of the disease in countries other than those in the Western Hemisphere were primarily in horses that originated from the Americas.^791-794 More recently, there have been reports of horses in France that developed neurologic deficits with positive S. neurona antibody titers that were native to France and had not resided in the United States.^795,796 Young standardbred, thoroughbred, and quarter horses are most often affected, although horses of any breed may develop the disease. There does not appear to be a gender predilection for EPM, and any age may be affected.⁷⁹⁷ The risk would appear to be higher in young horses, but horses as old as 30 years have developed the condition.⁷⁹⁷

The parasite produces inflammation and necrosis of the brain, brainstem, and spinal cord. Under light microscopy, the structure of the EPM agent resembles that of Toxoplasma gondii, but comparative electron microscopic analyses of the three agents show differences. The Sarcocystis agent of horses has been grown in explant cultures of monolayered bovine monocytes.^787,798 Antibodies in the sera or cerebrospinal fluid (CSF) can be detected using these specimens as probes of immunoblots of the cultured parasites. Sera from clinically affected cases recognized eight S. neurona—specific antigens,⁷⁹⁹ several of which are the basis for current diagnostic testing. DNA analysis has been very important in characterizing and classifying S. neurona. Using a random primed polymorphic DNA assay (RAPD), a unique sequence of base pairs was identified that distinguished S. neurona from eight related coccidia, specifically two Sarcocystis species, one Toxoplasma species, and five Eimeria species.⁸⁰⁰ This research demonstrated that unique DNA sequences could be successfully used as a species-specific probe for S. neurona, and that these probes permitted differentiation of S. neurona from other coccidia of equines.⁸⁰⁰

Clinical Signs

Descriptions of clinical signs of horses diagnosed with EPM may vary greatly because the organisms that cause this disease can affect any CNS tissue. Therefore, any horse exhibiting neurologic abnormality could be diagnosed with EPM.

Clinical signs recognized in the earliest studies of this disease still characterize neurologic abnormalities in horses with EPM. Early workers described horses with EPM as having an asymmetric ataxia and associated muscle atrophy.^801,802 Horses may have a sudden onset of clinical signs, or disease may progress slowly over several months.⁸⁰¹ Vague, intermittent lameness that is nonresponsive to therapy may be caused by EPM, and encephalitic signs typified by asymmetric cranial nerve deficits may also be seen in affected horses.⁸⁰¹ Gait abnormalities in horses with EPM include ataxia, tetraparesis, knuckling, circumduction, and crossing over. The abnormalities may be asymmetric. Depending on the location of the lesion in the spinal cord, areflexia, hyporeflexia, or hyperreflexia may be seen. Infections of the myelencephalon may result in head tilt, facial paralysis, circling, nystagmus, dysphagia, facial paralysis, and apparent blindness, with or without abnormal pupillary reflexes. Parasitic invasion of the ventral spinal rootlets or the radicles of the maxillary branch of the trigeminal nerve may result in neurogenic atrophy of the tongue and masticatory muscles (Fig. 35-9). This is often accompanied by focal areas of desensitization. Regional sweating (“strip sweating”) may be observed if the sympathetic tracts of the spinal cord are affected. Although EPM is typified by the presence of asymmetric, multifocal neurologic abnormalities, horses with EPM may have focal or symmetric signs.

Fig. 35-9 Unilateral atrophy of the tongue in a standardbred horse with protozoal myeloencephalitis. Atrophy also may occur in many other muscle groups of the head and the limbs.

Courtesy Dr. R.H. Whitlock.

Cerebral signs are rarely seen in horses with EPM. However, three horses with EPM presented to Ohio State University Veterinary Teaching Hospital displayed seizure activity and evidence of cortical electrical activity abnormalities on electroencephalographic (EEG) examination.⁸⁰³ Horses with cerebral neurologic signs often have a poor prognosis. However, seizure activity in horses with EPM may be treatable. Visual deficits and behavioral abnormalities have been reported in horses with EPM.⁸⁰⁴ Head shaking was also reported in a recent case series describing three horses diagnosed with EPM.⁸⁰⁵ Head shaking resolved in these horses after treatment for EPM. Recently, urinary incontinence and incoordination have been reported in three horses diagnosed with EPM. Resolution of the clinical signs were mixed in those cases.⁸⁰⁶

Differential diagnoses of the most common neurologic diseases of horses that resemble EPM include equine degenerative myelopathy (EDM), cervical spinal injuries, cervical vertebral stenosis or malformation (CVM), equine herpesvirus type 1 (EHV-1), and equine lower motor neuron disease (ELMND).

Another disease has become the number-one differential for EPM over the last 7 years. West Nile Virus (WNV) was first reported in the United States in 1999; however, the number of equine cases has increased consistently from 25 in 1999 to greater than 14,000 in 2002.⁸⁰⁷ Almost all the equine cases in 1999 had been diagnosed with EPM first before a definitive diagnosis of WNV was determined.⁸⁰⁸ Asymmetric neurologic deficits with profound weakness and ataxia make it difficult to differentiate WNV from EPM.

Clinical Pathology

A Western blot (WB) analysis for the diagnosis of EPM has been described and commercially marketed.⁷⁹⁹ Macrophage-cultured S. neurona is used as the antigen. After electrophoresis the blots are probed with suspect CSF or serum. Reactions are seen as bands developing on the blotted membrane. The sensitivity and specificity of WB has been reported as 89% based on 295 postmortem examinations.⁸⁰⁹ However, these figures are likely based on more severe cases. Although promising, exhaustive examinations of the sensitivity and specificity of the test in clinical cases are not yet available. Recent research suggests that the sensitivity is excellent, but the specificity in current clinical cases is much lower than originally reported.⁸¹⁰ There is no apparent serologic cross-reactivity between the parasites of EPM (S. neurona and N. caninum) and T. gondii. Use of the WB for antibody to S. neurona differs depending on the prevalence of the disease in the population studied.⁸¹¹ If the test is applied in the normal horse population, where the prevalence of EPM is likely less than 1%, the predictive value of a positive test is extremely low (<8%) based on the 89% sensitivity and specificity. However, given the presence of neurologic signs, the prevalence increases dramatically (50% at Ohio State), leading to a positive predictive value of about 90%. This suggests the test should not be applied in normal horses. A recent report demonstrates that the specificity of WB was much lower than originally reported.⁸¹² Another report attempted to demonstrate increased specificity for WB for S. neurona antibody detection by blocking the reaction using Sarcocystis cruzi antibody.⁸¹³ However, a double-blind investigation indicated that this increased specificity is questionable (William J.A. Saville, unpublished data).

Several new diagnostic tests are still measures of reactivity to antigens. Initially, a recombinant surface antigen (SAG1) protein was discovered, and an ELISA test was developed to test horses for S. neurona exposure.⁸¹⁴ However, no mention of controlled environment or heat-treated feed in this study makes it difficult to interpret the results. Further development of an ELISA test using a recombinant baculovirus-expressed SAG1 antigen was completed by others.⁸¹⁵ This test was able to detect both naturally and experimentally infected horses and other species. Another diagnostic test was developed using direct agglutination, the S. neurona agglutination test (SAT).⁸¹⁶ In mice the sensitivity of SAT was 100% and the specificity 90%.⁸¹⁶ In California an indirect fluorescent antibody (IFA) test was developed for detection of S. neurona antibody.⁸¹⁷ This test was compared with two currently commercially available WB tests, and the results suggested that the IFA was better than either WB test with regard to specificity.⁸¹⁷ Further evaluation of the IFA using both naturally and experimentally infected horses resulted in likelihood ratios that would be useful in diagnosing EPM.⁸¹⁸ The one concern regarding this test is that it may cross-react with horses recently infected with Sarcocystis fayeri, another common parasite infecting U.S. horses.⁸¹⁹ Further research with the IFA suggests that using this test on CSF of horses has no benefit if used on the serum in the diagnosis of EPM.⁸²⁰

More recently, it has been reported that surface antigens (SAGs) of S. neurona are similar to the SAGs of T. gondii.⁸²¹ The SAGs of S. neurona were named SnSAG1 through SnSAG4.⁸²¹ Recombinant proteins were developed to the four SnSAGs to produce a set of ELISAs for detection of antibodies to the SnSAGs.⁸²² Testing these ELISAs with confirmed cases of EPM serum and CSF samples suggested that the SnSAG2, SnSAG3, and SnSAG4 assays would be good tests in the diagnosis of EPM.⁸²² The ELISA to the rSnSAG1 demonstrated poor sensitivity (68.2%) and specificity (71.4%), likely because of the differences in strains of the parasite, some of which do not express antibody to SAG1.⁸²² There was also no cross-reactivity when testing horses infected with S. fayeri or Neospora hughesi.⁸²² The most recent test developed for diagnosis of EPM has been the S. neurona—specific IgM-capture ELISA.⁸²³ This test was developed using samples from previous experimental infection studies performed at Ohio State University.⁸²³ The IgM titers developed and peaked between weeks 2 and 3 postinoculation (PI), and the IgM response waned by 7 weeks PI.⁸²³ The IgM test is still a measure of exposure; however, it indicates an acute infection and therefore may be useful in the early diagnosis of EPM.⁸²³ Further evaluation of this diagnostic test is needed using large numbers of field tests.

Regardless of the tests developed in the past few years, they are primarily still tests of exposure, so diagnosis of EPM is problematic.⁸²⁴ Therefore, antibodies in serum or CSF must be accompanied by asymmetric neurologic deficits followed by rule-outs of other possible causes of the neurologic deficits.⁸²⁴

One of the difficulties in diagnosing EPM is caused by the large number of horses that have detectable quantities of antibody to S. neurona in the CSF for several months after therapy, or in horses that do not exhibit neurologic signs. S. neurona—specific IgG found in the CSF is assumed to be produced locally because the blood-brain-barrier (BBB) should prevent large molecules from freely entering the cerebrospinal space. However, antibody detected in the CSF could have been produced systemically if the BBB is compromised by disease or if peripheral blood contaminates the spinal fluid during sampling. To help determine whether IgG in CSF was produced locally or resulted from peripheral blood contamination, a series of tests were modified from those used in human patients with neurologic diseases. The albumin quotient (AQ) and the IgG index are calculated using concentrations of albumin and IgG found in serum and CSF of the patient.⁸²⁵ The AQ is a measure of BBB integrity, and the IgG index is a measure of intrathecal antibody production.⁸²⁵ Some reports suggest that horses with EPM usually have a normal CSF albumin concentration and a normal AQ, but the IgG index is usually elevated.⁸²⁶ It has been suggested that these indices, specifically an elevated AQ, can be used to distinguish true-positive WB tests in horses with EPM from false-positive tests resulting from blood contamination during sampling.⁸²⁶ False-positive results caused by blood contamination may alter the AQ and the IgG index by increasing the albumin and IgG concentrations in the CSF.⁸²⁶ It has also been recommended that the CSF indices may be used to monitor the response to therapy for EPM by identifying a decrease in the IgG index that would mark a decrease in intrathecal antibody production over time. However, small numbers of horses were used in the initial evaluation of these tests, with some inconsistencies in the reported results. Although the IgG index did decrease in 9 of 12 (75%) horses treated for EPM, 3 of 12 horses demonstrated an increase in the IgG index.⁸²⁷ The reliability of the CSF indices has been questioned by others.^828,829 One controlled investigation suggests the CSF indices are inconsistent, therefore, if used, they should be interpreted with caution.⁸¹⁰

Polymerase chain reaction (PCR) is available to aid in diagnosing EPM.^830,831 The sensitivity and specificity of PCR testing of CSF has been reported to be 83% and 100%, respectively, when histologically confirmed cases of EPM were used to validate the assay.⁸³² However, other research suggests that the sensitivity of PCR may be only about 40%.⁸³³ PCR requires that parasite DNA be intact. A strong inflammatory response favors enzymatic degradation of parasite DNA, and this process may affect sensitivity of the PCR test. Therefore, PCR may be useful early in the course of the disease and in chronic cases.⁸³² In addition, PCR analysis of CSF may be insensitive because the parasite is most frequently found in tissues and not floating freely in the CSF. Therefore, parasite DNA may not be present in CSF, even when adjacent tissue is infected.⁸³³ Toxoplasma gondii infections are difficult to detect in people for the same reasons.⁸³⁴ A recent study of clinical cases found that PCR alone was not useful for antemortem diagnosis of EPM based on the low sensitivity (12/151).⁸²⁹

Sarcocystis neurona DNA has been detected in blood samples. The presence of DNA in blood samples is thought to indicate recent ingestion of S. neurona sporocysts and subsequent infection. However, it is not known how long detectable amounts of DNA remain in the blood stream after infection.⁸³² Controlled investigations of PCR testing in horses with neurologic deficits as well as in normal horses are required to define the usefulness of this procedure.

Cerebrospinal fluid analysis has been used to aid in determining the etiology of neurologic diseases in the horse. Early studies suggested that horses with EPM had mildly elevated CSF protein concentrations, increases in numbers of mononuclear cells, and mild elevations in CSF enzyme activity (creatine kinase, CK; aspartate transaminase, AST).⁸³⁵ Two early studies reported marked elevations in the CSF CK activity in horses diagnosed with EPM.^836,837 However, more recent studies suggest that neurologic disease of horses cannot be reliably differentiated based on CSF leukocyte counts, CK activity, AST activity, or protein concentration.^810,838,839

Several serologic tests have been developed for detection of Neospora caninum antibodies in animal species.⁸⁴⁰ The three methods currently used are ELISA, IFA test, and direct agglutination test.⁸⁴⁰ However, these antibody tests are only measures of exposure to the organism. New diagnostic tests for N. hughesi used in experimentally infected equines included a whole-parasite ELISA, a recombinant-protein ELISA, a modified direct agglutination test, and an IFA test, which showed the most consistent results.⁸⁴¹ However, a recombinant NhSAG1 ELISA has been developed that suggests, with a sensitivity of 94.4% and a specificity of 95%, it may be an excellent diagnostic test for N. hughesi infections in horses.⁸⁴²

Pathogenesis and Pathologic Changes

Little is known about the pathogenesis of EPM. It is assumed that the horses ingest S. neurona and that the course of infection and disease is then similar to that observed in other host species infected with Sarcocystis species. Because the sporocysts of S. neurona are passed in the feces of the opossum, infective oocysts likely are introduced into the feed and water supply of intermediate hosts. Once ingested, the sporocysts excyst and release sporozoites, which penetrate the gut and enter arterial endothelial cells of various organs. Meronts develop and rupture the host cell, releasing merozoites into the bloodstream. This is probably followed by a second round of merogony throughout the body. In most sarcocystis-like diseases, this process results in the formation of sarcocysts in the muscle. Subsequent ingestion of the infected muscle tissue by the predator or definitive host completes the life cycle. Sarcocysts of S.neurona had not been found in affected horses, indicating that the horse is likely an aberrant, dead-end host.⁸⁴³ However, a recent report suggested that the horse may develop S. neurona sarcocysts and therefore may be a natural intermediate host.⁸⁴⁴ The fact that this has been seen only once in the literature is troublesome, and the inability to fulfill Koch’s postulates is also a problem. Horse muscle fed to naive opossums has failed to produce sporocysts after numerous attempts (William J.A. Saville; unpublished data).

Little is known about the life cycle of N. caninum or N. hughesi in horses. Recent reports have demonstrated that the definitive host of N. caninum is likely the dog.⁸⁴⁵ At present it is not known if the dog is the definitive host of N. hughesi. The dog is the definitive host and also can be an intermediate host, which is similar to T. gondii in cats.⁸⁴⁰ Unlike EPM caused by S. neurona, tachyzoites have been found in horse tissues, as well as tissue cysts in two horses reported to have EPM caused by Neospora.⁸⁴⁰ In addition, one case of neosporosis in a foal was determined to have been congenitally infected.⁷⁸⁰ Congenital infections have not been demonstrated in horses infected with S. neurona.

Sarcocystis neurona has been recovered from CNS lesions in several horses and subsequently propagated in culture in the laboratory.⁸⁴⁶ When administered to horses parenterally or introduced through the epidural space, cultured merozoites have not induced clinical disease in the horse.⁸⁴⁶ The merozoite stage of Sarcocystis species is not known to be transmissible to other animals.⁸⁴⁶ However, nude mice have been inoculated intraperitoneally with cultured merozoites and subsequently developed evidence of S. neurona—associated encephalitis.⁸⁴⁷ These mice were immunosuppressed strains, and intraperitoneal injection would not likely be the normal route of infection with S. neurona in horses. A better mouse model has recently been developed by feeding sporocysts from feral opossums to interferon-γ knockout (IFN-γ/KO) mice.⁸⁴⁸ These procedures in IFN-γ/KO mice also help to differentiate Sarcocystis species that are excreted in opossum feces; at least three species appear to be present.⁸⁴⁸ The mechanism by which the merozoites enter the CNS is currently unknown. The organism likely enters the CNS through infected leukocytes or through the cytoplasm of endothelial cells.⁸⁴⁶ Recent research may help in confirmation of this speculation. When co-cultured with equine peripheral leukocytes, S. neurona merozoites penetrated the cells within 5 minutes after starting the culture.⁸⁴⁹ This may be the mechanism for entering the CNS. In addition, a microneme protein for host cell invasion has been documented.⁸⁵⁰

There have long been anecdotal reports that the parasite may create immune suppression in the horse. One report demonstrated a strong association between health events and development of clinical signs of EPM.⁷⁹⁷ Earlier reports suggested that nitric oxide is important to resistance to intracellular parasites, decreased nitric oxide was reported in horses with experimental and naturally occurring cases of EPM.⁸⁵¹ Another report indicated that decreased levels of transforming growth factor beta (TGF-β) in CSF of horses may be important in development of EPM.⁸⁵² Two recent reports found a decrease in IFN-γ production in lymphocytes from EPM-positive horses compared with negative horses.^853,854 Other studies have corroborated these findings.⁸⁵⁵ Two reports in mice suggest that protection against S. neurona infections requires CD4 and primarily CD8 cells.^856,857 This phenomenon has been demonstrated in T. gondii infections as well.^856,857

Much more research is needed to elucidate the mechanisms and pathogenesis of S. neurona infections in horses.

Life Cycle of Sarcocystis neurona

Most specific details regarding the life cycle of S. neurona are currently unknown, although recent research has demonstrated that the opossum is likely the definitive host. The geographic distribution of opossums is similar to the geographic distribution of EPM, and areas with lower seroprevalence of S. neurona appear to coincide with regions outside the natural range of opossums.⁸⁵⁸ Further evidence that the opossum is the definitive host for S. neurona was obtained by experimental induction of EPM.⁸⁵⁹ When sporocysts from feral opossums were fed to horses, neurologic disease developed.⁸⁵⁹ This study has been repeated by other research groups.^860-865 However, induction of clinical EPM by feeding Sarcocystis falcatula sporocysts was not successful.⁸⁶⁶ More recent work has demonstrated at least three species of Sarcocystis sporocysts in feces from the opossum.⁸⁶⁷ Other studies suggest that four species of Sarcocystis sporocysts may be present in opossum feces.^868,869 Development of DNA probes that distinguish Sarcocystis species will better enable researchers to characterize sporocysts from opossum feces, using additional induction studies to help develop a reliable equine model for EPM.

Several recent studies have induced experimental infection in horses with S. neurona sporocysts. Initially, in a University of Kentucky study, naive foals were infected with 1 × 10⁶ to 4 × 10⁷ sporocysts orally, which resulted in mild to moderate neurologic deficits. The sporocysts were collected from wild-caught opossums, cleaned, and administered by nasogastric intubation. Unfortunately, the parasite was not cultured from the CNS tissues, resulting in the inability to fulfill Koch’s postulates.⁸⁵⁹ At the University of Florida, three studies used sporocysts detected using molecular DNA probes.⁸⁶⁹ One study administered 1 × 10⁶ S. neurona sporocysts and another 5 × 10⁵ sporocysts orally once daily for 7 consecutive days.^864,865 Both studies resulted in mild to moderate neurologic deficits, and no parasite was detected.^864,865 In another Florida study, sporocysts characterized as S. falcatula were administered to horses, with no development of neurologic signs and no seroconversion.⁸⁶⁶ This study corroborated that the opossum excreted more than one Sarcocystis species of sporocysts.⁸⁶⁶ Recently, a study at Ohio State University attempted to infect horses using 8 × 10⁴ S. neurona sporocysts with three different treatment groups.⁸⁶³ All nine horses in the infected groups developed neurologic signs; however, the most severe signs (mild to moderate) were seen in horses in the transport stress group. Unfortunately, as in the Kentucky study,⁸⁵⁹ Koch’s postulates were not fulfilled in the Ohio study.⁸⁶³

The previous studies attempted to mimic stress using dexamethasone, but the clinical signs were less severe, and the horses’ clinical signs appeared to improve.^859,863,864 In addition, regardless of the dose of sporocysts administered, some horses demonstrated an improvement in their clinical signs with no treatment. These results suggest that horses are capable of clearing large numbers of these organisms. Those findings may explain the high number of CSF antibody-positive horses with no evidence of neurologic deficits. Equally troubling, after orally inoculating horses with 1 × 10⁸ S. neurona sporocysts, clinical signs of neurologic deficits were readily detectable; however, no parasite was found in the CNS at 7 or 14 days postinfection (PI) (W.J.A. Saville and J.P. Dubey, unpublished observations). In the natural intermediate host, the raccoon, S. neurona was readily detectable in the CNS 7 days PI.⁸⁷⁰

Additional infection studies have been attempted in horses. A horse with severe combined immunodeficiency disease (SCID) received characterized sporocysts of S. neurona orally. Although a parasitemia was detected in this horse, the first detection of the parasite in an experimentally challenged horse, the horse did not develop evidence of neurologic dysfunction. This suggests that development of clinical signs of EPM requires an intact immune system, which also supports anecdotal evidence that EPM is a neuropathologic disease in horses.^860,871

Studies at Ohio State University (OSU) have further characterized S. neurona infection in horses. The infectious sporocysts were produced using laboratory-raised opossums and infected raccoons and the life cycle as previously reported.^870,872 The second trial was done to determine the effect of sporocyst dose on development of clinical signs of EPM.⁸⁶² Horses in the group that received 1 × 10⁶ sporocysts seroconverted earlier and developed more consistent clinical signs than those infected with lower doses of sporocysts. Horses were transported a second time and developed worsening clinical signs after residing at the Veterinary Teaching Hospital at OSU.⁸⁶² Therefore, another trial was carried out testing the effect of a second transport after infection with sporocysts immediately on arrival at the first study site.⁸⁶¹ The results demonstrated more significant clinical signs in the horses not transported a second time, refuting the hypothesis from the 2002 study. Neither the 2002 study nor the 2004 study resulted in detection of the parasite in the tissues of infected horses. Another study, in 2005, demonstrated that S. neurona could be detected in the blood of an experimentally infected, immunocompetent horse.⁸⁷³ The horse had been infected daily for 98 days, and S. neurona was detected from the blood of one of six horses tested, demonstrating that the parasite could be detected in immunocompetent, experimentally challenged horses.⁸⁷³ Because of the lack of parasite detection in equine infection models at most sites, researchers at OSU decided to attempt culture early in the infections rather than at the end of the infection period. Eight naive horses negative for antibody to S. neurona were included; six were infected with sporocysts derived from laboratory production using the opossum-raccoon cycle, and two were control animals.⁸⁷⁴ Parasite was cultured from mesenteric lymph node, liver, and lung at 1, 2, and 7 days PI; 2, 5, and 7 days PI; and 5, 7, and 9 days PI, respectively.⁸⁷⁴ Although no parasite was detected in CNS tissue, evidence of infection was present at 7 and 9 days PI.⁸⁷⁴ This recent OSU study was able to fulfill Koch’s postulates and shows evidence that the parasite can invade the tissues very quickly after ingestion, as demonstrated in previous studies in two different species of animals.^844,848,870 Further work needs to be done to examine an equine model for the disease.

Unlike most Sarcocystis species, S. neurona may aberrantly infect a large number of intermediate hosts. Although the full range of intermediate hosts for S. neurona has not yet been identified, several species of animals and birds have been reported to exhibit symptoms similar to those seen in horses with EPM. Several reports indicate that an S. neurona—like organism infected and caused neurologic disease in dogs, sheep, cats, mink, raccoons, a striped skunk, a golden hawk, Pacific harbor seals, sea otters, chickens, a Grant’s zebra, Canada lynx, and a Fisher.^875-888 The harbor seals and sea otter had evidence of sarcocysts in the muscle that were S. neurona positive.^878,879 This seems to be the first evidence for potential S. neurona sarcocysts to date, although the significance of this finding was not well understood. This positive reaction to anti—S. neurona antibody could be caused by cross-reactivity to other Sarcocystis species. A recent report indicates that S. neurona cycles normally between the opossum and various intermediate host species. Even though the opossum is the definitive host and does shed the parasite in its feces, the opossum does not develop antibodies to S. neurona.⁸⁸⁹ Sarcocysts are found in the muscles of five species: infected domestic cats (Felis domesticus), nine-banded armadillos (Dasypus novemcinctus), striped skunks (Mephitis mephitis), raccoons (Procyon lotor), and sea otters (Enhydra lutris nereis).^872,890-893 It was thought that the domestic cat was only a laboratory intermediate host, but studies in Missouri and Ohio suggest that the cat is a natural intermediate host as well;^895,894 this also seems to hold true for the striped skunk.

Recently, the life cycle of S. neurona has been completed in the laboratory.⁸⁹² Previous reports of serum prevalence of antibodies to S. neurona in striped skunks suggest that they are likely a natural intermediate host as well.⁸⁹⁶ When muscle from wild-caught raccoons and road-killed armadillos was fed to laboratory-raised opossums, it resulted in shedding of sporocysts infective for ponies, horses, and IFN-γ/KO mice.^872,891 High seroprevalence of S. neurona antibodies in armadillos tested (100%) from three states and raccoons (57%) tested from four states supports that these two species are natural intermediate hosts.^891,897 After ingestion of the infected muscle, opossums shed sporocysts in their feces. Small numbers of sporocysts were found after feeding sea otter muscle, but the sporocysts were infective for IFN-γ/KO mice.⁸⁹⁰ The role of the sea otter as a natural intermediate host is likely limited because it is a marine mammal. Results of these life cycle studies demonstrate that many species may be potential intermediates hosts for S. neurona. This wide host range is atypical for Sarcocystis species. This host range behavior is similar to that of T. gondii, which is phylogenetically close to S. neurona.^898,899

Based on the number of intermediate hosts determined so far, a number of isolates of S. neurona are postulated to exist. Recent work found that there are differences between South American and North American isolates.^900,901 Also, evidence indicates that the U.S. group could be divided into northern and southern U.S. groups, which suggests geographic groupings. Other work has demonstrated differences in the SAG1 gene in different isolates of S. neurona, with 73% to 100% sequence similarity.⁹⁰² This is in contrast to the SAG1 gene of Neospora species, with 96% to 98% similarity.⁹⁰² These differences in the isolates was confirmed when monoclonal antibodies developed against immunodominant proteins of S. neurona failed to detect all isolates.⁹⁰³

Based on the estimated numbers of opossums in North America, the poor survival rate of these animals, and the small areas in which they travel, S. neurona may be transmitted by routes other than direct contact with opossum feces. Experiments performed by researchers in the 1980s indicate that some transmission may occur through birds.⁹⁰⁴ In experiments attempting to characterize the life cycle of S. falcatula, birds were apparently infected by aerosol spread.⁹⁰⁴ Vector transmission was also demonstrated by the recovery of sporocysts after budgerigars, canaries, white mice, and chickens were fed opossum feces.⁹⁰⁵ The recovered sporocysts were then fed to budgerigars to assess the viability of the sporocysts. Four of six budgerigars died, demonstrating that the sporocysts were viable. These experiments suggest that sporocysts might be transmissible between intermediate hosts. Considering the apparent wide range of natural and aberrant intermediate hosts for S. neurona and S. falcatula, transmission of infectious organisms between intermediate hosts implies that control of disease caused by these organisms may be extremely difficult. Insects such as flies and cockroaches may also be transport vectors for S. neurona. Early work demonstrated that flies and cockroaches may act as transport vectors for T. gondii.^906,907 In addition, fatal pulmonary disease developed in psittacine birds fed cockroaches after the cockroaches had been fed opossum feces.⁹⁰⁸ Although this suggests that insects may play a role in transmission of S. neurona, further investigation is necessary to determine which insects are actually involved in its life cycle.

Stress may play a role in the development of EPM,^843,899 but limited evidence is available to support this hypothesis. The severity of EPM may be related to the size of the infective dose, immune competency of the host, and the environmental stresses to which the horse is exposed.⁸⁴⁶ A similar association between immunosuppression and disease has been documented in other species with EPM-like symptoms. For example, recent mouse models have been developed for EPM using nude mice and IFN-γ/KO mice, both of which are immunocompromised strains.^847,848 Raccoons have been identified that were concurrently infected with a Sarcocystis-like protozoan and canine distemper virus (CDV).^884,909 Interestingly, CDV is known to be immunosuppressive and has often been associated with cerebral toxoplasmosis in dogs, foxes, and raccoons.⁹⁰⁹ Immunocompromised people are often infected with T. gondii, and stress plays a major role in the recrudescence of the clinical signs of T. gondii—associated encephalitis.⁹¹⁰ Infections with either N. caninum or T. gondii can cause T-cell hyporesponsiveness to the parasite antigen. It has also been demonstrated that an intact T-cell response, specifically, appropriate interleukin-12 (IL-12) and IFN-γ production, is necessary for resistance against either N. caninum or T. gondii. The parasite may therefore facilitate further infection by compromising host immune responses.⁹¹¹ Recent evidence suggests that neuropeptides called neuroimmune proteins (NIPs) are released from the CNS when an animal is stressed, which may lead to suppression of lymphocyte production and function.⁹¹² Stress leads to high circulating glucocorticoid concentrations, which are also immunosuppressive.⁹¹² The combination of high resting concentrations of glucocorticoids and an increase in NIP release may result in immunosuppression and facilitate development of clinical disease in horses infected with S. neurona. Recent evidence from a controlled investigation at OSU demonstrated that health events before diagnosis of EPM were strongly associated with the disease.⁷⁹⁷ Transport stress and induction of the disease in an experimental equine model provide supporting evidence that stress may play a role in the pathogenesis of EPM.⁸⁶³ It has long been known that transport is a stressor in horses and other species.^913-917 Horses are transported year-round to equestrian events in the United States, sometimes across the country and to other countries. Further controlled investigations are needed to examine the role of stress in the development of clinical signs of EPM in horses.

Diagnosis

The diagnosis of EPM has been difficult because of a lack of understanding regarding its pathogenesis and the variety of clinical signs. Postmortem examination was the first method used to diagnose EPM definitively, and many still consider it the “gold standard” for diagnosis. Grossly, the CNS lesions identified postmortem are described as multifocal areas of hemorrhage to light discoloration of the brain or spinal cord.⁸⁴³ Histology often reveals a marked mononuclear perivascular cuffing with necrosis and loss of neurons, with infiltration of monocytes, lymphocytes, some eosinophils, and rarely, neutrophils.^802,835 Protozoan organisms can be seen in some of the lesions but are often difficult to detect.⁸³⁵ Difficulty in detecting the organisms increases if the animal has been treated with antiprotozoal medications.⁹¹⁸ Immunohistochemical staining techniques can be used to identify parasites definitively in situ.^919,920 Postmortem examination is also the definitive diagnostic test for EPM caused by N. caninum (N. hughesi).⁸⁴⁰ Inmmunohistochemistry is also a useful tool for identification of the Neospora organisms.⁸⁴⁰ However, a significant problem with this diagnostic method is that, by definition, it cannot be applied in horses antemortem and therefore cannot be applied to most clinical cases. A reliable diagnostic test that can be used for antemortem diagnosis is needed to better understand EPM and appropriately manage horses with this disease.

Epidemiology

Little is known about the epidemiology of EPM, although more and more knowledge is being accumulated about this disease.

A small study from one county in Pennsylvania indicated that the seroprevalence was approximately 45% of the horse population (95% confidence interval, 36.3% to 54.3%), and prevalence increased with age.⁹²¹ Another report found an overall seroprevalence of 45% among horses in Oregon, with differences in seroprevalence among geographic regions.⁸⁵⁸ In Oregon the seroprevalence ranged from 22% in the eastern arid region of the state to 65% in the coastal region. A third study reported a 53.6% prevalence of serum antibodies to S. neurona in Ohio horses.⁹²² The Ohio study demonstrated an increase in prevalence with age of the horse, and greater prevalence in southwestern Ohio versus northeastern Ohio. The geographic differences in Ohio may have been related to climatic differences and freezing days in various regions of the state.⁹²² These studies suggest that in many areas of the United States, approximately 50% of the horses may have serum antibodies to S. neurona.⁹²³ Another study suggested horses are exposed to S. neurona in the eastern half of the United States at a rate 10% to 15% higher than the exposure rate in the western half of the country.⁸³² More recently, antibodies to S. neurona were found in 33.6% of various equid serum submitted to a laboratory in Colorado. As previously reported, the prevalence increased with age; prevalence was 26% in horses age 1 to 5 years versus 37% in 10-year-old horses.⁹²⁴ Seroprevalence was lowest during the colder months, as reported previously. Another report on seroprevalence of S. neurona in horses demonstrated 27% in California, 28% in Florida, 54% in Missouri, 0% in Montana, and 0% from New Zealand.⁹²⁵ A Michigan study indicated a seroprevalence of 60%, with lower rates in colder areas, which corroborates earlier findings.⁹²⁶ Another study tested two populations of horses for S. neurona antibodies: the wild horse population in Wyoming and horses from western Canada.⁹²⁷ Using the WB test yielded 18 of 276 Wyoming horses positive and 0 of 243 Canadian horses. These results are difficult to interpret in the Wyoming horses because of the range of the opossum; the Canadian results fit with its range. Two recent reports suggest that the seroprevalence of S. neurona antibody in horses in Argentina and Brazil are 35.5% and 35.6%, respectively.^784,785 Another recent study examined S. neurona exposure in Brazilian horses using an rSnSAG4 ELISA, resulting in a seroprevalence of 69.6%.⁹²⁸ Another study examined American-born horses exported to India for S. neurona exposure.⁹²⁹ Of the 86 horses tested in this study, 42 were still positive, even though some of the horses had been in India for 13 years. This work suggests that antibody to S. neurona has an extremely long half-life, or that chronic infection occurs with this parasite.

These results suggest that exposure to S. neurona is common, but that geographic differences may exist.

Little work has been performed regarding the prevalence of antibody to N. caninum/N. hughesi in horses. However, recent work found a seroprevalence of 23.3% in sera examined from two U.S. horse slaughterhouses and a lack of antibody detection in Argentina and Brazil.^784,785,930 Because of the low numbers of horses involved, these studies may not reflect the true prevalence of N. caninum/N. hughesi antibody in horses. A more recent study found seroprevalence to Neospora of 2% to 3% in Missouri and California.⁹²⁵ Antibodies to N. caninum were found in 31.1% of Wyoming horses.⁹²⁷ However, testing of the Brazilian horses using the rNhSAG1 ELISA demonstrated a seroprevalence rate of 2.5% for N. hughesi.⁹²⁸ Based on these varying results, More work is needed regarding N. caninum/N. hughesi exposure.

No formal studies on the incidence or prevalence of EPM in the United States have been done until recently. Based on the number of cases diagnosed postmortem at the University of Kentucky, the incidence of EPM may be increasing.⁸⁴⁶ The number of samples submitted for immunoblot analysis suggests that several hundred new cases of EPM might be diagnosed in the United States each year.⁸⁴⁶ The estimated incidence of EPM based on accessions to the University of Kentucky diagnostic laboratory was 1% or less of all horses each year.⁸⁴³ The number of U.S. cases has only recently been enumerated by the U.S. Department of Agriculture (USDA), which provide a baseline for future reference. Based on the USDA study, the average incidence of EPM was 14 ± 6 cases per 10,000 horses per year.⁹³¹ The incidence was examined based on primary use of the horse in the operation, and the lowest incidence was found in farm/ranch horses (1 ± 1 cases/10,000 horses/year). Incidence in pleasure horses increased to 6 ± 5 cases/10,000 horses/year. A marked increase followed in breeding horses (17 ± 12 cases), racing horses (38 ± 16 cases), and competition/show horses (51 ± 39 cases). The racing horses did not include horses at racetracks. These estimates reflect a similar incidence of EPM as previously reported, if not lower, and provide a baseline for the future.

Regarding the incidence of neosporosis in horses, no controlled investigations have been performed. There have been six reports of neosporosis in horses caused by N. caninum (N. hughesi).^778-783 However, only four cases were in horses with neurologic signs, one was in an aborted fetus, and one was related to an intestinal problem.

Equine protozoal meningoencephalitis has been reported from a number of U.S. states, as well as from Canada, Mexico, Panama, Argentina, and Brazil.^784-790 EPM has also been reported in England among horses imported from the eastern United States.⁷⁹⁴ EPM was diagnosed in an 8-month-old Arabian horse in South Africa that had been imported from the United States approximately 5 months before the onset of signs.⁷⁹³ The most recent report was a California horse that developed clinical signs of EPM after 10 months in Hong Kong.⁷⁹² An American-born horse developing EPM has also been reported in Japan.⁷⁹¹ Neurologic disease has been reported in horses in France, both American-born horses and horses native to France.^795,796 EPM is thus primarily a disease of the Western Hemisphere.

Several authors have suggested prevalence of disease may be high among standardbred horses.^{801,802,918,932} However, two of these authors also suggested that this apparent predilection may be caused by the environment in which horses were kept rather than breed characteristics.^801,918 Another case series reported that disease was most common in thoroughbreds.⁷⁸⁸ A controlled investigation into risk factors for development of EPM did not find a breed predilection, but occupations such as racing and showing demonstrated increased risk compared with breeding and pleasure horses.⁷⁹⁷ This finding was corroborated by the recent National Animal Health Monitoring System (NAHMS) study.⁹³¹

Early reports on EPM suggested that young horses had an increased risk of disease.^788,802,918 A consistent theme among reports was that at least 60% of the affected horses were 4 years old or younger. An OSU study also found increased risk in young horses, although increased risk was seen in horses older than 13 years as well.⁷⁹⁷

Historically, EPM has been reported as a “sporadic” disease; more than one case is rarely reported on farms.^801,933 In reports of EPM cases from Panama, all affected horses were stabled at the same location, although this is not a common occurrence.⁷⁸⁷ Also, an outbreak was reported on a farm in Kentucky.⁹³⁴ An Ohio study suggested an increased risk for EPM if the disease was previously diagnosed on the farm (>2.5 times higher), which suggests clustering of cases may occur.⁷⁹⁷

Several other risk factors for development of EPM have been reported. The Ohio study found an increased risk if opossums were seen on the farm and with the presence of woods on the farm, seasonal effect, or a health event before development of clinical signs of EPM.⁷⁹⁷ The seasonal effect increased the risk of EPM as the temperature increased, with the highest risk in the fall.⁷⁹⁷ Risk decreased if a creek or river was present on the farm and if the feed was kept protected from wildlife access.⁷⁹⁷ The NAHMS study found an increased risk if opossums were seen (vs. never seen) on the premises and even higher risk if the opossums were seen frequently.⁹³¹ Risk also increased with increased numbers of horses, purchased versus homegrown grain, use of wood chips or shavings as bedding, presence of rats and mice on the premises, and increased human population density. A lower risk was seen when woods were within 5 miles of the premises and when surface water was used as the primary drinking source. As in the Ohio study, the highest risk for disease was in the fall of the year. It is difficult to explain some of the findings from these studies, but management apparently plays a role in development of clinical EPM.

Recent results demonstrate that transplacental transmission of both S. neurona and N. hughesi is unlikely. After following horses at three breeding farms in California and one farm in Kentucky, investigators concluded that there was no detectable risk of transmission of either parasite.⁹³⁵

Treatment and Prognosis

Because of the slow development of a consistent equine model for induction of EPM, and because clinical patients require medication due to the severity of the disease, treatment regimens have evolved empirically. Until recently, recommended therapy had not changed since EPM was originally identified. However, recent use of liquid combination therapies with questionable stability resulted in a lack of response and consistently longer duration of treatment. This has led to the development of novel treatments.

The standard therapy for horses with EPM is a combination of sulfadiazine and pyrimethamine, both antifolate medications. Based on the description of the pathologic lesions of EPM and identification of organisms that resemble T. gondii, the first recommendations for treatment of EPM were extrapolated from therapy used to treat toxoplasmosis in humans.^936,937 Numerous changes have since been made with regard to dosage and duration of the therapy.^{896,933,938,939} Most recommendations were empirically based on clinical impressions rather than controlled clinical trials.⁸⁹⁶ More recently, some therapeutic recommendations have been based on pharmacokinetic data.^940,941 One study tested pyrimethamine, trimethoprim, sulfonamides, and combinations of these drugs against S. neurona merozoites in tissue culture.⁹⁴² Pyrimethamine was demonstrated to be completely inhibitory and coccidiocidal at 1.0 μg/mL. The same was true for trimethoprim at 5.0 μg/mL. None of the sulfonamides alone had activity at 100 μg/mL. Sulfonamides (5.0 or 10.0 μg/mL) in combination with pyrimethamine (0.1 μg/mL) improved activity against S. neurona.⁹⁴² However, these findings are based on in vitro studies, and further work is needed in controlled clinical trials in horses. Controversy surrounds the duration of therapy required to treat horses with EPM effectively; initially, recommendations were based on clearing of specific IgG from the CSF, as indicated by a negative WB. However, many horses remain CSF positive for antibody to S. neurona for months after therapy. Many clinicians have adopted the recommendation that the medications be continued at least 2 weeks after resolution of signs or 4 weeks after a plateau of the clinical signs. Current recommendations for the pyrimethamine/sulfadiazine combination is 20 mg/kg of sulfadiazine once or twice daily and pyrimethamine at 1 mg/kg daily orally for at least 150 to 180 days. Horses with EPM are often treated for long periods with medications that act by inhibiting folate metabolism. Some suggest that complete blood counts should be monitored for signs of folic acid deficiency in horses treated for EPM. Potential side effects of treatment with antifolate medications include bone marrow suppression, anemia, colitis, and even teratogenesis. Most of the anemias are mild and improve after withdrawal of the medication. One other side effect of trimethoprim-sulfamethoxazole and pyrimethamine, a commonly used combination in the past, is its effect on reproductive function in pony stallions.⁹⁴³ Although it may not affect semen quality, testicular volume, sperm production efficiency, erection, or libido of healthy stallions, it may induce changes in copulatory form and agility and alter the pattern and strength of ejaculation.⁹⁴³ Therefore, caution should be used when treating stallions for neurologic disease believed to be EPM.

Recently, triazine derivatives have been used to treat EPM. Two of these drugs, diclazuril and toltrazuril, were originally designed for use as herbicides and have been used in other countries in the prophylaxis of coccidiosis in poultry and swine. The response to therapy in horses with EPM was slightly better than the response documented for the standard therapy.⁹⁴⁴ The pharmacokinetics of both diclazuril and toltrazuril have been demonstrated.⁹⁴⁵ Currently, diclazuril is only available as a ration premix, so large volumes must be given daily. Another disadvantage is the poor palatability of diclazuril in its present form. One advantage to use of these compounds is an appreciably shorter duration of therapy. Diclazuril is administered at 5 mg/kg for a minimum of 28 days.⁹⁴⁶ Recent in vitro testing for activity of diclazuril against S. neurona has been demonstrated.⁹⁴⁷ Diclazuril may need to be administered by nasogastric tube daily.⁹⁴⁶ Toltrazuril is another coccidiostat becoming increasingly popular because of its ease of use and good absorption orally in horses.⁹⁴⁸ Toxicity studies of toltrazuril in horses at 50 mg/kg for 10 days resulted in mild anorexia and depression.⁹⁴⁹ The current recommended dose is 5 to 10 mg/kg for a minimum of 28 days.⁹⁴⁶ Further in vitro evidence indicates that ponazuril, a metabolite of toltrazuril, is effective against S. neurona.⁹⁵⁰

Nitazoxanide (NTZ) is another novel treatment recently used in the treatment of EPM. NTZ is a 5-nitrothiazole with a broad spectrum of activity against bacterial, protozoal, and helminthic parasites.⁹⁴⁶ It has been shown to kill S. neurona in cell culture.⁹⁴⁶ Toxicity studies showed that when horses were given two times the recommended dose, they became lethargic after 1 week of daily dosing.⁹⁴⁶ When horses were given NTZ at four times the recommended dose, they became significantly ill, with one death.⁹⁴⁶ At present, the suggested dose schedule is 25 mg/kg once daily for the first week and 50 mg/kg once daily for the next 23 days.⁹⁴⁶ Two of these three new EPM medications, Marquis and NTZ, have been approved by the U.S. FDA.

The prognosis for horses diagnosed with EPM is similar regardless of the treatment used. Most reports suggest an approximate improvement rate of 70% when using the standard therapy,^804,951,952 but earlier work suggested the success rate of therapy was about 50%.⁹³⁹ Less than 25% of affected horses may return to their original function, although little objective information exists on this issue.⁸⁰⁴ A recent study with diclazuril resulted in approximately 75% improvement in horses severely affected with EPM.⁹⁴⁴ In the diclazuril study, approximately 30% of the horses (11/36) treated either returned to their original level of performance before EPM diagnosis or improved their level of performance.⁹⁴⁴ An efficacy study of 70 horses given NTZ found 63% of the horses met the criteria for success after treatment.⁹⁴⁶ A growing concern is the percentage of horses which have a relapse in clinical disease after cessation of therapy. Some horses will relapse days, weeks, or even months after cessation of therapy, but the mechanism of relapse is unknown.⁸⁹⁶ Relapse may be caused by recrudescence of a truly latent stage of the parasite, presence of a small, persistent focus of infection, or perhaps reexposure to the parasite.⁸⁹⁶ Anecdotal estimates of the relapse rates range from 10% to 28% of treated horses.^804,951,952 In a controlled study performed at OSU, relapse rates were 19%, close to previous anecdotal reports.⁹⁵³ The relapse rate using diclazuril for the treatment of EPM was less than 5%.⁹⁴⁴

Identification of protein activity in S. neurona merozoites has demonstrated two potential targets for therapy, including serine protease and enolase.^954,955 In the future, better treatments might be developed against certain proteins to remove or reduce relapse rates and improve the resolution of clinical signs of EPM.

Before recognition of EPM, corticosteroids were widely recommended for treatment of neurologic diseases in horses. However, corticosteroids should be used with caution in horses with suspected EPM because the host immune response to the organism could be adversely affected.^{789,794,936,937} NSAIDs and DMSO have also been routinely used in the treatment of horses with EPM since the mid 1980s.^794,804

Because antibody to S. neurona persists in CSF for long periods in some horses with EPM, and because some horses may not mount a sufficient immune response to clear the organism, immune stimulants (nonspecific T-cell—stimulating compounds) have been recommended.⁸⁰⁴ Unfortunately, no controlled trials have examined the efficacy of these treatments.

Supplementation with folic acid, folinic acid, and brewer’s yeast has been recommended for treatment of presumed folic acid deficiency, particularly in pregnant mares.^794,939 However, folic acid supplementation has been discouraged by other investigators because of poor absorption and the potential for toxic effects on bone marrow activity.⁸⁰⁴ Toxicity has also been reported in newborn foals born to mares that were treated for EPM with antifolate medications and concurrently supplemented with folic acid.⁹⁵⁶ These foals showed evidence of bone marrow aplasia and hypoplasia, renal nephrosis or hypoplasia, and skin lesions. Another case report involved an adult horse being treated for EPM.⁹⁵⁷ A cause-and-effect relationship between folic acid supplementation and these developmental abnormalities has not been conclusively demonstrated. At present, however, folic acid supplementation should not be used, particularly in pregnant mares, until controlled clinical trials can be performed to corroborate or refute these findings.

Use of additional supplements, such as vitamin E and thiamine, that may facilitate healing of nervous system tissue have been recommended for treating horses with EPM.^804,951 However, clinical trials have not been performed to establish the efficacy of this supplementation.

Prevention

Based on the proclivity for transportation to equestrian events and the nature of the horse business, prevention of clinical cases of EPM will be difficult. A complicating factor is that the parasite is widespread throughout much of the United States. A killed—S. neurona vaccine was developed and released on a conditional license in 2000. The vaccine has been shown to induce both humoral and cell-mediated immunity in the horse.⁹⁵⁸ However, efficacy of the vaccine to prevent EPM using an equine model have not been completed to date. Because development of other parasite vaccines is extremely difficult, efficacious vaccines most likely will not be developed for many years.^949-962

Based on age as a risk factor for the disease, close monitoring for evidence of neurologic disease in high-risk age-groups (young horses, old horses) may help detect EPM early. The seasonal risk for EPM should raise the index of suspicion that it may be the cause of the clinical signs when horses are presented for neurologic disease in the warmer months. This seasonal risk factor is compounded by many major horse competitions occurring in the fall. Therefore, monitoring of horses subsequent to transport and competition may be helpful. Access to feed and water by wildlife (opossums) and pests (mice, rats) should be restricted by using rodent-proof containers, which may help to prevent some cases of EPM. Forages should also be protected from wildlife access by keeping the forages in enclosed facilities to prevent access to opossums.⁷⁹⁷ Some research suggests that preventing bird access to facilities may help to prevent some cases of EPM, although it is not known what role birds may play in its pathogenesis. Ingesting sporocysts may result in passage through the intestinal tract of birds and may be infective for other species of wildlife or horses.^904,905 Horses may develop EPM after some other health event, as confirmed by a recent controlled investigation.⁷⁹⁷ Therefore, monitoring of broodmares close to foaling and horses that develop a major illness or injury is important because it may assist in the early diagnosis of EPM cases.

Focusing prevention on manipulation of risk factors for the disease, such as keeping opossums out of the feed and water, is very important. However, one also must consider the intermediate host’s role in EPM. As discussed previously, several species of mammals have been reported to act as natural or laboratory intermediate hosts in the life cycle of S. neurona.^872,891-893 These animals are not a threat unless they are dead. Therefore, veterinarians should encourage horse owners to pick up dead cats, armadillos, skunks, and raccoons on their property. It is important to dispose of the carcass so that opossums cannot eat them and excrete more infectious organisms to infect horses. Picking up these carcasses should be done carefully with an inverted plastic garbage bag, plastic gloves, or some other instrument.

These protozoan parasites are extremely difficult to kill in the environment. A recent study tested the most common disinfectants used in veterinary medicine, including povidone-iodine, chlorhexidine, formalin, two different strengths of NaOH, and high-temperature steam.⁹⁶³ The S. neurona sporocysts thrived on the disinfectants and the lower concentration of NaOH. At the higher concentration of NaOH, the parasites did not survive the treatment, however, most barn materials could not withstand this agent. The parasite could not withstand temperatures of 60°C (140°F) for 1 minute or higher temperatures for shorter periods. Therefore the only alternative for cleaning horse facilities is use of steam.

Recently, medications to prevent development of clinical signs in IFN-γ/KO mice and in horses were attempted. The first study used pyrantel tartrate, a daily anthelmintic prophylactic treatment, and concluded that it does not prevent S. neurona infections in mice.⁹⁶⁴ A similar study in horses tested for seroconversion in serum and CSF and days to seroconversion.⁹⁶⁵ There was no difference in the groups receiving and those not receiving pyrantel. Another study used the triazine-derivative anticoccidial diclazuril;* mice treated before or up to 7 days after infection did not develop clinical signs, and no parasites were recovered from the mice.⁹⁶⁶ This study suggests that anticoccidial drugs may be useful in the prevention of S. neurona infections in animals. A second triazine derivative, ponazuril, was tested in IFN-γ/KO mice.⁹⁶⁷ A single dose prevented abnormal neurologic signs and death, depending on when it was administered. This further corroborates the potential for prevention of EPM using anticoccidial drugs. In a further study on prevention of EPM in horses, two dose ranges of ponazuril were administered daily based on the recommendations for treatment; 71% of horses given 2.5 mg/kg and 40% given 5 mg/kg developed neurologic deficits.⁹⁶⁸ Seroconversion was decreased in the 5-mg/kg group compared with the controls. Therefore, this study needs to be repeated to attempt other doses of ponazuril for prevention of clinical signs of EPM, because coccidiostats may be one method of prevention useful in the future.

Definition and Etiology

Ehrlichia ruminantium is a rickettsial parasite that causes a fatal encephalitis in goats, sheep, and cattle.⁹⁷⁹ The disease originated in sub-Saharan Africa and has spread to cattle in the West Indies (Guadeloupe, Antigua, and Marie Galante),⁹⁸⁰ where it has become an economically important tick-borne disease of cattle. E. ruminantium is transmitted by Amblyomma ticks.^981,982 Although a number of Amblyomma species have been implicated in the transmission of heartwater disease, the most important agents are A. hebraeum and A. variegatum. The intermittent feeding behavior of the tick makes it particularly resistant to treatment with acaricides. Amblyomma ticks require three separate blood meals to complete their life cycle. The gravid females fall from the host and lay the eggs in rotting vegetation, particularly in areas where the hosts are bedded for the evening. Recently hatched larvae crawl onto foliage and await a host. After their first feed the larvae detach, molt into nymphs, and await a second host. After refeeding the nymphs detach and molt into adults. The adults remain under rotting vegetation until they are activated by carbon dioxide exhaled by a large mammal. They are further attracted to the host by pheromones from male ticks that remain permanently attached to the host. Animals that do not have male tick infestations are poor attractants for nongravid females. Once attached to the proper host, the females seek the male, breed, feed, and fall from the host when it lies down for the evening. Ticks that feed from Ehrlichia-infected hosts develop ovarian infections and transmit the agent to their offspring. This serves to perpetuate the agent over successive seasons.⁹⁸³

Many species of vertebrates, including snakes, iguanas, lizards, and birds, are reservoirs for E. ruminantium because these animals may serve as the first two hosts for the Amblyomma tick.⁹⁸⁴ A vector-wildlife cycle facilitates the survival of E. ruminantium even when domestic livestock are absent from the environment.⁹⁸⁵

Clinical Signs

Animals with the peracute form of E. ruminantium infection die suddenly without premonitory signs. The acute form of the disease is characterized initially by fever, anorexia, depression, and respiratory distress. Cyanosis also may be noted. Nervous system signs, which may appear within a few days, include hyperesthesia, snapping closure of the eyelids, rapid extension of the tongue, behavioral changes, muscular fasciculations, hypermetria, ataxia, conscious proprioceptive deficits, and head pressing. As the disease progresses, the animals become recumbent and comatose. Convulsions may occur terminally. These episodes are characterized by opisthotonos, nystagmus, chewing movements, and frothing at the mouth. Mild forms of the disease are characterized by transient diarrhea, malaise, and fever, with no CNS involvement. The mortality rate in sheep ranges from 6% to 80%. Animals that recover are immune to reinfection for at least 58 months.⁹⁸⁶ Losses may reach 60% of susceptible cattle and 40% of goats. The mortality rate among Angora goats may exceed 90%.

Pathophysiology

After inoculation into a ruminant, the rickettsial agent infects reticuloendothelial cells and proliferates by binary fission within membrane-bound vacuoles.^987,988 Release of the parasite from degenerating macrophages and neutrophils causes successive waves of parasitemia that infect endothelial cells and cause vasculitis.^988,989 In the cell the developmental stages of the Ehrlichia organism resemble those of chlamydia and include elementary, reticulate, and intermediate bodies, which can be differentiated microscopically.⁹⁹⁰ Nervous system lesions may be caused by permeability changes in the cerebral capillaries. Changes in the other soft tissues include hydropericardium, hydrothorax, and subcutaneous edema.⁹⁷⁹

Except for Angora goats, which are highly susceptible to Ehrlichia infection, animals reared in indigenous areas usually have a high level of immunity and do not succumb to the infection.⁹⁹¹ Animals that survive the initial infection become asymptomatic but remain rickettsemic for as long as 223 days (sheep), 246 days (cattle), and 8 days (goats). Calves under 3 weeks of age, lambs under 8 days of age, and kids under 6 weeks of age are inherently resistant to E. ruminantium infection, regardless of the amount of colostral protection they have received.^992,993

Necropsy Findings

Pathologic changes of heartwater disease include hydropericardium, hydroperitoneum, hydrothorax, pulmonary edema, perirenal edema, hemorrhages in the pleura and peritoneum, and hemorrhagic enteritis. Microscopic changes include microgliosis, necrotizing vasculitis in the brain, hemorrhage, edema of the neuropil, microcavitation, and focal necrosis of the granular layer of the cerebellum. In clinical cases the parasite can only be definitively diagnosed by biopsy of the cerebral cortex or by collection of the cortical tissues at postmortem examination.⁹⁹⁴ Simple techniques for collection of such biopsy specimens have been described.^995-997 Squash preparations of the biopsied material should be stained with either methyl green pyronine or Giemsa stain before microscopic examination of the tissues.^998,999 A cloned DNA probe that identified E. ruminantium DNA has been developed, but the clinical usefulness of the test is unknown.¹⁰⁰⁰

Diagnostic tests developed to identify infected livestock include an indirect fluorescent antibody (IFA) test using infected bovine aortic endothelial cells for indicators, several ELISAs, and tests using PCR methodology.^1001-1004 Of these, PCR is the most accurate.

Treatment

The administration of oxytetracycline (6 to 10 mg/kg IV twice daily for 3 or 4 days) may be beneficial for treatment of the early stages of the disease. The long-acting formulation of oxytetracycline also is effective, but for best results it should be administered as soon as the animal becomes febrile. Addition of dimethyl sulfoxide (DMSO) to the treatment regimen may be helpful: DMSO improved respiratory function in E. ruminantium—infected sheep, although it reduced their appetites.¹⁰⁰⁵ Treatment usually is futile if the first dose of oxytetracycline is administered after the onset of neurologic symptoms.¹⁰⁰⁶ Despite the depository nature of the long-acting formulation, two or more doses 48 hours apart are needed to achieve a good clinical response. Cattle should be re-treated if they develop a fever after the first dose has been administered.¹⁰⁰⁷ Animals with nervous system signs frequently die despite intensive antibiotic therapy. Angora goats are highly susceptible to heartwater disease. In South Africa, Angora producers routinely treat all animals every 14 days during the summer with oxytetracycline. Nevertheless, the number of deaths caused by heartwater disease is directly related to the number of antibiotic treatments administered. Animals that remain essentially tick free never develop adequate immunity to the Ehrlichia organism.

A small degree of tick infestation and exposure of the animals to low numbers of the agent, combined with judicious oxytetracycline therapy, would seem to favor the development of immunity over time.¹⁰⁰⁸ One method of immunization uses a controlled infection of a virulent strain (Onderstepoort Ball 3 strain) of the Ehrlichia organism and treatment with long-acting oxytetracycline (800 mg per adult goat) at the beginning of clinical disease and 10 days later.¹⁰⁰⁹ Vaccination with this isolate produces immunity against exposure to homologous but not heterologous strains of Ehrlichia.^1010-1012 More recent strategies have included vaccines utilizing inactivated organisms, live attenuated organisms, and fragments of E. ruminantium nucleic acid, but none has had great success.^1013-1017

Prevention and Control

Control of ticks on cattle pastures is the most desirable means of controlling heartwater disease. Complete eradication of the ticks in most regions of sub-Saharan Africa is neither possible nor desirable. Cattle that are reared in areas where Ehrlichia infection is endemic develop acquired immunity over time. In these areas the ticks should be sufficiently abundant to permit a low level of heartwater infections in most cattle, yet not so populous as to introduce severe, overwhelming infections. Integrated methods for tick control have been recommended. These include exclusion of wildlife from paddocks, artificial induction of host resistance to ticks, application of insecticidal ear tags, and conventional acaricide application. Application of insecticides as the sole method of tick control has not proved highly effective. Resistance to the acaricides may develop with prolonged use. The dips are expensive and frequently are used at improper concentrations. Moreover, some species are not highly attracted to ruminants unless other ticks are already attached. Treatment of these animals would be ineffective and wasteful. A novel osmotic pump loaded with ivermectin that delivers 60 μg/kg/day kills Amblyomma hebraeum and reduces the number of fertile eggs shed. Data on the efficacy of the drug under field conditions have not been published.

Definition and Etiology

Cerebral theileriasis is an encephalitic disease of cattle caused by the piroplasma parasites Theileria annulata and Theileria parva.^1018,1019 Theileriasis is seen mainly in Africa (Kenya and Tanganyika) and India, where it is characterized by a high mortality rate and is known as East Coast Fever. Theileria sergenti is found in Korea and Japan. Theileria mutans, a parasite of cattle in the southwestern United States, is relatively nonpathogenic.¹⁰²⁰ A mild form of theileriasis, locally called “January disease,” occurs in cattle in Mozambique. This condition is caused by the subspecies Theileria parva bovis. Corridor disease is caused by Theileria parva Lawrence, and Theileria annulata is the cause of Mediterranean Coast fever or tropical theileriasis. Other species associated with cerebral theileriasis include Theileria mutans and Theileria taurotragi.¹⁰¹⁸

East Coast fever probably originated in buffalo in eastern Africa and later spread to cattle. Spread of the disease to southern Africa probably occurred through introduction of infected cattle from eastern Africa. European breeds of cattle (Bos taurus) develop a more severe disease than do comparably infected Indian breeds (Bos indicus). Both types of cattle appear to be equally susceptible to infection, but differ in their ability to control the course of disease.¹⁰²¹ Theileria organisms are transmitted to susceptible cattle by the ticks Hyalomma anatolicum, Hyalomma lusitanicum, and Amblyomma hebraeum.¹⁰²²

Trypanosoma species manipulate the immune system both to evade destruction and to promulgate infection. Organisms escape the host immune response by altering immune system activation so that infected cells avoid recognition and destruction.^1023-1025 Clonal expansion of infected lymphocytes is promoted, and these cells disseminate throughout the body.¹⁰²⁶

Clinical Signs

The clinical signs of Theileria infections include lymphadenopathy, nasal discharge, lacrimation, tachycardia, fever, subcutaneous edema of the face, gangrenous dermatitis, sloughing of facial skin, dyspnea, pallor, CNS disorder, and emaciation.¹⁰²⁷ The neurologic syndrome is characterized by ataxia, hypermetria, conscious proprioceptive deficits, depression, head pressing, hyperesthesia, blindness, nystagmus, circling, and aggressiveness. Terminally the animals become recumbent and develop opisthotonos, tonic-clonic seizures, and coma. In rare cases the parasite may localize in the spinal cord. The CNS signs result from vasculitis and lymphocytic inflammation of the brain.^1028-1030 Clinically recovered animals become persistently infected.

Necropsy Findings

The postmortem lesions of theileriasis include capillary engorgement, scattered punctate hemorrhages on the surface of the brain, thrombosis of the meningeal vessels, hemorrhage in the cerebral ventricles, pulmonary edema, peripheral lymphadenopathy, and infarctions of the kidney and spleen. The brain of affected animals appears to have a yellow hue.¹⁰³⁰ Microscopically, blue cytoplasmic inclusion bodies (Koch’s blue bodies) are seen in the lymphocytes adjacent to the hemorrhagic areas.

The nervous form of theileriasis is difficult to diagnose definitively because the parasite is only sporadically visible in sections of nervous tissue from the infected animals. Parasitemia occasionally can be detected by microscopic examination of blood smears from infected calves. Reliable blood tests currently are not available. Biopsy of the cerebral cortex has been recommended as a confirmatory test for the disease. The CSF of affected animals is normal or has an increased protein concentration ranging from 1400 to 12,452 mg/dL, with normal numbers of white blood cells.¹⁰²⁹

Treatment

Parvaquone (Clexon),* 10 to 20 mg/kg intramuscularly (IM) in two injections 48 hours apart, and buparvaquone (Butalex),^† at a single dose of 2.5 mg/kg, are approximately 90% effective for the treatment of theileriasis.¹⁰³¹ Some studies have found buparvaquone to be the most effective treatment.^1032,1033 Parvaquone can be combined with furosemide, which improves survival of cattle that develop pulmonary edema, a common consequence of theileriasis.¹⁰³⁴ Menoctone (10 mg/kg IV or IM) also is curative. Single doses were effective, but repeating the treatment daily for 5 days eliminated posttherapeutic recrudescence.¹⁰³⁵ The disease is exotic to the United States.

Prevention

Prevention of Theileria infection has largely been based on the use of live attenuated vaccines, which produce a solid, cell-mediated immunity.¹⁰³⁶ These vaccines are used in an “infection and treatment” manner, whereby vaccination must be combined with administration of long-acting tetracyclines.¹⁰³⁷ Recent efforts at vaccine development have focused on the creation of subunit vaccines, using a variety of Theileria antigens. While promising, more work is still needed.^1038-1041 Use of vaccines, however, is still not on a large scale.

The napthoquinones are the preferred chemotherapeutic agents in vaccinated animals and include parvaquone, halofunginone lactate,* and buparvaquone (see Treatment).

Control

Tick control is vital. Weekly spraying with coumaphos and insertion of ear tags impregnated with cypermethrin (Decum)^† have been effective for controlling ticks and preventing Theileria infections in calves in Tanzania. The calves became infected with Theileria parva by 3 months after the tags were removed. Woodford¹⁰⁴² suggested that the intensive tick control could minimize the incidence of theileriasis until the calves could be successfully vaccinated. However, inadequate or incorrect application of products and tick resistance to chemicals present challenges to effective tick control.

Definition, Etiology, and Clinical Signs

Trypanosomiasis is a hemoprotozoan disease of African cattle that also infects the central nervous system (CNS). The disease is transmitted to cattle by the bite of the tsetse fly (Glossina species). The agents that infect cattle include Trypanosoma vivax, Trypanosoma congolense, and Trypanosoma brucei. An especially severe neurologic form of the disease has been caused by the inoculation of cattle with T. congolense, followed 1 year later by infection with T. brucei, or by simultaneous inoculation with the two agents.¹⁰⁴³ The encephalitic signs develop 2½ to 5 months after infection and include ataxia, conscious proprioceptive deficits, knuckling, depression, circling, and head pressing. Some animals may show signs of behavioral change, lose the herd instinct, and develop hyperesthesia and constant repetitive movements. Other signs associated with progression of trypanosomiasis are semicoma, coma, recumbency, opisthotonos, and intermittent tonic-clonic convulsions, which occur 2 to 3 days before death. Affected animals are emaciated, anemic, and icteric at death.¹⁰⁴⁴

Experimental infection with trypanosomes causes a marked fever (as high as 40.5°C [105°F]). Other common nonneurologic signs are anemia, petechiation of the mucous membranes, occult fecal blood, melena, and epistaxis. Chronic weight loss without other clinical signs may be seen in some animals. Hematologic abnormalities include anemia, hypoalbuminemia, hyperbilirubinemia, and increased plasma concentrations of aspartate transaminase and urea nitrogen.

Acutely infected animals may develop a thrombocytopenia and prolongation of the prothrombin and partial thromboplastin times, indicating that DIC may be responsible for the vascular changes that occur before death. The anemia is characterized by increased mean corpuscular volume and mean corpuscular hemoglobin, with increased serum iron concentrations early in the infection.^1045,1046 One study did not demonstrate a change in the concentration of white blood cells or protein in the CSF of animals with the encephalitic form of trypanosomiasis.¹⁰⁴³ However, another study showed significant alterations in cattle infected with T. bruceii.¹⁰⁴⁷ These included an increase in total protein (range, 37 to 44 mg/dL) and pleocytosis (range, 0 to 3060 mononuclear cells/μL). The abnormalities in the CSF may be present in infected cattle not currently displaying clinical neurologic signs. Antitrypanosomal antibody may be found in the CSF of affected cattle using IFA tests.

Pathology

The pathologic lesions of trypanosomiasis are a nonsuppurative encephalomyelitis, serosanguineous pericardial fluid, serosal hemorrhages, pulmonary edema, centrilobular coagulative necrosis, splenomegaly, necrotizing myocarditis, and glomerulonephritis.¹⁰⁴⁸ Macroscopic lesions of the CNS include subtle thickening and grayish discoloration of the meninges. The meningeal vessels are congested. Microscopic lesions of the CNS include mild to moderate diffuse meningoencephalitis, plasmacytic and lymphocytic perivascular cuffing, nodular gliosis, and mononuclear choroiditis. The pathogenesis of the encephalitis is not understood.

Trypanosomes may be cultured from the blood and CSF of infected cattle. The number of hematogenous parasites is highest in animals with a dual infection by T. congolense and T. brucei. The clinical diagnosis of trypanosomiasis may be confirmed by inoculating blood or CSF specimens into laboratory mice and observing the recipients for parasitemia with direct darkfield examination of the patient’s blood. Identification of motile trypanosomes in the buffy coat zone of a microhematocrit capillary tube using darkfield illumination apparently is the most accurate of all diagnostic methods. Trypanosomes can be differentiated by their morphologic features, manner of attachment to erythrocytes, and type of motility.¹⁰⁴⁹ Species-specific antibodies can be detected in serum using either an antigen-capture ELISA or an IFA test in which the column-purified trypanosomal antigen is fixed with acetone or formalized saline. Titers of 1:200 to 1:2000 were consistent with acute infection. The test is not typically used for field diagnosis of trypanosomiasis.^1050,1051 Infected cattle develop acquired resistance to homologous but not heterologous isolates of trypanosomes.

Treatment and Control

Of the drugs available for the treatment of trypanosomiasis, isometamidium chloride* is most often used. The recommended dosage ranges from 0.25 to 1 mg/kg IM; a single dose of 1 mg/kg exerts a protective effect for up to 6 months.¹⁰⁵² The higher dosage (1 mg/kg) was required to obtain increased weight gain and prevent recurrent infection. The treatment was particularly effective when combined with weekly surveillance, followed by treatment of confirmed infected animals.¹⁰⁵³ Side effects of isometamidium include tachycardia, salivation, lacrimation, pollakiuria, muscle fasciculations, convulsions, diarrhea, and in rare cases, death. The drug apparently does not cause abortion in pregnant cows or otherwise affect the calf.¹⁰⁵⁴

Other drugs used include suramin sodium,^† diminazene aceturate^‡ (7 mg/kg), quinapyramine sulfate and homidium chloride^§ (1 mg/kg). These drugs provide residual protection from reinfection for approximately 2 months; however, recurrent infection and resistance, especially to diaminazene, have been reported.^1055-1058 Combinations of these drugs (e.g., diaminazene followed by isometamidium) reduce the number of resistance-related therapeutic failures. Relapses also undoubtedly occur because many of the chemoprophylactic drugs are unable to penetrate the blood-brain barrier in sufficient concentrations to eliminate the parasite in the CNS tissues.

Because of the variable responses to treatment and the ability of Trypanosoma organisms to develop drug resistance, all control programs must include methods of controlling the tsetse fly.¹⁰⁵⁷ Current recommendations for fly control include application of insecticides with residual pyrethroids. Several methods of pyrethroid application have been investigated, including inclusion in a visual baited target (deltamethrin)¹⁰⁵⁹ or application of a pyrethroid pour-on formulation (Bayticol pour on, 10 mL/100 kg).* Both methods have been highly effective for reducing the tsetse fly population and trypanosome infection rates. Animals can be dipped in Deltamethrin dip every 2 weeks. A single application of the chemical has residual activity for as long as 52 days; however, to ensure maximum killing, application every 14 days is recommended. Because the tsetse fly prefers to feed from the ventral torso or the legs, insecticide-impregnated ear tags have been ineffective for preventing infection.¹⁰⁶⁰ Other fly control methods include aerial spraying with endosulfan, ground-based spraying with 4% dichlorodiphenyltrichloroethane (DDT), or use of scented insecticidal traps.¹⁰⁵⁹ However, cost and environmental concerns have limited the usefulness of these techniques.

Selection of resistant lines of cattle is possible. The taurine N’Dama and West African shorthorn breeds have innate resistance to trypanosomiasis and are the sole breeds of cattle in areas of tsetse fly range.¹⁰⁶¹ Resistant breeds of small ruminants include the Djallonke, Red Maasai, Blackhead Persian, and East African sheep and goats.¹⁰⁶² Imported breeds of livestock usually cannot be maintained, even in areas of low tsetse fly risk, without intensive drug therapy.¹⁰⁶³

Definition and Etiology

Polioencephalomalacia (PEM) is a common and important neurologic disease of ruminants^1064,1065 with a worldwide distribution. An animal with clinical manifestations of PEM often is referred to as having “polio” or being a “sleeper” or “brainer.”

Polioencephalomalacia is a descriptive term for histologic lesions^1064,1066 that may arise from a variety of etiologies. Literally, the name means softening or necrosis (malacia) of regions of the gray matter (polio) of the brain (encephalo). Thus a definitive diagnosis of PEM requires appropriate histologic examination of brain tissue. The possible etiologies of PEM include (but are not limited to) excessive sulfur consumption,^1067-1069 presumably manifested through elevated ruminal sulfide production;^1070,1071 altered thiamine metabolism;¹⁰⁷² so-called salt poisoning or water deprivation;¹⁰⁷³ amprolium administration;^1074-1076 the molasses-urea diet;¹⁰⁷⁷ and lead intoxication.¹⁰⁷⁸

Clinical Signs

PEM appears to manifest both subacutely and acutely.^1064,1071 In the subacute form, signs may develop within hours or over several days. In the early stages of the disease, the affected animals detach from the herd or flock, become anorectic, and stagger. They often appear blind, walk with the head held erect, and demonstrate a slight hypermetric gait. Occasionally, affected animals are excitable and charge around their enclosure, which may present a significant hazard to the veterinarian and animal handlers. Other early signs of PEM can include diarrhea, hyperesthesia, and muscle tremors, which are most obviously observed as ear flicking or facial twitching. Progression of the condition is associated with cortical blindness, head pressing, opisthotonos, dorsomedial strabismus, miosis, repetitive chewing, profuse ptyalism, and odontoprisis^{1064,1068,1079-1081} (Figs. 35-10 and 35-11). Despite the defective menace response, the animals usually have normal palpebral reflexes. Affected animals may also develop a variable nystagmus, strabismus (Fig. 35-12), and head tilt. The rectal temperature is normal unless excessive muscular fasciculations have developed. The pulse and respiratory rates are usually increased. An odor of hydrogen sulfide may be detected on the breath if the PEM is associated with excessive sulfur consumption.

Fig. 35-10 Feedlot steer with clinical manifestations of polioencephalomalacia. The steer has adopted a sawhorse posture.

Fig. 35-11 Calf with advanced signs of polioencephalomalacia showing abnormal head posture and depressed sensorium.

Fig. 35-12 Abnormal pupillary angle associated with strabismus in a calf with polioencephalomalacia. Because the calf’s head is tied upward, the eye has rotated ventrally, although this is actually dorsomedial strabismus.

Although most of these animals respond favorably to aggressive therapeutic intervention, clinical signs may progress to recumbency, tonic-clonic convulsions, and death. In facilities with certain types of fencing, such as cables, affected animals may push or press with such force that they die of asphyxiation.

In the acute form of PEM, animals are found recumbent and comatose.^1064,1071 These animals often experience episodic tonic-clonic convulsions, and they remain recumbent and hypertonic between seizures. The prognosis is poor for acutely affected animals or those with advanced subacute manifestations. Survivors may remain irreversibly decorticated and are culled because of poor performance, chronic anorexia and ataxia, or blindness. However, mildly affected animals may remain as productive members of the herd.

Because the clinical manifestations of PEM may be subtle and nonspecific, they can be confused with other disorders. PEM often is temporarily associated with lactic acidosis that develops after consumption of excessive amounts of readily fermentable carbohydrates. Animals with lactic acidosis may appear ataxic and obtunded in addition to having a foul-smelling, watery stool and a distended, fluid-filled rumen. Concurrent PEM may not be diagnosed, or producers may confuse PEM for lactic acidosis or primary ruminal tympany in acutely affected animals that have remained laterally recumbent for some time.

The major differential diagnoses for PEM include enterotoxemia type D (focal symmetric encephalomalacia form), Haemophilus meningoencephalitis (thrombotic meningoencephalitis), coccidiosis with nervous system involvement, listeric meningoencephalitis, vitamin A deficiency, ethylene glycol poisoning, locoism, rabies, and infectious bovine rhinotracheitis encephalitis (calves only).

Clinical Pathology

Although a definitive diagnosis depends on histologic confirmation, a presumptive diagnosis may be made antemortem based on the history and clinical signs or on a definitive diagnosis in herdmates of affected animals. If a diagnosis of PEM is made, either presumptive or definitive, attention should be focused on identifying the likely causes so that exposure of herdmates to etiologic agents can be mitigated or eliminated. The investigation may proceed at the animal, herd, and environmental level to identify evidence supportive of sulfide toxicity, thiamine deficiency, lead toxicity, or water deprivation—salt toxicosis.

Sulfide concentrations in the ruminal fluid and gas cap in experimentally induced sulfur-associated PEM have been shown to be high.^1082,1083 However, unpublished data supported a finding of a decrease in gas cap sulfide concentrations in naturally developing PEM associated with increased sulfur consumption.¹⁰⁸⁴ This probably is caused by the rapid metabolism of sulfate to sulfide in the rumen and resultant absorption or eructation of hydrogen sulfide (H₂S). Animals with naturally developing PEM are likely to have been anorectic for some time, resulting in a decrease in oxidized and reduced forms of ruminal sulfur.

Estimation of the rumen gas cap H₂S concentration in clinically healthy penmates of affected cattle is an effective chute-side diagnostic procedure that can indicate excessive sulfur consumption.¹⁰⁷¹ This method provides real-time results that may aid direction of further animal and environmental investigations. In short, an area in the left paralumbar fossa is prepared for ruminocentesis. An 18-gauge, 3½-inch spinal needle is inserted through the body wall and into the rumen gas cap. A modified gas sampler is attached to the spinal needle by means of an extension set, and a known amount of gas is drawn through an H₂S detector tube.¹⁰⁸³ It is important to adjust the values to account for any dead space of the sampling instrument, such as the extension set and other modifications. H₂S concentrations greater than 1000 ppm are indicative of excessive sulfur consumption.¹⁰⁸⁵

Appropriate blood samples may be analyzed for lead concentration and possibly estimation of thiamine status. Thiamine status generally is evaluated using one of several available methods, including determining the total blood thiamine concentration using a thiamine-dependent Lactobacillus bioassay.¹⁰⁸⁶ The erythrocyte thiamine pyrophosphate concentration may be measured by high-performance liquid chromatography (HPLC).¹⁰⁸⁷ The value of estimating all phosphorylation forms of thiamine (free, diphosphate, and triphosphate) is questionable. Table 35-9 shows the reference ranges for the total thiamine concentration in normal and affected cattle.

Table 35-9 Mean Concentration of Thiamine in Tissues*: 95% Confidence Intervals in Clinically Normal Cattle and Sheep and in Patients with Polioencephalomalacia

Another method of evaluating thiamine status is determining erythrocyte transketolase activity. This is a sensitive and specific measurement of active thiamine status.¹⁰⁸⁸ Transketolase catalyzes the reaction between xylulose-5-P and ribose-5-P to form sedoheptulose-7-P and 3-phosphoglyceraldehyde in the pentose phosphate pathway. Normal mean transketolase activity has been reported to range from 0.301 to 2.9 mmol pentose/hr/10⁹ RBCs (mean, 0.782). Transketolase assays often are reported as the mean thiamine pyrophosphate effect (Table 35-10). This test compares the specific activity in the active (holoenzyme) and inactive (apoenzyme) forms with the activity of the two forms after addition of thiamine to the homogenates. A large increase in specific transketolase activity after addition of the thiamine pyrophosphate suggests thiamine deficiency. Theoretically, in animals with thiamine-associated PEM, the concentration of holoenzyme is decreased, and that of the apoenzyme is increased. Thiaminase may be detected in the rumen and feces of affected animals, but the value of this assay is questionable.^{1072,1087,1089,1090} If thiamine deficiency is identified in an affected animal, caution should be used in interpreting the results because a period of anorexia may result in a decrease in ruminal de novo synthesis;^1092,1091 thus, affected animals may have a thiamine deficiency that develops secondary to PEM.

Table 35-10 Mean (and 95% Confidence Range) Values of Erythrocyte Transketolase as Percentage of Thiamine Pyrophosphate Effect in Erythrocytes of Normal Cattle and Sheep and in Patients with Polioencephalomalacia

From Edwin EE et al: Diagnostic aspects of cerebrocortical necrosis, Vet Rec 104:4, 1979.

Changes in CSF of affected animals usually are vague. They include mild pleocytosis (5 to 50 WBCs/dL) with vacuolation and increased protein concentrations (>50 mg/dL).^1072,1093

Electrophysiologic studies of affected animals show a normal latency and decreased amplitude of the late peaks of the visual-evoked potentials. These changes reflect a decreased population of neurons capable of responding to the photic stimulation.¹⁰⁹⁴ Electroencephalographic (EEG) changes in some animals include constant high amplitude (50 to 60 mV) and slow activity (1 to 4 Hz). Another change is diffuse lowered activity, which is consistent with diffuse necrosis.¹⁰⁹⁵

Environmental investigations should include evaluation of all practical feed and water sources for sulfur concentrations or the possibility of lead contamination. The diet should also be checked for molasses and urea content.

Pathogenesis

The fundamental lesion with PEM appears to be neuronal edema with secondary compartment syndrome. In most cases the edema is postulated to result from adenosine triphosphate (ATP) depletion and insufficient function of the sodium-potassium pump. With salt intoxication, aberrant accumulation of intraneuronal osmolar substances, followed by mass movement of water into neurons, offers a different pathway to cerebral edema. In either case, the bony calvarium limits expansion, and neuronal swelling leads to pressure necrosis. Additional neuronal dysfunction independent of edema and compartment syndrome cannot be ruled out.

The brain is susceptible to PEM because of its high energy and oxygen requirements. Brain tissue is only about 2% of body weight but accounts for 20% to 30% of body glucose utilization and 20% of oxygen utilization in adults.¹⁰⁹⁶ More than 85% of glucose used by the brain is energy substrate, and the blood-brain barrier prohibits the switching to many alternate energy substrates. Compared with liver or muscle, the brain has relatively small glycogen stores, only about a 10-minute supply. It is entirely dependent on circulating oxygen, and under normal conditions, the extracellular fluid contains approximately 100 times as much glucose as the oxygen required for complete aerobic glycolysis. Thus, oxygen delivery or utilization is much more rate limiting for ATP production under most circumstances than glucose availability,¹⁰⁹⁷ and factors that inhibit oxygen utilization (e.g., H₂S, lead) could have profound effects on neuronal ATP production.

One intriguing but controversial theory on cerebral energy metabolism could further explain the high neuronal susceptibility to ATP depletion. By this theory, glucose is reduced anaerobically to lactate in astrocytes.¹⁰⁹⁸ This lactate diffuses into neurons, which are then absolutely dependent on aerobic glycolysis for ATP production. Even if this theory is false, it is unlikely that anaerobic glycolysis alone could meet neuronal ATP needs.

Much of the early work on PEM focused on the importance of thiamine (vitamin B₁, thiamin, aneurin). In adult ruminants, thiamine is produced by rumen microbes at a rate that is marginally faster than the rate of consumption; very little is stored.¹⁰⁹⁹ Preruminants depend on dietary thiamine. Thiamine compounds play several important roles in the glycolytic pathways. Thiamine pyrophosphate (thiamine diphosphate) is an important coenzyme for transketolase, the rate-limiting enzyme in the hexose monophosphate pathway (pentose phosphate shunt) of glycolysis, and the α-ketoacid dehydrogenases of the Krebs cycle. The role of thiamine in the hexose monophosphate pathway was long thought to be key in the development of PEM, but this pathway actually accounts for less than 3% of cerebral glycolysis.¹¹⁰⁰ It is unlikely that impairment of this pathway alone could lead to such severe disease. However, decreased function of the Krebs acid cycle through inactivity of the α-ketoacid dehydrogenases could probably cause the necessary reduction in ATP production. Thiamine triphosphate may also play a role in neuronal function independent of its enzymatic function.

The dependence of ruminants on microbial thiamine production has lead to investigation of factors that might decrease production, absorption, or function. The mechanisms proposed include ruminal production of bacterial thiaminases, production or ingestion of inactive thiamine analogs, ingestion of preformed plant thiaminases, decreased intake of preformed thiamine by preruminants, impaired absorption or phosphorylation of thiamine by rumen bacteria, increased fecal excretion of thiamine, and decreased ruminal production of thiamine diphosphate. The most frequently reported inactive thiamine analog is amprolium.^1074-1076 Two types of bacterial thiaminases have been described. Thiaminase I, which is produced by Bacillus thiaminolyticus or Clostridium sporogenes,¹¹⁰¹ catalyzes the cleavage of thiamine at the methylene bridge between the pyrimidinyl and the thiazole ring. A basic cosubstrate is required to combine with the pyrimidinyl derivative to form a new compound¹¹⁰² (Fig. 35-13). Many common feedlot medications, including benzimidazoles, levamisole, and promazines, appear to be able to serve as this cosubstrate.

Fig. 35-13 Enzymatic cleavage of thiamine by thiaminase I and II. Thiaminase I attaches a picolinium base to the pyrimidine ring structure, whereas thiaminase II catalyzes the hydrolysis of thiamine at the methylene bridge. (From Edwin EE, Jackman R: A rapid radioactive method for determination of thiaminase activity and its use in the diagnosis of cerebrocortical necrosis in sheep and cattle, J Sci Food Agric 25:357, 1974.)

Several plants also appear to make a thiaminase similar to thiaminase I. These include bracken fern (Pteridium aquilinum)^1103,1104 horsetail (Equisetum arvense),¹¹⁰⁴ and Nardoo fern (Marsilea drummondii).¹¹⁰⁵ Of these, only the Australian Nardoo fern has been strongly linked to outbreaks of PEM. Thiaminase II is produced by Bacillus aneurinolyticus, which proliferates in response to excessive grain intake.¹¹⁰⁶ The enzyme catalyzes the hydrolysis of the methylene bridge between the two ring structures of the thiamine molecule. The specific relationship of this thiaminase to the clinical syndrome of PEM as seen in the field is unclear.

Complete correlation has not been established among production of ruminal and fecal thiaminase, tissue and plasma concentration of thiamine, and development of clinical encephalopathy.^1089,1107 Some affected animals may show normal amounts of thiamine in the plasma but have greatly decreased levels in the erythrocytes and other tissues, supporting the diversity of causes of PEM.¹¹⁰⁸

Dietary sulfur and sulfates are an important factor in the development of many cases of ruminant PEM. Beef cattle require 0.15% to 0.20% sulfur on a dry matter basis.¹¹⁰⁹ Sources of sulfur include elemental sulfur,¹⁰⁶⁷ feed additives such as gypsum and ammonium sulfate,^1068,1110 feedstuffs such as corn-processing by-products,¹¹⁰⁹ cruciferous crops,^1111,1112 molasses,¹⁰⁷⁷ and fertilizers. Water can be an important contributor to sulfur intake, usually in the form of sulfates.^{1070,1113,1114}

There are two primary metabolic pathways of sulfur in the rumen.^1114-1119 The assimilatory pathway involves reduction of sulfate to sulfides and incorporation into sulfur-containing organic compounds such as cysteine and methionine.¹¹¹⁷ These are ultimately incorporated into microbial crude protein. The dissimilatory pathway is an energy-producing pathway in which microorganisms use sulfate as a terminal electron acceptor, similar to how mammals use oxygen.¹¹²⁰ The end product is liberated sulfide ion. At a ruminal pH of 5.2, 97.2% of sulfide ions are in the form of H₂S and move freely to the rumen gas cap.^1116,1121 H₂S is readily absorbed and transported to the liver and oxidized to sulfate.^1121,1122 Some H₂S may be lost through eructation,^1082,1121 but the significance of this route has been questioned.¹¹²³ Excess sulfur is excreted in the urine and large intestine^1124,1125 or recycled to the rumen.^1126,1127 A period of adaptation is required for maximum H₂S production after exposure to sulfur.^{1118,1124,1127-1129}

Because sulfur and sulfate demonstrate low cellular toxicity, it is unlikely that sulfur-associated PEM results from a sulfate or sulfur toxicity. However, sulfides are highly toxic.^1130-1132 Sulfur-associated PEM is more likely to occur secondary to a sulfide toxicity.^{1069,1071,1082}

It has been proposed that the pathogenesis of sulfur-induced PEM involves inhibition of cytochrome-c oxidase, an enzyme in the electron transport chain; this chain is important for regenerating the intermediaries of aerobic glycolysis and the final round of ATP production. With oxygen availability already limiting aerobic glycolysis in the brain, inhibiting the electron transport chain could have a profound effect on neuronal ATP production.

For highly toxic sulfide to reach the brain, it must escape hepatic oxidation. This might be achieved by two mechanisms. A surge in ruminal sulfide generation usually follows a period of adaptation to high-sulfur diets. This may overwhelm the hepatic detoxification capacity. As an alternative possibility, cattle inhale the majority of eructated ruminal gas.¹¹³³ These inhaled gases can contain high amounts of H₂S, and if absorbed via the pulmonary route, the H₂S would completely bypass the hepatic circulation. However, this concept has been questioned.¹¹³⁴

In feedlot cattle, a summer peak in PEM cases was associated with consumption of water containing 2500 mg/L of sulfate.¹⁰⁷⁰ The total sulfur intake of these steers was estimated to be 0.6% on a dry matter basis during the hottest days of the year. Most PEM cases occurred between 15 and 35 days after arrival in this feedlot. In another investigation, 11% of weaners consuming a diet containing 0.9% sulfur on a dry matter basis developed clinical manifestations of PEM.¹⁰⁷¹ Lesions were confirmed in one steer that died. Addition of gypsum (calcium sulfate) to feeder steer rations at a final concentration more than 2% organic sulfate results in a significantly greater risk of developing PEM. The addition of sodium sulfate (0.6% to 0.8%) to diets may induce PEM within 11 days.^{1082,1128,1135,1136} High sulfur concentrations in well water in combination with accumulation in forage has been traced to an outbreak of PEM in Canada.¹⁰⁷¹ One survey described an outbreak of cerebrocortical necrosis in cattle eating diets containing 7200 mg/kg of sodium sulfate.¹¹¹⁴

The recommended maximum tolerance level of sulfur is 0.4% of dry matter intake.¹¹⁰⁹ Other effects of excessive sulfur intake are decreases in feed intake and weight gains. Accurate diagnosis of sulfur toxicity requires measurement of sulfur in all food and water sources. Sulfur from water must be included when calculating total sulfur intake. One third of the molecular weight of sulfate is sulfur. Therefore, if an animal drinks 30 L of water a day containing 2000 mg/L of sulfate, this contributes 60,000 mg of sulfate, or 20 g of sulfur. Furthermore, if this animal consumes an average of 10 kg of dry matter daily at 0.15% sulfur, the feed or forage contributes 15 g of sulfur, making the total sulfur intake 35 g, or 0.35% on a dry matter basis. Water therefore may be a substantial source of sulfur.

Some have suggested that sulfides result in thiamine destruction, thereby directly implicating a thiamine deficiency in the pathogenesis of sulfur-associated PEM. Rumen thiamine production was slightly reduced by the inclusion of excessive sulfur in the ration.¹¹³⁷ However, the authors deemed the reduction clinically insignificant. Apparently, sulfur-associated PEM occurs independent of thiamine status.

PEM induced by feeding of molasses and urea is thought to be related to the high sulfur content of the molasses and to the depletion of propionate and other glucogenic precursors induced by the foodstuff.¹⁰⁷⁷ It is not considered to be caused by an underlying thiamine destruction. The tissue thiamine concentrations of animals with molasses-related PEM are normal,¹¹³⁸ and signs are preventable by concomitant feeding of glycerol, which is converted to glucose in the rumen. Outbreaks of PEM in range cattle have been associated with ingestion of the plant Kochia scoparia.^1139,1140 The pathogenesis of this condition is unknown; however, some have suggested that the plant has the capacity to accumulate sulfur in the forage.¹¹¹⁴

Some cases of PEM cannot be linked to either problems with thiamine or sulfur toxicosis. Other compounds, such as lead, affect electron transport in a manner similar to sulfides and therefore also impair ATP production. Water intoxication creates a similar histologic lesion, but the pathogenesis of edema relates more to the generation of osmotic compounds and fluid shifts with rapid fluctuations of blood and neuronal osmolality.

Epidemiology

PEM has a worldwide distribution. The condition is seen both in individuals and as herd outbreaks. In one instance, approximately 2000 of 2200 sheep grazing were clinically affected with PEM.¹⁰⁶⁷ No predilection by gender or breed is seen, although anecdotal reports suggest that heifers are less likely to develop PEM than steers in a feedlot environment. The condition affects cattle, sheep, goats, deer, camels, and camelids.^{1068,1074,1141-1144} Although PEM is seen predominantly in animals that eat a high-concentrate supplement, the condition also can occur in unsupplemented animals on pasture. The inciting cause of PEM can be identified in some cases.

One report from the United States indicated a predominance of cases in the summer in range cattle.¹⁰⁶⁴ The age range for susceptibility to PEM has been reported as 3 weeks to 5 years in sheep, 3 weeks to 8 years in cattle, and 2 months to 2½ years in goats. The peak age of incidence is 18 months or younger in cattle and sheep, but this depends on the production system.^{1064,1071,1145} The incidence of PEM has been reported as high as 90% in some sheep flocks, with mortality of 1% to 10%.¹⁰⁶⁷ The incidence of PEM is high in sheep exported by sea from Australia to the Middle East. In these cases the underlying disturbance was thought to be associated with a thiamine deficiency caused by a lack of rumen synthesis secondary to the shipboard conditions.¹¹⁴⁶

Necropsy Findings

In cases of sulfur-associated PEM, rumen contents may have an odor of H₂S. In other cases there may be evidence of grain overload, coccidiosis, or respiratory disease. Some of these lesions relate to management conditions, but may also relate directly or indirectly to PEM. The macroscopic pathologic brain lesions of PEM include cortical swelling, softening, flattening, and yellowish discoloration of the gyri. Necrotic areas of the cerebral cortex autofluoresce under ultraviolet light (365 nm).^1147,1148 Severe cases show herniation of the cerebellum through the foramen magnum or the occipital cortex under the tentorium cerebelli. Animals necropsied months after recovery may show cerebral atrophy and submeningeal cortical cysts. The major microscopic lesion is a diffuse laminar necrosis. Other changes include intracellular and intercellular edema, neuronal necrosis, gliosis, and neuronophagia.^1066,1080

Treatment and Prognosis

Regardless of the underlying cause, animals with the subacute form of PEM often respond favorably to parenteral administration of thiamine hydrochloride. These animals may remain blind and may have depressed sensorium for weeks or months.¹¹⁴⁹ Thiamine should be administered at 10 to 20 mg/kg IM or subcutaneously (SC) three times daily. In severe cases, IV administration of the first dose might be warranted. If given IV, thiamine should be diluted in 5% dextrose or other isotonic fluid and administered slowly to avoid adverse reactions. If no improvement occurs initially, the treatment should be continued for at least 3 days. In some patients, recovery may take as long as 7 days, but most patients show improvement by 24 hours. A single administration of sodium dexamethasone at 0.25–1 mg/kg IM or IV or 1 g/kg of mannitol in a 20% solution IV may be beneficial in reducing cerebral edema. Anecdotal reports indicate that feedlot animals that have recovered from PEM are at increased risk of respiratory disease; therefore, prophylactic antimicrobial administration may be indicated. Convulsions may be controlled with phenobarbital, pentobarbital, or diazepam. Specific dosing regimens are listed in Tables 35-7 and 35-8.

Animals with the acute form of PEM usually have more severe cortical and deep gray matter lesions than animals with the subacute form.¹⁰⁷¹ These animals generally do not respond to therapeutic regimens. Ruminants with PEM secondary to the molasses-urea diet do not appear to respond to treatment with thiamine. They may respond more favorably to parenteral administration of glucose or enteral or parenteral administration of a glucose precursor.

Prevention and Control

Thiamine supplementation may not prevent outbreaks of PEM. Ultimately, the best way to prevent outbreaks is to manage the dietary intakes of susceptible animals appropriately. Ruminants should be allowed an adequate period of adaptation to high-concentrate rations. All feedstuffs and water sources should be carefully analyzed on a routine basis with an estimate of total sulfur intake. If excess sulfur consumption is a factor, steps should be taken to remove sources such as high-sulfur hay, ammonium sulfate, and molasses. If the excess sulfur intake is unavoidable, steps can be taken to limit its effects. Older members of the cow herd could be used to graze the high-sulfur pastures, and younger, more susceptible animals could be kept to lower-sulfur pastures or given hay supplementation. Personnel should be trained so that animals with PEM can be identified early in the disease and treated appropriately.

Thiamine may be supplemented (3 to 10 mg/kg of feed) in rations in which the concentrate/fiber ratio is high, but this has little or no effect in preventing PEM. Other recommendations for preventing PEM include addition of brewer’s yeast to the ration and gradual adaptation of ruminants (at least 2 weeks) to high-concentrate diets. If present as a feed-limiting additive, gypsum should be removed from the diet. Elimination of supplementation and rotation of pastures have been sufficient for controlling some outbreaks.¹¹⁴⁵ Supplementation with cobalt in trace-mineral salt mixes may be necessary in deficient areas.

Definition and Etiology

Salt poisoning is a common CNS disease of livestock. Salt-rich solutions ingested over time can cause production-related losses and even death. Ingestion of water containing more than 7000 mg/L of total dissolved salts is likely to result in acute salt poisoning.^1152,1153 Water that contains less than 3000 mg/L of total dissolved salts is considered safe for consumption. Salt poisoning can be associated with water deprivation.^{1152,1154,1155} Provided that access to free water is constantly available, animals may tolerate as much as 13% dietary salt intake.¹¹⁵⁶ The total dietary salt concentration should never exceed 4%.

The acute toxic dose of oral sodium chloride (NaCl) for cattle and horses has been reported as approximately 2.2 g/kg body weight and for sheep about 6 g/kg.¹¹⁵⁶ With water restriction, the toxic dose of salt is considerably less, and poisonings have resulted from ingestion of 0.9% NaCl in water-restricted cattle.¹¹⁵⁷ Chronic toxicity can occur at lower dietary salt levels than acute toxicity. Ingestion of water with salt concentrations above 1% uniformly results in toxicosis if no other source of ion-free water is provided.^1158,1159

Ingestion of water containing 0.7% salt lowers the fertility of females,¹¹⁵⁹ and water containing 0.25% salt suppresses milk production in cattle.¹¹⁶⁰ Dairy calves have been poisoned by daily feeding of 4 L of milk replacer containing 2.6% NaCl.¹¹⁶¹ Animals are most susceptible to salt poisoning during the summer because of the increased insensitive loss of water at that time. Salt poisoning in ruminants is most frequently a syndrome of “water intoxication”: a period of restricted access to low-salt water, followed by unrestricted access to water.^{1152,1154,1155}

Clinical Signs

Rapid ingestion of large amounts of salt causes gastrointestinal and neurologic signs.^{1158,1161-1166} These include mucohemorrhagic diarrhea and colic, head-neck extension (“star gazing”), blindness, aggressiveness, hyperexcitability, psychomotor seizures (paddling and loss of consciousness), vocalization, ataxia, proprioceptive deficits, head pressing, constant chewing movements, nystagmus, muscle twitching, and coma. Death occurs as a result of respiratory failure. Before the onset of neurologic signs, cattle with chronic salt toxicosis may appear to be depressed and dehydrated. Table 35-11 summarizes the spectrum of clinical effects associated with different levels of salt intake.

Table 35-11 Effects of Different Salt Concentrations in Drinking Water on Performance in Cattle

From McCoy CP, Edwards WC: Sodium ion poisoning in livestock from oilfield wastes, Bovine Pract 15:152, 1980.

Excessive salt intake also may interfere with productivity in the absence of acute neurologic signs. In one study, cattle were given either tap water (196 ppm of dissolved salts) or saline (2500 ppm NaCl), and their milk production was measured.¹¹⁶⁰ Cows given tap water had a greater fluid intake and a significantly greater lactational persistence and daily milk production than did cows given saline. The serum concentrations of sodium and potassium were normal in the animals fed saline.

The clinical diagnosis of salt poisoning depends on the demonstration of exposure to toxic concentrations (>7000 ppm or 0.7% of sodium), the presence of water deprivation, or the determination of serum or cerebrospinal fluid (CSF) sodium concentrations greater than 160 mEq/L. CSF/serum sodium ratios greater than 1 also suggest salt poisoning. The serum sodium concentration may vary, depending on whether the patient had recently been given ion-free water before measurement. Some animals with acute neurologic lesions may be normonatremic if they have recently drunk to repletion with ion-free water, whereas others that have not had ion-free water may be hypernatremic. The CSF sodium concentration in salt-poisoned animals is consistently elevated and may exceed 200 mEq/L.¹¹⁶² Ruminal sodium concentrations above 0.36% to 0.5% or brain sodium concentrations above 150 mEq/g or 1800 ppm¹¹⁶⁷ also suggest salt poisoning in cattle.^{1162,1163,1168} Rapid intake of low-sodium water during the rehydration phase of the disease may cause intravascular hemolysis with resultant hemoglobinuria; such findings raise the index of suspicion of salt poisoning.¹¹⁵³

The concentration of acetylcholinesterase in plasma and RBCs is decreased in animals that have been ingesting excessive salt (>0.49% of diet).¹¹⁶⁹ The decrease is first seen after 4 months of continuous ingestion of the high-salt diet.

Pathophysiology

The pathogenesis of salt poisoning involves the deposition of sodium ions in the CNS parenchyma and the CSF, which occurs either acutely from ingestion of a large quantity of salt or chronically after long periods of reduced water consumption. The ionic sodium accumulates in the CSF and neurons by passive diffusion. The resulting hyperosmolality reduces energy-dependent sodium transport mechanisms and the anaerobic glycolytic pathways.¹¹⁷⁰ These mechanisms normally provide energy by which the sodium ion is removed from the cell cytoplasm.¹¹⁶⁵ The thirst receptors are triggered in response to the hyperosmolality. The animal is permitted to drink ion-free water to repletion, and the fluid is absorbed from the gastrointestinal tract, resulting in expansion of the extracellular fluid and a return to normal plasma osmolality. Water then diffuses from the blood into the relatively hyperosmolar CSF and neurons, resulting in CNS edema, increased intracranial pressure, and acute encephalopathy. If the patient has had sudden access to a large quantity of salt, the hyperosmolality in the intestine results in saline catharsis and diarrhea.

Epidemiology

Animals are tolerant of high dietary salt levels if they have concomitant access to fresh drinking water. Feedstuffs that are common sources of excessive salt include whey, saline-preserved fish or fish meals, bakery by-products, and certain milk replacers.¹¹⁷¹ Confined calves may be poisoned by improperly formulated milk replacers or oral electrolyte replacements.^1162,1171 Cattle eagerly ingest large amounts of oil well sludge, which is a potential source of salt for cattle in the western and southwestern United States.^{1166,1172,1173} Brine is used extensively as a flush during the drilling of oil wells. Effluents from drilling rigs may contain as much as 100,000 ppm of salt. The effluents are also contaminated by heavy metals and magnesium salts, which complicate the clinical syndrome of salt poisoning. Concurrent neurologic disease may predispose animals to reduced water intake, as occurred in a group of goats with locoweed (Oxytropis species) poisoning who became water deprived because of reluctance to move to a water source. They developed clinical and pathologic evidence of salt poisoning when moved subsequently to an area where water was easily available.¹¹⁵⁴ Salt poisoning caused by water restriction may occur either inadvertently from freezing of water sources in northern climates or from intentional water restriction of veal calves.¹¹⁶² Ingestion of brackish or tidal water is a cause of salt poisoning in cattle pastured on the coastal regions of the world.

Necropsy Findings

The pathologic changes of salt poisoning include cerebral edema and softening and flattening of the cortical gyri. Microscopic lesions include laminar cortical necrosis, poliomalacia, and occasionally, meningeal or perivascular infiltration of eosinophils. Perivascular infiltration of eosinophils is not as reliable an indicator of salt poisoning in ruminants as in pigs because most affected ruminants show perivascular cuffing of mononuclear cells.

Treatment

Treatment of animals affected by salt poisoning is difficult. Many animals die even after intensive medical treatment. Prognosis depends on severity of clinical signs at the start of treatment: animals that already have significant cerebral edema have a guarded prognosis. Therapy should be aimed at limiting the ingestion of nonionic water and attempting to remove intracellular solutes slowly from the brain while simultaneously controlling cerebral edema and attendant CNS signs. To prevent brain swelling and herniation of the brain through the foramen magnum or the tentorium cerebelli, slow reduction of the CSF and plasma sodium is imperative. Slow intravenous (IV) administration of normal or hypertonic saline is the mainstay of treatment.

Adult cattle should receive normal or hypertonic saline intravenously (IV) at maintenance rate (7% body weight in adult cattle, 10% in neonates).¹¹⁵³ Slight underestimation of fluids is preferable to excess fluid administration. Serum sodium should be monitored regularly and correction effected over 24 hours or longer. Oral fluids can be introduced gradually toward the end of the first day of treatment; salt should initially be added to oral fluids to make them isotonic to blood. When IV replacement is not feasible, isotonic to hypertonic oral fluids should be administered at a maintenance level divided into four to six feedings daily. Access to low-salt water is gradually allowed after 3 to 4 days. Any worsening of clinical signs is an indication to increase the salt level in fluids and may require IV mannitol (0.5 to 2.0 mg/kg as a 20% solution) or oral glycerin (1 mL/kg diluted 50:50 with water) to reduce cerebral edema.¹¹⁵³

In calves, treatment can be accomplished by administration of hypertonic saline concomitantly with feeding of 2 to 4 L of fresh milk daily. First, the plasma sodium concentration is measured, and the calf then is given 1 to 2 L of a hypertonic saline solution IV. The molar strength of the sodium ion in the IV fluid should be equal to or slightly greater than that of the plasma. If the hypotonicity of the whole milk is not counterbalanced by treatment with hypertonic saline, the CNS will rapidly expand as a result of absorption of free water. The plasma electrolyte concentration is measured twice daily. If the plasma sodium concentration declines too rapidly, 1 L of hypertonic saline solution is infused over several hours. The concentration of sodium in this fluid should be greater than the most recent plasma sodium measurement and less than the beginning plasma sodium concentration. If the calf develops nervous twitching, salivation, head-neck extension, stiff forelimbs, or convulsions, 0.5 to 1 g/kg of mannitol is immediately infused IV. A blood administration set is used to filter insoluble mannitol crystals. The calf should have no access to fresh water.

The use of solutions containing 5% glucose is dangerous and probably contraindicated because this represents ion-free extracellular fluid, which can exacerbate brain edema. Administration of corticosteroids (dexamethasone, 0.25 to 1 mg/kg by slow IV injection twice daily for 2 to 3 days) may be helpful in animals with acute cerebral edema. However, potential benefits should be weighed against the possibility of inducing extrarenal sodium retention. If the neurologic signs diminish and the plasma sodium level returns to normal, the animal may be given fresh ion-free drinking water. Thiamine (10 mg/kg by slow IV injection) may be a useful adjunctive therapy.

Control

Cattle should be fenced away from polluted ponds and oil wells. Cattle on coastal pastures should have access to fresh well water. The total daily dietary salt intake should not exceed 4% of dry matter intake. Drinking water must contain less than 7000 ppm of sodium unless the dietary sodium load is reduced correspondingly. Oral rehydrating fluids for calves should be dissolved in strict accordance with the manufacturer’s recommendations and should not be administered for longer than 3 consecutive days.

Definition and Etiology

Vitamin A (retinol) is found in green plants and can be synthesized by the small intestinal mucosal cells from plant carotenoid precursors. Precursors of vitamin A are usually fed in cattle rations as β-carotene or as retinoids (retinyl palmitate or acetate). Carotenoid in forage is converted to retinol in the liver and gut. Vitamin A deficiency occurs primarily in growing ruminants in feedlots. Deficiency develops under these conditions because the growing animal has a higher requirement for the vitamin, and feedlot-reared animals may have limited access to succulent plants. The vitamin is labile in foodstuffs and is essentially depleted after several years of storage. Diets that are naturally low in vitamin A include cereal grains, beet pulp, and cottonseed hulls. Conditions in which the immune system is challenged, as when exposure to pathogens is high, also increase the requirement for vitamin A.

The clinical signs of vitamin A deficiency in cattle are related to increased intracranial pressure and ill-thrift caused by secondary infections. Signs include intermittent convulsions, depression, and blindness. The usual dietary or management conditions that favor the vitamin deficiency include grazing on dry pastures or cereal grains other than corn, exclusive feeding of cereal grains that have been stored at high temperature and humidity, or prolonged feeding of mineral oil as a preventive for frothy bloat.

Clinical Signs

The neurologic signs of vitamin A—depleted animals are age dependent. Signs in deficient calves include anorexia, ill-thrift, blindness, diarrhea, and pneumonia. The syndrome in adults is characterized by “star-gazing” posture, blindness, diarrhea, anisocoria, nystagmus, strabismus, exophthalmos, loss of pupillary light reflexes, and intermittent tonic-clonic convulsions. The seizures last for only a few minutes and are followed by partial recovery.^1174-1176 Animals may die during seizures. Stimulation of the animals frequently precipitates seizures.^1177,1178 Death often is preceded by hyperesthesia and coma.^{1174,1175,1179} Inadequate vitamin A supplementation of calves is associated with unthriftiness, intermittent fevers, and a higher incidence of diarrhea.¹¹⁸⁰ Vitamin A—deficient adults appear to be in good body condition unless parasitism or some other nutritional deficiency is superimposed on the low vitamin A intake.^1174,1181 Secondary factors that can influence the appearance of the animals include concomitant nutritional deficiencies, parasitism, and pneumonia.

The ocular changes of vitamin A deficiency are characteristic. The pupils become dilated and unresponsive. As papilledema develops, the optic disc becomes pale and its borders become indistinct, particularly in the upper quadrants,^{1174,1176,1182} giving the appearance of an inverted heart. The swollen disc may cast a shadow on the adjacent retina. The color of the disc becomes faded. In advanced cases the disc may become atrophic and appear dull, gray, flattened, and smaller than normal. The retinal blood vessels become tortuous or appear to be occluded as they course over the disc. Retinal detachment and subretinal hemorrhages are possible.¹¹⁷⁴ Corneal changes are an uncommon clinical finding.^1174,1175

Reproductive disturbances can occur, including malformed fetuses, abortions, loss of libido, testicular degeneration, and decreased sperm counts. Calves born to vitamin A—deficient dams are blind, have domed foreheads and thickened carpal joints, and are weak at birth.¹¹⁸³

Vitamin A deficiency can be clinically differentiated from polioencephalomalacia (PEM) and salt poisoning by comparing the menace response with the pupillary light reflex. Calves with lead poisoning and PEM generally have intact pupillary light reflexes because of the proper functioning of the mesencephalon and optic nerves, whereas vitamin A—deficient cattle have absent pupillary light responses because of retinal degeneration and constriction of cranial nerve II at the level of the optic foramen.

Clinical Pathology

Assay of vitamin A and carotene concentrations in the plasma and feed is the most direct method of diagnosing the dietary deficiency. The concentration of plasma vitamin A and β-carotene in normal animals ranges from 25 to 85 μg/dL and 150 and 397 μg/dL, respectively.¹¹⁷⁵ Plasma concentrations of vitamin A—deficient and β-carotene—deficient animals usually are less than 7 and 70 μg/dL, respectively. Papilledema first occurs when plasma concentrations of the vitamin fall below 18 μg/dL.¹¹⁸⁴ Ataxia and blindness occur when the serum vitamin A concentration ranges from 4.87 to 8.88 μg/dL.¹¹⁷⁷ The hepatic concentration of vitamin A and carotene in normal calves ranges from 60 to 200 and from 4 to 800 μg/g of tissue, respectively. In deficient calves the hepatic concentrations of the vitamin A and carotene nutrients range from 2 to 14 and from 0.5 to 32 μg/g, respectively.^1181,1184 There are no consistent changes in the blood chemistry analysis or the hemogram of deficient animals. Increased CSF pressure (>200 mm Hg) may occur; however, standardization of the measurement for all forms of anesthesia and methods of measurement is difficult.¹¹⁸⁵ Changes in the CSF of vitamin A—deficient animals include a mononuclear cell pleocytosis (40 to 50 nucleated cells/dL) and an increased protein concentration (140 mg/dL).¹¹⁸⁶

Pathophysiology

Vitamin A is responsible for the regeneration of rhodopsin in the retina and the maintenance of tissue integrity. The vitamin has effects on osteoblasts and osteoclasts, epithelial tissues, the choroid plexus, and reproductive tissues. The arachnoid villi and the retina are most sensitive to a deficiency of the vitamin. Vitamin A deficiency causes a thickening of the dura mater, resulting in diminished CSF absorption from the arachnoid granulations and the nerve rootlets. Narrowing of all the bony foramina of the skull occurs in immature animals, although bone remodeling does not occur in adults. The combined effects cause an increase in CSF pressure.¹¹⁸¹ In severe cases the brain may herniate through the foramen magnum. Closure of the optic foramen may lead to transection of the optic nerve. The high CSF pressure is transmitted into the optic nerves and results in papilledema.

Three causes of blindness have been associated with vitamin A deficiency. Nyctalopia is presumably caused by the decreased formation of vitamin A aldehyde in the regeneration of the visual pigment rhodopsin; this type of blindness usually is reversible. Degenerative changes in the outer retinal layers also cause blindness; this is reversible if treated in the early stages. The third cause is associated with stenosis of the optic foramen and compression of the optic nerve; this condition is irreversible.¹¹⁸⁷ An experimental study has shown that humoral immune function also is impaired in sheep with vitamin A deficiency. The pathogenesis of this condition is unclear.¹¹⁸⁸

Epidemiology

The vitamin A requirement of all species ranges from 40 to 110 IU/kg daily.^1189-1191 The minimum recommended daily dose of vitamin A for growing calves up to 1 year of age, for pregnant sheep, and for growing horses is 40 IU/kg. Pregnant cattle and pregnant or lactating horses require 40 to 50 IU (13.76 to 17.2 μg/kg) of vitamin A daily.¹¹⁸⁹ Lactating cattle require 80 IU (27.5 μg/kg) of vitamin A daily. Horses are susceptible to vitamin A deficiency, but the condition is rarely seen in that species. This is thought to be the result of differing conditions of management rather than an inherent resistance to the deficiency. The daily dietary requirement for carotene is 0.12 mg/kg.¹¹⁸⁹ Pasture forage, silage, and properly cured hay (<1 year old) contain large amounts of carotene. Common constituents that have low concentrations of vitamin A are sorghum, brewer’s grain, and wheat straw.

Livestock are protected from short-term deprivation of vitamin A by their ability to accumulate the vitamin in the liver; however, it is estimated that the intake required to initiate storage is at least three times the minimum daily intake.¹¹⁸⁹ Vitamin A—replete cattle fed a diet devoid of vitamin A require approximately 180 days before they begin to show clinical signs. During that time the cattle grow and fatten normally and show no adverse effects. Offspring born to these animals may show severe deficiencies. Papilledema and blindness develop rapidly after the hepatic stores are depleted.¹¹⁸⁹ An interesting sexual dimorphism in susceptibility to dietary deficiency of vitamin A was found in one study of feedlot cattle; although all cattle were fed the same deficient diet, only steers had clinical signs.¹¹⁹⁰

Vitamin A deficiency may be categorized as a primary or a secondary condition. Primary deficiencies of vitamin A develop in cattle confined in drylot corrals or pasture on dry grass forage for prolonged periods or when cattle are kept indoors and fed unsupplemented, vitamin-depleted cereals and dry forage in which the activity of carotene has been destroyed. Also, there is a seasonal difference in the concentration of vitamin A in feedstuffs. For example, cattle grazed on green pastures are consistently replete with the vitamin, whereas those grazed on dry pastures at the end of summer may become marginally deficient. Approximately 80% of the vitamin A concentration of hay is lost during field curing.¹¹⁹²

Destruction of carotene is hastened by many environmental and physical factors, including heat, sunlight, trace mineral supplements, and humidity. In one study, exposure of nine different supplements to trace minerals in a humidified atmosphere (60% relative humidity) at 28°C (82.2°F) resulted in depletion of 47% to 92% of the total vitamin A after 1 week of incubation.¹¹⁹³ Improper storage has been implicated as the cause of the depletion of vitamin A in one field case.¹¹⁸⁹ Other factors that affect the stability of vitamin A in feedstuff include pelleting and exposure to rancid fat in the feed. The addition of gelatin to vitamin premixes has been recommended to stabilize the vitamin A activity in feed.^1184,1194

Secondary deficiencies of vitamin A result from interference with vitamin absorption, inhibition of the conversion of β-carotene to retinol (vitamin A) in the small intestine, or an increased requirement in the face of limited vitamin intake. The conversion of carotene to retinol is impaired in vitamin A—deficient patients.¹¹⁷⁹ Sheep may be more resistant to vitamin A deficiency because they convert β-carotene more efficiently than cattle.¹¹⁷⁹ Extensive destruction of preformed vitamin A by microflora appears to occur in the rumen and the abomasum.^1194-1197 Microbial destruction, fever, lactation, high ambient temperatures, and inadequate dietary energy may increase the daily requirement for vitamin A.¹¹⁹⁸ Females are slightly more resistant to the vitamin deficiency than males, presumably because of the interconversions of estrogenic hormones into vitamin A.

Secondary deficiencies of vitamin A may be caused by impaired vitamin absorption, which may occur from long-term feeding of mineral oil. Ingestion of highly chlorinated naphthalenes (X disease) causes severe vitamin A deficiency as a result of interference with the conversion of carotene to vitamin A. Some in vitro evidence indicates that a high level of dietary nitrates inactivates intraruminal vitamin A by oxidation. This may not be clinically important, however, because studies performed in vivo failed to show a greater requirement for the vitamin when animals were fed subtoxic doses of nitrates.^1197-1202 Other factors that can affect availability of vitamin A or the requirement for the vitamin include diets with low forage content, high proportion of corn silage to hay in the diet, increased exposure of animals to pathogens, and periods where immunocompetence is reduced (e.g., peripartum period).¹¹⁹¹ Challenges to the immune system increase the requirement for vitamin A.

Necropsy Findings

The major pathologic changes in the fundus of vitamin A—deficient calves include papilledema, small flame-shaped hemorrhages around the optic disc, venous congestion in the area of the swollen optic disc, degeneration of the retinal ganglion cells, focal retinal thinning, and fusion of parts of the retina to the choroid plexus.¹¹⁸² Other changes associated with vitamin A deficiency include doming of the frontal bones, enlargement of the carpi, cerebellar and cerebral compression, partial transtentorial herniation of the cerebellum, cystic dilation of the hypophyseal cleft, focal ruminal hyperkeratosis, and increased keratinization of the squamous epithelium of the penile and the preputial mucous membrane.^1203,1204 Corneal ulceration and clouding have been observed in the eyes of calves with naturally occurring deficiencies.^1175,1185 Vitamin A deficiency also can cause anasarca, squamous metaplasia of the salivary ducts, degeneration of the germinal testicular epithelium, degenerative changes in the intestinal epithelium in lambs, and reduction in intramuscular fat in cattle.^1204-1206

Microscopic changes in the CNS include attenuation of the optic nerve with necrosis and demyelination. Focal accumulations of phagocytic cells containing lipofuscin and hemosiderin are present in the necrotic area. The optic nerve is attenuated along its entire length. Gliosis and focal vacuolization of the nerve also are seen, as is a focal loss of granular and molecular layers and Purkinje’s cells in the cerebellum. The meninges are thickened by fibrosis and mononuclear cell inflammation. The microscopic changes in the bones include wider than normal spacing of the central canals and reduction of osteoclastic lacunae.

Treatment and Control

Cattle with severe blindness caused by damage to the retina or optic nerves do not regain their vision when treated with vitamin A; however, cattle with acute encephalopathy and simple papilledema may respond favorably after a short period of vitamin supplementation.¹¹⁸⁹ Affected cattle should receive 440 IU/kg (1 IU = 0.4 μg) of vitamin A parenterally and then 6000 IU/kg parenterally every 50 to 60 days until the diet has been enriched. High-dose oral therapy is important because carotene and oil suspensions of vitamin A are not efficiently used when administered by parenteral injection.¹²⁰⁷ Administration of large doses orally is important because conversion of β-carotene to vitamin A is inhibited in deficient calves. The recommended concentration of vitamin A in milk replacers for preruminant calves is 11,000 IU/kg dry matter.¹¹⁸⁰

Prophylactic dietary supplementation of vitamin A should be considered in all cattle that lack access to green feed. Dietary supplements could include leafy, freshly cured hay, green pasture, or 0.5 to 2 kg of alfalfa meal daily. Concentrate feeds formulated with exogenous, stabilized vitamin A are commercially available. Vitamin A powder* may be added to the drinking water at a rate of 425,000 U/50 gallons. This treatment should be continued for as long as the dietary deficiency exists. The recommended vitamin A requirements in cattle are 80 IU/kg for growing animals and 110 IU/kg for adult animals (including pregnant and lactating cows). Recommended concentrations (IU/kg dry matter) of vitamin A in feed are 2200 IU for feedlot cattle, 2800 IU for pregnant cows, and 3900 IU for lactating cows.¹¹⁹¹

Subclinical deficiencies in ewes have been treated with a vitamin-mineral premix containing 0.3 to 0.27 kg iodine, 20.6 kg zinc, 7.9 kg copper, and 1644.5 million IU vitamin A per ton of feed. Addition of this premix to the diet of a group of sheep increased their productivity, as measured by viability, birthweight and rate of gain of lambs, and amount and quality of wool.¹²⁰⁸