EQUINE ENDOCRINE AND METABOLIC DISEASES

Pathophysiology

The pathologic hallmarks of PPID are hypertrophy, hyperplasia, and microadenoma or macroadenoma formation in the pituitary pars intermedia that results in an increased secretion of POMC peptides. Horses with PPID develop enlarged pituitaries that may reach five times normal weight. As the pars intermedia expands, it compresses the adjacent pituitary lobes and hypothalamus, often resulting in a loss of function of these tissues. In contrast, the pars intermedia remains active in horses with PPID, secreting relatively large quantities of POMC-derived peptides into the peripheral circulation. Horses with disease may have as much as a 40-fold increase in plasma concentration of pars intermedia POMC-derived peptides.²³ Clinical signs of disease likely result from a combination of increased circulating POMC peptides and loss of neuroendocrine function of adjacent tissues.

Evidence indicates loss of dopaminergic inhibition is critical in the pathology of PPID. Dopamine and dopamine metabolite concentrations in the pars intermedia of PPID horses are decreased eightfold compared with age-matched controls.⁶ Systemic supplementation of dopamine or a dopamine agonist to horses with PPID results in a decrease in plasma concentration of POMC peptides.²³ Several investigators have reported that horses treated with the dopamine agonist pergolide show improvement in both clinical signs and biochemical abnormalities associated with disease.^24-26 Immunohistochemistry of formalin-fixed tissue showed a fivefold decrease in pituitary dopaminergic nerve terminals (P < .001) and a 50% reduction in the number of dopaminergic periventricular cell bodies (P < .01) in the hypothalamus of PPID animals.²⁷ This evidence suggests that a loss of functional periventricular dopaminergic neurons or “dopaminergic neurodegeneration” occurs in horses with PPID (Fig. 41-3). Loss of periventricular dopaminergic inhibition of the pars intermedia in other species results in pathologic changes similar to those of PPID. Surgical disruption of the periventricular hypophyseal dopaminergic tracts in rats results in increased expression of pars intermedia melanotropes.²⁸ In addition, D₂ dopamine receptor knockout mice develop pars intermedia lesions similar to PPID.²⁹ These data suggest PPID is primarily a disease of hypothalamic origin rather than the consequence of a spontaneously forming pituitary adenoma.

Fig. 41-3 Pathophysiology of equine pituitary pars intermedia dysfunction. Loss of functional dopaminergic periventricular neurons leads to a decrease in dopamine at the pars intermedia. This in turns results in disinhibition of the melanotropes of the pars intermedia. The outcome is hypertrophy and hyperplasia of the pars intermedia and increased systemic release of the pars intermedia POMC-derived peptides, α-melanocyte—stimulating hormone (α-MSH), β-endorphin (β-END) and corticotrophin-like intermediate lobe peptide (CLIP).

One potential cause for dopaminergic neurodegeneration is oxidative stress. Oxidative stress is modification of cellular components including proteins, DNA, and cell membrane lipids because of excessive exposure to exogenous or endogenous sources of free radicals. This cellular damage ultimately leads to cell death or, in the case of neurons, neurodegeneration. Chronic exposure to oxidants in excess of an animal’s antioxidant capacity results in accumulation of functionally impaired cellular components. Dopaminergic neurons are particularly vulnerable to oxidative damage, because dopamine metabolism itself produces free radicals. Horses with PPID have evidence of oxidative damage, including accumulation of pars intermedia 3-nitrotyrosine²⁷ and decreased plasma thiol.³⁰ This oxidative damage does not appear to be the result of impaired antioxidant capacity, as the systemic and pituitary antioxidant capacity of horses appears unchanged.³¹

Clinical Signs

Equine PPID affects many aged equids, resulting in a variety of clinical signs including hirsutism, laminitis, muscle atrophy, fat accumulation, polydipsia, polyuria, secondary infections, lethargy, infertility, persistent lactation, hyperhidrosis, and metabolic abnormalities including hyperglycemia (Fig. 41-4).³² The most unique clinical manifestation of PPID is an abnormally long, curly hair coat that fails to shed, referred to as equine hirsutism. Often horse owners may report the horse shedding its winter coat slowly or incompletely in the year(s) prior to development of full hirsutism. Hair may be initially retained along the legs or under the mandible. The mechanism responsible for development of hirsutism in horses with PPID has not been investigated. The onset of hirsutism in an aged horse or pony is considered essentially pathognomonic for PPID.

Fig. 41-4 Typical horse with pituitary pars intermedia dysfunction (PPID). This 22-year-old Morgan mare shows obvious hirsutism. Other clinical signs of PPID included laminitis and weight loss despite an excellent appetite.

Laminitis secondary to PPID is reported to occur in 24% to 82% of diagnosed cases and frequently necessitates euthanasia in affected animals.^32-35 When adult horses with laminitis of unknown origin were examined, 70% had increased ACTH, suggesting that laminitis in these animals may have been the result of undiagnosed PPID.³⁶ This study suggests PPID may be underdiagnosed, especially when laminitis is the presenting complaint and hirsutism is absent. The pathogenesis of laminitis in PPID is not currently understood but is the subject of ongoing investigation. It has been suggested that laminitis secondary to PPID is the consequence of high circulating cortisol concentration. However, the inability to induce experimental laminitis in normal horses using corticosteroids indicates the pathogenesis is more complex. Recent data have shown that glucose deprivation in the laminar tissue of the hoof, as would occur with insulin resistance (IR) secondary to endocrinopathy or exogenous glucocorticoid administration, may result in failure of the energy-dependent reactions required to maintain the laminar attachments known as hemidesmosomes.³⁷ Inflammatory cytokine and matrix metalloproteinase activation has been suggested to have a role in acute laminitis associated with endotoxemia or grain overload and may similarly contribute to laminitis in horses with endocrine dysfunction.^38,39

In the author’s experience, weight loss caused by muscle mass atrophy is the most common and possibly earliest clinical sign of PPID (Fig. 41-5). Despite weight loss, PPID horses often have a potbellied appearance, and as a consequence owners may fail to notice the lost weight. Weight loss and muscle atrophy may result from several factors including poor dentition, poor nutrition, heavy parasite burden, minimal exercise, and protein catabolism induced by increased cortisol activity. Histologic evidence of type 2 myofiber atrophy has recently been documented in muscle biopsies from horses with PPID, consistent with corticosteroid-associated muscle atrophy in other species.⁴⁰ Response to treatment with the dopamine agonist pergolide was assessed in three horses. Pergolide treatment was associated with an improvement in myofiber type composition, although myofiber ratio and cross-sectional area were no different. Improvements in management of PPID horses often result in significant weight gain, even in the absence of pharmacologic treatment.

Fig. 41-5 Horse with pituitary pars intermedia dysfunction (PPID). The major clinical sign of disease in this horse with PPID was weight loss. Muscle mass atrophy along the dorsum, with a potbelly appearance, is characteristic of PPID.

Despite weight loss and muscle mass atrophy, horses with PPID often have abnormal accumulations of fat, most notably in the crest of the neck, tailbase, sheath, and superorbital fossa. This fat accumulation typically predates the weight loss and has a similar pattern as that observed in horses with equine metabolic syndrome (EMS). The similarity of these two diseases, in both the breeds that are predisposed and clinical signs, has resulted in misdiagnosis of PPID in animals with EMS. There has also been speculation that animals with EMS or sustained obesity with IR may be at greater risk for developing PPID as they age. Although epidemiologic data are currently lacking to support this association, client-provided anecdotal data suggest this may warrant more critical assessment.

Polyuria and polydipsia (PU/PD) have been estimated to occur in 17% to 76% of cases and are typically mild in severity.^32-34,41,42 The mechanism responsible for PU/PD may include (1) compression of the pars nervosa resulting in decreased arginine vasopressin (AVP; antidiuretic hormone) production; (2) osmotic diuresis secondary to hyperglycemia and glucosuria; or (3) factors that are cortisol induced. Cortisol is thought to cause PU/PD in other species by interfering with the secretion and/or action of arginine vasopression.⁴³ Evidence suggests that ACTH and cortisol may inhibit the renin-angiotensin-aldosterone axis, as well. The mechanism of PU/PD in horses with PPID has not been extensively examined. Glucosuria is not a common finding in horses with PPID. In one study, two PPID horses with marked hyperglycemia were found to have water consumption similar to that of normal horses.³⁵ These findings suggest it is unlikely that osmotic diuresis is a major mechanism of PU/PD in the PPID horse.

Horses with PPID have been reported to be more susceptible to infection, including endoparasitism, bacterial sinusitis, skin infections, foot abscesses, and respiratory infections.³² The morbidity and mortality of secondary infection in equine PPID has been reported to range from 27% to 48%.^33-34,41,42 In the absence of parasite control, a heavy parasite burden is common. Routine, quantitative fecal egg counts are recommended to ensure an adequate anthelmintic program. Vigilance on the part of the owner and veterinarian is important in both prevention and early recognition of infections, as they may be clinically insidious. Bronchopneumonia was found at necropsy in 7 of 19 horses with PPID.³⁶ Bronchopneumonia should be considered in the PPID horse with fever or tachypnea and ruled out in horses with intermittent hyperhidrosis.

Diagnosis

Equine PPID is both common and life-threatening; therefore early and accurate diagnosis and intervention are imperative. Antemortem diagnosis of PPID currently relies on testing hypothalamic-pituitary-adrenal axis responsiveness or measurement of endogenous plasma concentrations of POMC-derived peptides, such as ACTH. These tests have been the subject of recent evaluation, and the search for new testing strategies is ongoing.

The overnight dexamethasone suppression test (DST) has been considered the gold standard for antemortem PPID diagnosis. In the unaffected horse, intramuscular administration of dexamethasone decreases release of ACTH from the pars distalis, resulting in a serum cortisol concentration of less than 1 μg/dL (27.59 nmol/L) 19 hours after dexamethasone administration (Fig. 41-6).²² Horses with PPID fail to suppress serum cortisol concentration as a result of ACTH production from the pars intermedia. Originally this test was reported to have a sensitivity and a specificity of 100%.²² However, a recent report suggests the reliability of the test has been overestimated.⁴⁴ When the DST was performed three times at 30-day intervals in seven horses with clinical signs of PPID, only one of the seven horses tested positive for disease on all 3 days. After an initial positive result, five of the seven horses suppressed normally on each subsequent test date, indicating either false-positive results at the initial test period or false-negative results at subsequent testing. These findings are consistent with the observations of the author. Horses with a normal dexamethasone suppression response have been found to have an increased plasma concentration of ACTH or α-MSH and postmortem histologic evidence of pars intermedia adenomatous hyperplasia. Although it has not been critically assessed, a loss of feedback inhibition by glucocorticoid may be a late event in the disease progression, and the high sensitivity originally reported may reflect a case selection bias toward horses with advanced disease. Despite these limitations, the overnight DST remains the test of choice for diagnosis of PPID in the animal when the risk of laminitis is minimal and more intensive testing (DST and TRH) is not practical. In horses in which clinical examination suggests early PPID and diagnostic test results are normal, repeat testing is recommended 6 months later or sooner if clinical signs progress.

Fig. 41-6 Dexamethasone suppression test. , Cushing’s; , control; ↑, 40 mcg of dexamethasone intramuscularly. A, Overnight. B, Standard.

Seasonal variation in DST results has been recently documented.⁴⁵ Clinically healthy horses and semiferal ponies residing in Pennsylvania had a normal DST in January, but when the same animals were tested in September, 40% of the horses and 21% of the ponies failed to suppress. Other diagnostic tests have also been shown to be affected by season. Plasma ACTH concentration measured in September was significantly higher than in January or May.⁴⁵ In plasma samples collected in September, 85% of horses and 97% of semiferal ponies had ACTH concentrations greater than reference range. These animals would have been falsely diagnosed with PPID. In contrast, 100% of the horses and 98% of the ponies had ACTH concentrations within reference range when measured in January or May. Similarly, plasma α-MSH was increased twofold in horses residing in Prince Edward Island, Canada and elevenfold in semiferal ponies in Pennsylvania when measured in the fall compared with spring, summer, or winter concentrations.¹⁷ The effect of season on diagnostic testing needs to be more extensively explored using a larger number of animals in diverse geographic locations.

The TRH stimulation test is also used for diagnosis of PPID, particularly in horses with a history of laminitis. Horses with PPID show an increase in serum cortisol concentration 30 to 90 minutes after TRH administration, whereas normal horses do not.⁴⁶ TRH is believed to be a physiologic releasing factor of the equine pars intermedia.³ In healthy horses, α-MSH (a pars intermedia product) increased over 600% after TRH administration. The exaggerated cortisol release after TRH administration in horses with PPID may be the result of failure of the enzyme prohormone convertase 2 to keep up with the POMC production in the hypertrophic pars intermedia melanotropes, resulting in preferential accumulation and secretion of ACTH. Evaluation of the predictive value of the TRH stimulation test has not been critically assessed in a large number of horses. In one study, 5 of 15 horses without clinical or histologic evidence of PPID had a greater than 50% increase in cortisol 30 minutes after TRH administration.³ These horses would have been falsely identified as having PPID. Historically, TRH stimulation test was limited by the lack of availability of TRH approved for use in the horse and the high price of human-approved products. The current availability of compounded TRH and its apparent lack of adverse effects have resulted in this test being employed more frequently in field practice.

A diagnostic test that may have better performance than either the DST or the TRH stimulation test is the combined dexamethasone suppression and TRH stimulation test. Sensitivity of the combined dexamethasone suppression and TRH stimulation test has recently been critically evaluated in 42 horses, using histology for diagnosis.⁴⁷ Disease was defined as the presence of a discrete mass in the pars intermedia. Horses were included into the study based on availability (horses donated during the time span of the project). This differs from previous studies in which inclusion was based on presence or absence of clinical signs of disease. The method of selection used in the current study is more random and therefore more appropriate for determining sensitivity and specificity of a diagnostic test. Based on histology, prevalence of PPID was 40% in horses of all ages (2 to 33 years) and significantly correlated with age. Sensitivity of the combined test was reported as 88% and specificity as 76%. This test has the disadvantage of requiring multiple sampling over 2 days and an increased cost compared with other methods.

Endogenous concentrations of POMC-derived peptides are also useful in diagnosis of PPID. Increased plasma concentrations of ACTH, α-MSH, and β-END have all been shown to have a sensitivity and specificity of approximately 80% to 90%.^34,48 However, because the DST was used as a gold standard, it is likely these studies overestimate the validity of measurement of POMC-derived peptide concentration in the diagnosis of PPID. Measurement of ACTH is perhaps the most commonly used method for diagnosis of PPID in ambulatory practice because it requires collection of only a single plasma sample and poses no risk to the patient.

Imaging of the pituitary using computed tomography (CT) has also been examined in a limited number of cases as a method for documenting pituitary enlargement.^49,50 Accuracy of CT at estimating width, height, and length of the equine pituitary gland in disarticulated heads from 25 normal horses was determined to be 81% to 93%, 58% to 71%, and 88% to 99% respectively.⁵¹ Accuracy for volume was calculated to range from 43% to 53% in the same study.⁵¹ Although magnetic resonance imaging is the preferred diagnostic imaging modality for pituitary masses in humans, there are no reports of its use in diagnosis of PPID in the horse.

Necropsy Findings

Postmortem examination of the horse with PPID reveals a grossly enlarged pituitary resulting from hypertrophy and hyperplasia of the pars intermedia. The normal horse’s pituitary typically weighs 1 to 3 g; the affected horse’s pituitary may be two to five times this weight. Enlargement may be the result of an adenoma (>1 cm), which often contains areas of hemorrhage and necrosis. Alternatively, microadenomatous (≤1 cm) hyperplasia may be present. Melanotropic tumor cells are pleomorphic (polyhedral or spindle shaped) with eosinophilic, granular cytoplasm.^52,53 Cells are organized into nodules, rosettes, bundles, or follicular structures separated by fine septal tissue. Pigment deposition is common in the pars nervosa, and hemosiderin may be observed when hemorrhage is present. Other lesions include compression of the pars distalis, pars nervosa, or, in the case of large tumors that outgrow the sella turcica, compression of the optic chiasm or hypothalamus. Other gross lesions may include those related to disease complications such as laminitis, intestinal parasitism, pneumonia, or sinusitis.

Treatment and Prognosis

Treatment of PPID is aimed at improving general health and reducing the risk of disease complications such as laminitis and immunosuppression. Management practices should be optimized for care of an aged horse. Diet and feeding practices, dental and hoof care, and deworming schedule should be assessed. Feeding of a pelleted diet designed for senior horses and frequent deworming and correction of dental abnormalities are useful in maintaining the animal’s weight and improving overall general health. Body clipping the horse with hirsutism during the warm weather is critical to limit hyperhidrosis and avoid hyperthermia. Vigilant observation for evidence of infection or laminitis followed by early intervention is important in avoiding protracted illness.

Pharmaceutical therapies for PPID function by decreasing the concentration of circulating POMC peptides and/or cortisol, which theoretically should reduce the risk of disease complications beyond what can be achieved by management alone. Ideally, treatment should also reverse or retard the hyperplastic growth of the pars intermedia, thereby limiting compression of adjacent tissues. However, minimal data are available evaluating the long-term efficacy of medical therapy for PPID.

The current preferred drug for the treatment of PPID is pergolide, a dopamine agonist. Several reports have indicated that pergolide improves clinical signs and diagnostic test results in treated animals.^24-26 An initial dose of 0.002 mg/kg orally every 24 hours has been recommended.³² If no response is observed in 4–12 weeks, the dose is increased in 0.002-mg/kg increments monthly until clinical signs and biochemical abnormalities normalize. A total dose of 0.01 mg/kg should not be exceeded.³² Complications associated with pergolide use in the horse include anorexia, colic, and diarrhea. These are typically dose dependent and resolve spontaneously after a reduction in dose. Once an effective dose is established, endocrine testing should be repeated every 6 to 12 months to ensure hormonal control is maintained.

Cyproheptadine, a drug with antiserotoninergic, antihistaminergic, and anticholinergic activity, was one of the original drugs used to treat horses with PPID. However, several studies using measurable outcomes failed to show consistent efficacy of the drug.^24,26 In addition, although historically inexpensive, cyproheptadine has increased significantly in cost, making pergolide the more rational treatment choice. Cyproheptadine may be useful as an adjunct therapy in horses resistant to pergolide monotherapy. Cyproheptadine may be added at 0.3 to 0.5 mg/kg orally once daily to horses that show minimal response to 0.004 to 0.006 mg/kg of pergolide.³²

A newer treatment available in Europe is trilostane, a competitive inhibitor of 3β-hydroxysteroid dehydrogenase (HSD). Trilostane blocks cortisol production by the adrenal gland. In a report of 20 clinical cases diagnosed using the combined DST suppression and TRH stimulation test, trilostane at a dose of 0.4 to 1 mg/kg once daily resulted in improvement of clinical signs and normalization of cortisol after TRH administration 30 days after starting therapy, although baseline cortisol remained unchanged.⁵⁴ Adverse effects of trilostane were not reported in this study. The effectiveness of trilostane as a monotherapy or in combination with pergolide remains to be evaluated in a large number of horses. One potential limitation of trilostane as a monotherapy in the treatment of PPID is the lack of downregulation of the pars intermedia melanotropes. Therefore a continued elevation in plasma POMC peptide concentration and enlargement of the pars intermedia would be an expected outcome.

The prognosis of horses with PPID is not well documented. Many horses live for years after diagnosis, particularly if receiving optimized management. Anecdotal reports of PPID horses that have responded to pergolide for more than 4 years suggest treatment may remain effective long term. As with all diseases, early recognition, appropriate intervention, and avoidance of complications are the keys to a positive outcome.

Description of Disease

Anhidrosis is characterized as an inability to sweat in response to appropriate stimuli. It occurs in geographic areas that experience hot, humid weather for prolonged periods of time. Clinical signs are especially likely to occur when nighttime temperatures do not fall below 70° F. Classically, anhidrosis affects horses that are not accustomed to these environmental conditions and are moved to a hot, humid area. However, partial or complete anhidrosis is also diagnosed in horses that have grown up in the same areas. It appears that horses that are worked or exercised in hot, humid conditions are more likely to develop the condition than horses that are not worked. Stress may also contribute to onset of the problem. Epidemiologic studies performed in the mid 1980s concluded that there was no sex or breed predisposition to the condition.^63,64 However, it is the author’s subjective impression that thoroughbreds (and Appendix registered quarter horses) and warmbloods are more likely to be affected.

Etiology and Pathophysiology

The cause of anhidrosis is unknown. Presumably there is an abnormality in stimulation or production of sweat. Equine sweat glands produce sweat as an ultrafiltrate from plasma; water and electrolytes are secreted into sweat gland ducts, and this fluid is transported to the skin surface as sweat. Physiologic stimulation of sweating in horses is achieved by activation of β₂-adrenergic receptors, both by direct neural stimulation and from circulating catecholamines.^65-67 Sweat glands from anhidrotic horses do not respond normally to direct stimulation, and there is histologic evidence of sweat gland atrophy.^68,69 However, it is unknown whether the observed sweat gland atrophy is the primary cause of anhidrosis or merely secondary to disuse. Histologic examination of skin from anhidrotic horses also showed no evidence of neural disruption to the sweat glands,⁶⁹ and circulating concentrations of epinephrine are actually higher in anhidrotic horses than in horses that sweat normally,⁷⁰ suggesting that the problem is more likely caused by decreased ability of sweat glands to respond to stimulation, rather than failure of the thermoregulatory system to perceive the need to sweat or failure of stimulation to sweat. Most theories of pathogenesis of anhidrosis suggest downregulation or desensitization of the β₂-adrenoreceptors, but to date no studies have demonstrated either one of these or an alternative mechanism.

In addition to a neural mechanism, there may also be an endocrine component to this disease. Pregnant mares were found to be somewhat less at risk of developing anhidrosis than nonpregnant mares.⁶⁴ An association of anhidrosis with hypothyroidism most likely stems from the observation that thyroid supplementation helped anhidrotic horses racing in Hong Kong in the 1950s.⁷¹ However, the author has not observed any improvement in anhidrotic pleasure horses supplemented with thyroid hormones, and measurement of thyroid hormones at rest and in response to TRH showed no difference between normal horses and horses with anhidrosis.⁷²

There is evidence in some humans that acquired idiopathic anhidrosis may have an immunologic pathogenesis. Serum IgE concentration was increased in one reported case.⁷³ Sweat gland atrophy with infiltration by lymphocytes and mast cells and IgG and C3 deposition in the basement membrane has been described.^73,74 Steroid therapy improved the ability of one of these patients to sweat. An immune-mediated basis for anhidrosis has not been studied in the horse. Basement membrane was noted to be thickened in skin biopsies from 10 anhidrotic horses.⁶⁹ However, inflammation (i.e., infiltration with white blood cells) was noted in only one of six anhidrotic horses housed at ambient conditions.⁶⁹

Results of a recent study showed that expression of the water channel aquaporin-5 was decreased in sweat glands from anhidrotic horses compared with sweat glands from horses that sweated normally.⁷⁵ Although these results may help explain why hypohidrotic or anhidrotic horses cannot produce as much sweat as normal horses, it is not clear whether loss of these water channels is a cause or a consequence of decreased sweat gland secretory capacity.

Clinical Signs and Differential Diagnoses

Early clinical signs of hypohidrosis include exercise intolerance, particularly in warm humid weather, and tachypnea, initially during or after exercise and then at rest as the severity of disease increases. Owners may call the veterinarian thinking the horse has a respiratory problem. As hypohidrosis worsens or the horse becomes anhidrotic, owners realize the horse is not sweating as much as they would expect for the level of work it is performing, or it is taking longer than normal to cool out after exercise. Areas of residual sweat production often include under the mane, in the axilla or inguinal areas, and under the saddle. Body temperature can increase to dangerous levels if signs are not recognized, and affected horses continue to work in hot weather. The hair coat often becomes dry and thin in chronically affected horses, particularly over the face (Fig. 41-7) and cannon bones.

Fig. 41-7 Chronic anhidrosis often causes facial hair loss or thinning.

Clinical Pathology and Diagnostic Tests

Hypohidrosis or anhidrosis can be included in a differential diagnosis of exercise intolerance based on history and clinical signs. The diagnosis can be confirmed by performing an intradermal sweat test.^68,76-78 Six serial tenfold dilutions of a β₂-adrenergic agonist such as terbutaline (10⁻³ w/v to 10⁻⁸ w/v) are prepared. Each dilution (0.1 mL) is injected intradermally along the neck or in the pectoral region. In normal horses a localized area of sweat will appear at the site of injection at all concentrations, except perhaps the most dilute concentration (i.e., 10⁻⁸). The amount of sweat produced at each site is proportional to the concentration of terbutaline injected (Fig. 41-8). Sweating will first be noted in normal horses within 5 minutes of injection at the higher concentrations. Onset of sweat production may be delayed and amount of sweat reduced in hypohidrotic horses, or hypohidrotic horses may sweat only at the higher concentrations of terbutaline. Anhidrotic horses do not sweat, even at the highest concentration.

Fig. 41-8 Terbutaline intradermal sweat test. Six serial tenfold dilutions of terbutaline are injected intradermally along the neck. Normal horses, such as the one shown here, will sweat at the injection site of all concentrations, except perhaps the most dilute (10⁻⁸ w/v). In the normal horse, sweating will typically begin within 5 minutes of injection. Reduction in the amount or delay in onset of sweating is diagnostic for hypohidrosis. A complete failure to sweat in response to all terbutaline injections is diagnostic for anhidrosis.

Although failure to sweat in response to intradermal terbutaline injection confirms a diagnosis of anhidrosis, horses that are hypohidrotic may continue to produce sweat, even at the lower concentrations. For these horses, a lunge test will help confirm the diagnosis. Body temperature, heart rate, and respiratory rate are recorded. The horse is then lunged at a trot for 30 minutes on a hot day. During this time, the horse is observed for evidence of sweat production. Body temperature, heart rate, and respiratory rate measurements are made immediately at the end of lunging and every 10 minutes thereafter for the next 30 minutes. Although the heart rate response reflects the degree of fitness, the body temperature and respiratory rate responses correlate better to the horse’s ability to cool itself. If the horse’s respiratory rate is not back to what is was before the onset of lunging by 30 minutes after the end of the exercise, it is highly likely that the horse is having a problem cooling. Rarely, horses unable to secrete fluid sweat are observed to secrete salt crystals after a lunge test (Fig. 41-9).

Fig. 41-9 Salt crystals secreted in response to exercise (lunge test) in an anhidrotic horse. After a lunge test, this horse excreted salt crystals (shown here) without a fluid component to the sweat. The author has observed this in two horses diagnosed with anhidrosis.

The most likely differential diagnoses for hypohidrosis include various respiratory diseases. In my experience, some hypohidrotic or anhidrotic horses also have mild airway inflammation, documented most often by increased mast cells or eosinophils in bronchoalveolar lavage fluid. It is unknown whether the two problems coexist coincidentally or if there is a mechanistic link. Either way, it is likely that airway inflammation would decrease the ability of a horse to use its respiratory system to help cool itself, and this is likely to be more critical in a horse that cannot sweat.

Treatment, Prevention, and Prognosis

The only consistently successful treatment is movement of the horse to a cooler environment, although this is not always practical or feasible. Other environmental or dietary alterations may or may not be helpful. For hypohidrotic horses or horses that are having their first episode of anhidrosis, it is important to stop any workload and decrease the stress level. Concurrent disease conditions such as airway inflammation should be appropriately treated. Shade, fans, misting fans, and window air conditioners can be used to try to decrease the temperature in the horse’s local environment. Applying water by hose or sponging the horse with water during the hot part of the day can take the place of sweat. Electrolyte supplementation (especially KCl) has been advocated,⁷⁸ and various products are available commercially that appear to help some but not all horses. One such supplement contains L-tyrosine, ascorbic acid, niacin, and cobalt and claims to improve sweat production by supplying precursors to dopamine synthesis (One AC, MPCO, Phoenix, Ariz.). The theory is that dopamine improves skin vasodilation, resulting in increased blood flow to sweat glands during exercise. To the author’s knowledge no studies at present support or refute this claim. Other treatments that have been used with mixed success include acupuncture and Chinese herbs.⁷⁸

Medical therapy of anhidrosis has primarily been unrewarding. Anecdotally, the β₂-adrenergic agonist clenbuterol increases sweat production in hypohidrotic horses and may be beneficial if used sparingly, only when weather conditions are particularly bad (i.e., extremely high heat and humidity). However, traditional recommendations are to avoid use of β₂-adrenergic agonists in hypohidrotic horses because their use may precipitate complete anhidrosis.⁷⁹ There is increasing evidence in human medicine that combination treatment of chronic airway disease with both β₂-adrenergic agonists and corticosteroids results in a synergistic effect, with corticosteroids helping to prevent desensitization and tolerance to β₂-adrenergic agonists, and β₂-adrenergic agonists potentiating the antiinflammatory effects of the corticosteroids.^80,81 Thus, combination therapy may be useful in hypohidrotic horses, but to date no controlled studies have been performed. Corticosteroid therapy may also be useful if there is an immune component to the pathogenesis of anhidrosis.

Once a horse has had an episode of anhidrosis, there are certain measures that can be taken to try to prevent recurrence of the problem in subsequent years. Before the hot part of the year begins, start the horse on any supplements that have been helpful previously and make certain the horse is cardiovascularly fit. Also, make sure that any respiratory problems are under control before the onset of hot weather. As the hot season approaches, try to avoid procedures (e.g., dental procedures) that might require administration of heavy doses of α₂ sedatives that stimulate the horse to sweat. Plan to do these procedures during the cooler parts of the year. During hot periods, work the horse only during the cool parts of the day. Use external water sources to decrease the need for the horse to sweat.

The prognosis for hypohidrotic horses is guarded for future athletic performance in a hot environment. It is the author’s opinion that the longer an anhidrotic horse has been left untreated or unmanaged, the less likely the horse will be to sweat again despite any of the treatments described earlier. However, if an underlying disease or problem can be identified and treated successfully in a horse that is just becoming hypohidrotic, the likelihood that the horse will sweat better the following season is improved.

Other Alterations of Thyroid Function

Certain drugs, diets, physiologic, or pathologic states can alter thyroid hormone synthesis, metabolism, or binding, resulting in altered serum concentrations of thyroid hormones. For example, fasting lowers circulating concentrations of thyroid hormones in many species studied, including the horse.⁹⁷ Phenylbutazone or dexamethasone administration, strenuous exercise, and diets high in energy, protein, zinc, and copper have also been shown to alter circulating concentrations of thyroid hormones in horses.^98-105

Nonthyroidal Illness Syndrome

Nonthyroidal illness syndrome has been described in humans, dogs, and cats with systemic illnesses but has not been studied extensively in horses. A preliminary report by the author suggests nonthyroidal illness syndrome is similar in horses compared with other species.¹⁰⁶ In humans, milder forms of illness result in decreases in serum concentrations of T₃, with T₄ remaining within the normal range or slightly decreased. As nonthyroidal illness becomes more severe, total T₄ decreases, and eventually free T₄ also begins to decrease. The magnitude of thyroid hormone suppression has been correlated to severity of disease and mortality.^107-109 Mechanisms by which thyroid hormones decrease during illness include decreased peripheral conversion of T₄ to T₃ by 5’-deiodinase, altered binding to serum carrier proteins, and hypothalamic-pituitary dysregulation or suppression.^109-113

Fescue Ingestion

Tall fescue (Festuca arundinaceae) is a perennial grass that is grown commonly in the Southeast because it is relatively easy to establish, has a long growing season, and has good disease and drought resistance, which allows it to survive hot, humid summers. Various reproductive problems have been described in mares consuming fescue that have been shown to be caused by alkaloids produced by an endophytic fungus (Neotyphodium coenophialum) that lives symbiotically on the fescue plant. These alkaloids act as a dopamine agonist (for a recent review, see Evans).¹¹⁴ Because TSH release from the pituitary is inhibited by dopamine,¹¹⁵ it has been suggested that fescue consumption could lead to secondary hypothyroidism. Fescue ingestion was proposed as the cause of lower serum TSH concentrations in mares and foals consuming endophyte-infected fescue on a central Kentucky farm, compared with neighboring mares and foals grazing mainly endophyte-free fescue pastures.¹¹⁶ However, when adult, nonpregnant horses were fed endophyte-infected fescue seed for 2 months there was no differences in baseline concentrations of thyroid hormones, TSH, or responses to administration of TRH.¹¹⁷ It appears that dopamine acts more as an acute modulator of TSH secretion, rather than as the primary control. In humans, although acute dopamine blockade results in increased TSH secretion and increased circulating thyroid hormones, chronic administration does not cause long-term alterations in thyroid hormone status.¹¹⁵ Therefore it is likely that compensatory mechanisms override any dopaminergic effect of chronic fescue ingestion in the horse.

Syndromes Historically Associated with Hypothyroidism

Assessment of Thyroid Function in Adult Horses

Because certain drugs and pathophysiologic states can lower serum concentrations of thyroid hormones in otherwise euthyroid horses, it is important that thyroid function tests not be performed while horses are ill, receiving certain drugs, or on thyroid hormone supplementation. The author recommends that thyroid hormone testing be performed in horses that have not received any medications for at least 2 and preferably 4 weeks before testing. If a horse has been receiving thyroid hormone supplementation without prior documentation of hypothyroidism, the author recommends weaning the horse off supplementation and then testing thyroid function once the horse has not received any supplementation for at least 4 weeks.

Tests that are currently available for assessment of thyroid function in the horse include measurement of total and free fractions of T₄ and T₃ and response of these hormones to administration of either TRH or TSH.^{98-100,125,129-132} Although results of TRH or TSH stimulation tests are thought to provide a better indication of thyroid status than single-point-in-time measurements of thyroid hormone concentrations, these tests are not routinely performed by ambulatory clinicians because of the impracticality of having to take multiple blood samples over time. In addition, TRH and TSH either are not readily available commercially for sterile, single-dose application or are prohibitively expensive.

For performance of a stimulation test, a control blood sample is obtained, TRH (1 mg to the average 450- to 500-kg horse) or TSH (5 IU) is given intravenously (IV), and subsequent blood samples are obtained. Most references say that T₃ should double at 2 hours and T₄ should double at 4 hours. The author routinely performs TRH stimulation tests and prefer to measure both T₃ and T₄ before and 1, 2, and 4 hours after TRH administration, in order to make sure the peaks are not missed. In 36 normal horses that participated in various studies the author conducted, the mean increase in total T₄ was 2.2 times at 4 hours. However, the range was 1.3 to 3.8 times. Increases in free T₄ and free T₄ by dialysis (mean and range) were 1.7 (1.1 to 2.1) times and 1.8 (1.1 to 2.8) times at 4 hours, respectively. Increases in total and free T₃ (mean and range) were 3 (1.1 to 10.3) times and 4.2 (1 to 53) times at 2 hours, respectively. These seemingly lower thyroid hormone responses to TRH injection are in agreement with findings in a recent report of levothyroxine administration to normal horses.¹³³ Closer examination of the data from the author’s horses revealed that in general the few individuals with small increases in T₄ or T₃ either started with higher resting values or had peaks that occurred later (T₄) or earlier (T₃) than 4 or 2 hours. Individuals with lower T₄ responses in general were not the same individuals with lower T₃ responses. Therefore one must be careful when interpreting results of a TRH stimulation test. Failure of T₃ to double by 2 hours or T₄ to double by 4 hours after TRH injection does not necessarily mean that the horse is hypothyroid. Individuals with high responses tended to be individuals with very low resting thyroid hormone concentrations.

If single-point-in-time measurement of thyroid hormones is the only option available for evaluation of thyroid status, measurement of free fractions of thyroid hormones (alone or in conjunction with measurement of total amounts of hormone) provides more useful information than measurement of total amounts of thyroid hormones alone. Measurement of serum TSH concentration in single samples will likely aid diagnosis of thyroid status once a TSH assay for the horse becomes commercially available.

During illness in humans, measurement of serum free T₄ by direct methods often underestimates values when compared with measurements of free T₄ after dialysis or ultrafiltration.^134-138 This also appears to be the case in dogs¹³⁹ and horses.¹⁰⁶ Therefore serum concentrations of free T₄ measured by equilibrium dialysis are more likely to reflect true thyroid status in ill horses, compared with other methods of free T₄ measurement. Measurement of fT₄D instead of fT₄ may help prevent equine clinicians from misdiagnosing ill horses as being hypothyroid.

Treatment of Hypothyroidism in Adult Horses

Management of horses that have been properly diagnosed as being hypothyroid or horses that have undergone thyroidectomy theoretically should be fairly straightforward. Serum thyroid hormone concentrations should be monitored and dosages of thyroid hormone supplementation adjusted to maintain serum thyroid hormones in the normal range. This is not necessarily as simple as it sounds. T₄ is the most common form of thyroid hormone supplementation. T₄ administration increases serum concentrations of T₄; however, serum concentrations of T₃ may not change or may actually decrease if T₃ is not also given or if the horse is not truly hypothyroid. In addition, T₄ dosage recommendations have been made based on thyroid hormone concentrations obtained after thyroid administration to normal horses with intact thyroid glands.¹⁴⁰ Thyroid hormone pharmacokinetics may be different in truly hypothyroid horses; pharmacokinetic studies in horses made hypothyroid by thyroidectomy or by administration of antithyroid drugs would be useful. Until such studies are performed, the following recommendations can be made. T₄ is available in several forms. Iodinated casein contains approximately 1% T₄ and is given at 5 to 15 g/horse/day by mouth [PO].⁹² The recommended starting dose of levothyroxine was 20 g/kg/day PO,^82,140 but a more recent study suggested that doses as high as 50 to 100 g/kg/day are well tolerated and may be necessary.¹³³ If a sensitive TSH assay becomes commercially available, dosages should be adjusted to normalize TSH.

Thyroid Function in Premature Foals

Premature human infants experience transient hypothyroxinemia, with serum T₄ concentrations correlated to gestational age.^148-151 In a study recently conducted by the author, serum concentrations of thyroid hormones and TSH at rest and in response to TRH were measured in foals to determine the possible contributions of an immature hypothalamic-pituitary axis and nonthyroidal illness to thyroid function. Three groups of foals were examined: (1) normal, healthy neonatal foals that were full term and not receiving any medications (normal foals); (2) premature neonatal foals (premature foals); and (3) full-term neonatal foals that were hospitalized for conditions similar to those encountered in premature foals (sick foals).¹⁴⁴ Both sick and premature foals received medications routinely used to treat their conditions, which included (but were not limited to) failure of passive transfer, sepsis, and perinatal asphyxia syndrome. Blood samples were collected for measurement of baseline concentrations of total and free thyroid hormones and TSH at predetermined ages. TRH stimulation tests were performed in foals at less than 3 days of age. Premature foals had significantly lower serum concentrations of total and free fractions of thyroid hormones than normal foals. Baseline serum concentrations of TSH were not different, but TSH responses to TRH were exaggerated in premature foals compared with normal foals. Serum concentrations of T₃ and TSH were similar in sick full-term foals and premature foals, but serum concentrations of T₄ in sick full-term foals were intermediate between those of premature and normal foals. These results suggest that sick foals experience nonthyroidal illness syndrome, primarily a low T₃ state. The effects of drugs commonly used to treat ill foals on serum thyroid hormone concentrations in the premature and sick foals in this study were not examined. More profound alterations in thyroid function in premature foals compared with sick full-term foals may be caused by an immature hypothalamic-pituitary-thyroid axis. It remains to be seen whether early thyroid hormone supplementation in premature foals might improve short-term survivability and preserve long-term athletic function. Traditional thought has been that administration of thyroid hormones to patients with nonthyroidal illness syndrome is not beneficial and might even be detrimental. However, these beliefs have recently been challenged,^109,110 and the issue remains controversial. Although results are variable, treatment of premature human infants with T₄ has resulted in improved IQ and neurologic development at 2 years of age,^148,149 and T₃ administration to human neonates with severe respiratory distress syndrome has been shown to improve survival.¹⁵²

Congenital Hypothyroidism in Foals

Two syndromes of congenital hypothyroidism have been described in foals. Hypothyroid foals with visible goiters have been produced by mares ingesting either too much or too little iodine and by mares ingesting goitrogenic plants.^153-155 These foals are born weak, with poor sucking and righting reflexes, hypothermia, and developmental abnormalities of the musculoskeletal system, including tendon contracture or rupture and delayed bone development, particularly of the small cuboidal bones of the carpus and tarsus. Thyroid hormone supplementation to more severely affected foals may improve survivability, but dosage recommendations are scarce. Irvine recommends basing the dose on secretion rate.¹⁴⁷ For oral administration of T₄, this would equal 10 × 0.22 × kilograms of body weight × 0.08 × plasma T₄ (g/L). This calculates to approximately 2.5 mg/day PO for a 1- to 3-day-old 50-kg foal. Because there is a time delay for T₄ to act and because T₃ is more active than T₄, T₃ supplementation at one third the calculated T₄ dose may provide more benefit initially.

A second syndrome of congenital hypothyroidism has been described in foals, primarily in the western parts of the United States and Canada.^156-160 This syndrome is characterized by thyroid gland hyperplasia, increased gestational length, and musculoskeletal abnormalities, including mandibular prognathia, flexural limb deformities of the front legs, ruptured digital extensor tendons, and incomplete ossification of the carpal and tarsal bones. Despite the prolonged gestation, other indicators of prematurity may be present, such as a silky hair coat. At the time of birth, baseline serum concentrations of T₄ and T₃ are usually within the normal neonatal ranges, but the response to TSH administration is decreased.¹⁶¹ The cause is unknown but suspected to be a dietary deficiency or toxicity of the mare during gestation. Likely candidates include nitrate, low iodine, low selenium, or goitrogenic plant ingestion by the mare.^159,161 Because thyroid hormone concentrations are normal at the time of birth, thyroid hormone supplementation is usually not administered. Supportive care to try to prevent collapse of the carpal and tarsal bones is recommended.

Hypocalcemic Disorders

Conditions associated with hypocalcemia in the horse are presented in Box 41-2. Most clinical signs of hypocalcemia are the result of increased neuromuscular excitability and decreased smooth muscle cell contractility (Box 41-3). A decrease in extracellular Ca²⁺ concentrations increases cell membrane Na⁺ permeability, decreasing the resting membrane potential, thus making muscle cells and nerve fibers more excitable. This results in spontaneous and continuous discharges, muscle fasciculations, tremors, tetany, and seizures. Tachycardia and cardiac arrhythmias may be present, although bradycardia may develop during severe hypocalcemia. Decreased serum ionized magnesium (Mg²⁺) concentration occurs frequently in horses with hypocalcemia and further increases neuromuscular excitability.

Synchronous diaphragmatic flutter (SDF) or “thumps” may occur in horses with hypocalcemia associated with gastrointestinal disease,¹⁹³ endurance exercise,²²⁷ hypoparathyroidism,^217,228 idiopathic hypocalcemia,²²³ tetany (lactation, transport),²²⁹ sepsis,¹⁹³ blister beetle toxicosis,²³⁰ and alkalosis.¹⁹⁰ SDF develops when depolarization of the right atrium stimulates action potentials in the phrenic nerve as it crosses over the heart. Clinically, affected animals have a rhythmic movement on the flank from diaphragmatic contractions that are synchronous with the heartbeat.

During alkalosis there is increased Ca²⁺ binding to albumin, resulting in ionized hypocalcemia. Exercising horses may develop alkalosis from hyperventilation (respiratory alkalosis) and chloride losses in the sweat (metabolic hypochloremic alkalosis). Hypomagnesemia is common in horses with SDF and hypocalcemia.

Hypocalcemic tetany is the development of sustained skeletal muscular contractions in horses with hypocalcemia. Although hypocalcemic tetany can occur in any horse with hypocalcemia, lactating mares and horses transported for long distances are at greatest risk. Lactation tetany may occur anytime in mares immediately before foaling up to the end of the lactation period. In particular, mares producing large amounts of milk and eating diets low in calcium, grazing lush pastures, or performing physical work (draft mares) are at risk. Clinical signs may include anxiety, depression, ataxia, muscle fasciculations and tremors, stiff gait, tachypnea, dyspnea, dysphagia, hypersalivation, and hyperhidrosis (see Box 41-3).¹⁹⁰

Hypocalcemic seizures, seen in foals and adult horses, are caused by decreased CNS extracellular Ca²⁺ concentrations leading to increased neuronal excitability. Clinical signs usually improve with calcium treatment, although some animals may require repeated treatments. In general, horses and foals with refractory hypocalcemic seizures have a poor prognosis for recovery.

Ileus and retained placenta in mares are both believed to result from decreased smooth muscle tone and contractility secondary to hypocalcemia. Whereas in skeletal muscle most of the Ca²⁺ required for contraction comes from the sarcoplasmic reticulum, in smooth muscle Ca²⁺ comes from the extracellular space. For this reason, any condition that results in hypocalcemia can decrease smooth muscle contractility. Retained placenta in mares has been reported to occur in up to 10% of foalings.²³¹

Hypoparathyroidism in horses is characterized by hypocalcemia, hyperphosphatemia, hypomagnesemia, and decreased serum PTH concentrations. Primary hypoparathyroidism results from decreased secretion of PTH, whereas secondary hypoparathyroidism results from magnesium depletion and sepsis. There are few documented cases of primary hypothyroidism in horses.^217,228 Hypoparathyroidism should be suspected in any horse or foal with refractory hypocalcemia. Clinical signs include ataxia, seizures, hyperexcitability, SDF, tachycardia, tachypnea, muscle fasciculations, bruxism, stiff gait, recumbency, ileus, and colic.^190,217,228 Laboratory findings include hypocalcemia, hyperphosphatemia, and low or normal serum intact PTH concentrations. Hypomagnesemia may be present in some horses and foals.^217,228 Secondary (functional or acquired) hypoparathyroidism per se has not been reported in the horse; however, some critically ill foals and horses with hypocalcemia have impaired parathyroid gland function, which we believe represents hypoparathyroidism secondary to sepsis and/or hypomagnesemia.

Critically ill foals may develop a form of hypocalcemia that is refractory to calcium treatment.²²³ These foals have low or normal PTH concentrations despite hypocalcemia, suggesting hypoparathyroidism. This condition has been called neonatal idiopathic hypocalcemia.²²³ It is believed that abnormal parathyroid gland function may result from increased inflammatory cytokines. IL-1 and IL-6 have been shown to increase CaR activation and decrease PTH secretion by equine parathyroid cells.¹⁹⁷ Prognosis for survival in foals with refractory hypocalcemia is poor.

Sepsis and endotoxemia are the most common cause of hypocalcemia in equine patients admitted to veterinary hospitals. Clinical observations also indicate that hypocalcemia is common in horses with severe gastrointestinal disease.¹⁹³ Hypocalcemia in septic patients often results from parathyroid gland dysfunction (insufficient PTH secretion) as well as intracellular calcium sequestration.^190,193 Inflammatory mediators known to be increased in horses and foals with sepsis and endotoxemia such as TNF-α, IL-1, and IL-6 decrease equine parathyroid cell sensitivity to Ca²⁺ and PTH secretion.¹⁹⁷

Horses under intense exercise develop electrolyte and acid-base abnormalities. Unlike humans, who may develop either hypocalcemia or hypercalcemia, hypocalcemia is a more consistent finding throughout exercise in the horse. Exercise-induced hypocalcemia may result from Ca²⁺ losses in the sweat, intracellular movement of Ca²⁺, increased Ca²⁺ binding to albumin, lactate, phosphate, and bicarbonate during alkalosis, and parathyroid gland dysfunction.²²²

Oxalate toxicity causes hypocalcemia by interfering with calcium absorption; a diet consisting of 1% oxalate or higher can reduce most intestinal calcium absorption.²³² It is important that the equine diet contain less than 0.5% oxalate or a calcium:oxalate ratio over 1.0. Clinical signs associated with oxalate excess are those of phosphate excess, calcium deficiency, and nutritional hyperparathyroidism. Oxalates are present in several grasses and toxic plants (see Table 41-4).

Equine cantharidiasis (blister beetle toxicosis) is a condition reported in the southwestern and midwestern United States and results from the ingestion of alfalfa contaminated with beetles (Epicauta species) that produce cantharidin (cantharidic acid). Clinical signs develop from the irritant effects of cantharidin on mucosal surfaces (gastrointestinal and urinary tracts). Cantharidin often causes acute hypocalcemia and hypomagnesemia. Therefore clinical signs of severe hypocalcemia (muscle fasciculations, SDF, ataxia, dyspnea, laryngeal spasm, and cardiac arrhythmias) may be present. It is unclear why these horses develop hypocalcemia; however, it may be a combination of severe gastrointestinal disease associated with acute renal failure and parathyroid gland dysfunction.

Hypocalcemia and hypomagnesemia are common findings in horses with acute renal failure. Reabsorption of Ca²⁺ and Mg²⁺ in the kidney is highly dependent on functional epithelial cells, and these cells are very susceptible to various insults (hypoxia, ischemia, toxins). The loss of epithelial cells and their absorptive capacity results in decreased reabsorption of Ca²⁺ and Mg²⁺.

The pathogenesis of hypocalcemia in exertional rhabdomyolysis is unknown. It is speculated that damage to muscle fibers during intense exercise results in Ca²⁺ influx and sequestration in the sarcoplasmic reticulum.

Hypercalcemic Disorders

Hypercalcemic disorders in horses are divided into two groups: (1) parathyroid gland—dependent hypercalcemia (develops because of parathyroid gland hyperfunction), and (2) parathyroid gland—independent hypercalcemia (develops despite parathyroid gland suppression). This distinction is clinically relevant in the differential diagnosis and in the interpretation of specific diagnostic tests, including intact PTH, PTHrP, Ca²⁺, PO₄, and vitamin D concentrations. Parathyroid gland—dependent hypercalcemia in the horse is limited to primary hyperparathyroidism, whereas parathyroid-independent hypercalcemia results from various conditions (secondary hyperparathyroidism, chronic renal failure [CRF], hypercalcemia of malignancy, and hypervitaminosis D).

Primary hyperparathyroidism results from an excessive and autonomous synthesis and secretion of PTH by the parathyroid gland that is not responsive to the negative feedback of Ca²⁺. Primary hyperparathyroidism has been reported in ponies and horses^218,233-236 and results from parathyroid adenomas or parathyroid hyperplasia. The elevated PTH concentrations increase renal Ca²⁺ reabsorption, decrease PO₄ reabsorption, increase 1,25(OH)₂D₃ synthesis, and increase bone resorption (osteodystrophia fibrosa). Laboratory findings include hypercalcemia, hypophosphatemia, hypocalciuria, and hyperphosphaturia. PTHrP concentrations are within normal limits (low or undetectable). Clinical findings include facial bone enlargement, lameness, and a poor body condition. Radiographic findings include decreased long and facial bone density, fibrous proliferation of the maxilla and mandible, and loss of the lamina dura surrounding the molars.²¹⁸ Endoscopic examination may reveal narrowing of the nasal passages.

Postmortem findings include enlargement of the maxilla and mandible, stenosis of the nasal passages, and loosening of premolars and molars. Histologic evaluation of the parathyroid gland is important to confirm the diagnosis of primary hyperparathyroidism; however, finding the parathyroid glands in the horse is a challenge because of their small size and variable location.¹⁹⁰ Although not documented in horses, treatment consists of surgical removal of the affected parathyroid gland.

Secondary hyperparathyroidism is characterized by excessive secretion of PTH in response to hyperphosphatemia and hypovitaminosis D from chronic renal failure (renal secondary hyperparathyroidism) or hyperphosphatemia and/or hypocalcemia from nutritional imbalances (nutritional secondary hyperparathyroidism). Renal secondary hyperparathyroidism is not a well recognized disease in the horse. Unlike humans and small animals, in which CRF results in hyperphosphatemia, horses with CRF often have hypophosphatemia. Moreover, the hypercalcemia in horses with CRF is the result of renal Ca²⁺ retention rather than increased PTH concentrations. The increased Ca²⁺ concentrations in turn decrease PTH secretion; therefore PTH concentrations in horses with CRF frequently are below or within the normal range.^216,237 In contrast, nutritional secondary hyperparathyroidism is a well documented pathologic condition of the horse.

Definition and Etiology

Horses fed diets low in calcium, high in phosphorus, or with a phosphorus:calcium ratio of ≥3:1 may develop nutritional secondary hyperparathyroidism, also known as bran disease, Miller’s disease, big head, osteodystrophia fibrosa, osteitis fibrosa, and equine osteoporosis.²³⁴ Pastures and toxic plants with high content of oxalates (see Table 41-4) predispose to secondary hyperparathyroidism.

Pathogenesis

Excessive dietary PO₄ reduces intestinal calcium absorption and results in hyperphosphatemia. Dietary oxalates form insoluble calcium oxalate (Ca[COO]₂), reducing calcium absorption. Both high-phosphorus and low-calcium diets induce parathyroid cell hyperplasia and stimulate PTH secretion in the horse.²³⁴ Hyperphosphatemia directly stimulates PTH secretion and inhibits renal 1,25(OH)₂D synthesis. Because 1,25(OH)₂D inhibits parathyroid cell proliferation, low 1,25(OH)₂D concentrations contribute to parathyroid cell hyperplasia and PTH secretion. Hyperphosphatemia also results in the formation of calcium phosphate precipitates, reducing blood Ca²⁺, and inducing additional PTH secretion. PTH increases osteoclastic activity, bone resorption, and bone loss (Fig. 41-15).²³⁸ There is facial bone loss with excessive accumulation of subperiosteal unmineralized connective tissue (osteodystrophia fibrosa) resulting in facial enlargement (big head) (Fig. 41-16). Because this is a condition of slow progression, the homeostatic mechanisms that regulate extracellular Ca²⁺ concentrations (PTH, vitamin D, calcitonin) in general are effective in maintaining Ca²⁺ within the normal range. Affected horses preserve normocalcemia at the expense of the skeletal reserves and do not develop clinical signs of acute hypocalcemia.

Fig. 41-15 Pathogenesis of nutritional secondary hyperparathyroidism in horses. Excessive dietary phosphorus reduces intestinal absorption of calcium and induces hyperphosphatemia. In addition, phosphate, oxalate, and phytate bind dietary calcium to reduce absorption. Both high-phosphorus and low-calcium diets induce parathyroid cell hyperplasia and stimulate PTH secretion. PTH increases osteoclastic activity and bone resorption, resulting in bone loss (osteodystrophia fibrosa).

Fig. 41-16 Big head. Two-year-old Belgium horse presented to the Ohio State University Veterinary Teaching Hospital with clinical signs consistent with nutritional secondary hyperparathyroidism, including facial bone enlargement and upper respiratory noise. The horse was fed excessive amounts of grain. There was narrowing of the nasal passages, loss of bone mass, and excessive accumulation of unmineralized bone matrix (osteodystrophia fibrosa) in maxillary and mandibular bones.

Clinical Signs

Clinical signs result from increased bone resorption and include unthriftiness, intermittent, shifting lameness, and a stiff gait. Younger animals may develop physitis and limb deformities. There is a typical and symmetric swelling of the facial bones; however, facial bone enlargement may not be evident in old horses (see Fig. 41-16). The facial changes and the increased bone resorption around molars and premolars may result in masticatory problems. These horses are physically weak and may be in poor body condition from the pain associated with lameness and mastication. In severe cases, teeth may become loosened and spontaneous fractures of long bones may occur. Upper airways obstruction, dyspnea, and epiphora may be present.^239,240

Laboratory Findings

Typical laboratory changes in horses with nutritional secondary hyperparathyroidism include hyperphosphatemia, hypocalcemia or normocalcemia (hypercalcemia is unusual), and increased intact PTH concentrations, especially if the animal is eating a low-calcium or high-phosphorus diet at the time of evaluation. The urinary fractional excretion of calcium is low (hypocalciuria), whereas the excretion of phosphorus is increased (hyperphosphaturia). Serum alkaline phosphatase activity and collagen degradation products may be increased. Laboratory findings may be within normal limits if the animal is eating a balanced diet.

Radiologic Findings

Decreased bone density is frequent; however, bone density must be decreased by 30% before it can be detected radiographically.²⁴¹ Decreased facial bone density along with fibrous proliferation is a consistent finding. Resorption of alveolar sockets and loss of the dental lamina dura may be present before other radiographic changes are present, and long bones are affected only in advanced cases.

Necropsy

There is increased bone resorption, bone fragility, accumulation of fibrous tissue around facial bones, obstruction of nasal passages, and parathyroid gland hyperplasia (see Fig. 41-16). Soft-tissue mineralization has been reported in affected foals.^241a

Treatment

Diet evaluation is indicated. Eliminate or reduce any grain-based diet and avoid high-containing oxalate feeds. The addition of alfalfa to the diet may be helpful. Supplementation with calcium carbonate (limestone; CaCO₃) or dicalcium phosphate may result in improvement.²¹⁹ Ground limestone, which contains no phosphorus, is recommended as a good source of calcium (35%). An affected animal may require a total of 100 to 300 g/day. The diet should have a Ca:P ratio of 3:1 or 4:1. Limestone may decrease feed palatability, and adding molasses should be considered. Supplementation with vitamin D has been proposed. Horses may require 9 to 12 months for complete recovery, although some bone changes may not regress. Confinement of severely affected horses is advised. The use of NSAIDs may be indicated in some animals.

Definition and Etiology

The ingestion or administration of ergocalciferol (vitamin D₂) or cholecalciferol (vitamin D₃) results in disturbances of calcium and phosphorus metabolism in horses.^221,242-244 Ingestion of plants containing 1,25(OH)₂D-like compounds results in typical clinical signs of vitamin intoxication.^243,245 The ingestion of Solanum glaucophyllum (Solanum malacoxylon) results in a condition known as “enteque seco” in Argentina and “espichamento” in Brazil.^242,245 In Hawaii, Solanum sodomaeum, and in the southern United States, jessamine (Cestrum diurnum) may cause hypervitaminosis D.²⁴³ In Europe the ingestion of golden oat (Trisetum flavescens) results in enzootic calcinosis.

Pathogenesis

Hypervitaminosis D increases the intestinal absorption and renal reabsorption of calcium and phosphorus. Hyperphosphatemia is the most consistent and early laboratory finding in horses with vitamin D intoxication.²⁴⁶ Serum calcium concentrations may be increased or within the normal range.^221,243,244 Hypervitaminosis D results in parathyroid cell atrophy and decreased PTH secretion. In addition, hypercalcemia contributes to decreased PTH secretion, lowering bone turnover. Azotemia and hyposthenuria may be present.²²¹

Clinical Signs

Most clinical findings in horses with hypervitaminosis D are the result of hyperphosphatemia. Affected horses often have weight loss, poor appetite, lameness, and painful stiffness and are reluctant to move. Acute death from severe cardiovascular mineralization has been reported.²²¹ Polyuria and polydipsia are frequent findings. In cases with hypercalcemia, mineral deposition in the kidneys may precede mineralization elsewhere, resulting in renal failure. Lameness is probably caused by calcification of ligaments and tendons.

Radiologic Findings

These horses often have increased bone density, decreased size of the medullary cavity, and increased calcification of soft tissues.

Treatment

Reducing dietary calcium intake and using calcium-binding agents such as sodium phytate have been proposed.²⁴⁴ Glucocorticoids are used in humans with hypervitaminosis D, as they may inhibit the vitamin D—mediated intestinal calcium absorption. In equids, glucocorticoids decrease intestinal absorption of calcium, increase urinary excretion of calcium, and decrease bone resorption.¹⁹⁰ Dexamethasone has been administered to horses with hypervitaminosis D with variable results. The prognosis for horses with hypervitaminosis D is poor.

Necropsy Findings

Postmortem examination may reveal mineralization of soft tissues. Mineralization of the endothelium of the aorta and pulmonary vessels and of the endocardium is frequent. Mineralization may be found in the kidney, liver, lymph nodes, lungs, ligaments, and tendons. Osteopetrosis of epiphyses and metaphyses may be present. Atrophy of the parathyroid gland can be severe.²⁴⁷

Definition

Ketosis is a condition characterized by abnormally elevated concentrations of ketone bodies in the body tissues and fluids.²⁵³ The ketone bodies are acetone (Ac), acetoacetic acid (AcAc), and β-hydroxybutyric acid (BHB), which is technically not a ketone body but is formed from AcAc. Ketosis has been categorized as type I or type II based on blood glucose and blood insulin.²⁵⁴ Type I ketosis is the classic primary or spontaneous ketosis, causing a reduction of glucose in the blood and liver (decreased glycogen) and an increased fat mobilization culminating in elevated ketone body accumulations from a negative energy balance during early lactation.^254,255 Type II ketosis involves high blood insulin and transient hyperglycemia secondary to overconditioning and fatty infiltration of the liver (also see Chapter 33 for discussions of fatty liver and pregnancy toxemia). Baird and co-workers²⁵⁶ stated that “a certain degree of ketosis is a natural state in the ruminant, and the ketotic animal may only represent the extreme of a normal metabolic range.” Ketosis becomes a disease condition only when the absorption and production of ketone bodies exceed their use by the ruminant as an energy source, resulting in elevated blood ketones, free fatty acids (FFAs), or nonesterified fatty acids (NEFAs) and decreased blood glucose. The clinical signs of ketosis tend to be vague and nonspecific. Therefore ketosis is classified as clinical or subclinical on the basis of levels of ketone bodies in the blood, urine, and milk and the presence or absence of clinical signs. Any disease process occurring in early lactation that reduces feed intake may cause secondary ketosis.

Etiology

Ketosis is a production disease of modern agriculture. Dairy cattle have been genetically selected for high milk yield, which has resulted in elevated milk production during early lactation. This milk production exceeds the capacity of the animal to ingest sufficient feed to meet requirements for energy.²⁵⁷ The input into the animal must equal or exceed the output to prevent a negative energy balance (Table 41-7).

Table 41-7 Summation of Components of Energy Balance in the Bovine

The milk production of dairy cattle peaks by approximately 4 weeks after parturition, but the dietary intake on a dry matter basis does not peak until 7 to 8 weeks.²⁵⁸ High-producing dairy cows will be in a negative energy balance for as long as 8 weeks, despite the provision of a high-quality, palatable diet. To offset the negative energy balance, the individual cow must mobilize body fat and protein stores in the form of triglycerides and amino acids for gluconeogenesis. Ketone bodies are normally produced by the liver and ruminal wall, although ruminal wall ketone production is insignificant during clinical ketosis.²⁵⁹ The mammary gland indirectly contributes to ketone body production by using glucose for lactose production.²⁶⁰ The liver, however, is the major source of overall ketone production during ketosis. All tissues in the normal cow can adapt to the use of ketone bodies as an alternate energy source except the liver. It must be stressed that normal, high-producing dairy cows will have some level of ketosis during the rising curve of their lactation until their energy intake balances milk production, despite the provision of good-quality feed. During this period cows lose 30 to 100 kg (65 to 220 lb).^261-263 The difficulty is in identifying and preventing the factors that move a cow from the normal level of ketone body formation into the subclinical and clinical categories.

Any disease condition that decreases dietary intake may cause secondary ketosis as a result of increased fat mobilization and ketone production. During the immediate postpartum period cows are susceptible to many diseases that are likely to reduce their normal feed intake. Ketosis may also be secondary to the ingestion of preformed ketones in the diet (silage high in lactic or butyric acid).²⁵⁸ Biogenic amines such as putrescine, tryptamine, cadaverine, and histamine contained in ketogenic silage may play a role, possibly by decreasing intake.^264,265 Cobalt deficiency has been implicated as a potential cause of ketosis.²⁶⁶ There is also a high incidence of ketosis in herds affected by fluorosis.²⁶⁷ Contamination of concentrates with low levels of lincomycin have been reported to cause herd outbreaks of clinical ketosis.²⁶⁸ The onset of bovine somatotropin (BST) use in dairies was speculated to increase the incidence of ketosis, but this has not been demonstrated.^269,270

Clinical Signs and Differential Diagnoses

Clinical ketosis is most commonly seen as a gradual loss of appetite and decrease in milk production over several days. Loss of appetite is usually sequential, with refusal of grain, then silage, and lastly forages. As feed intake decreases, weight is lost rapidly, and milk production drops. During early lactation, reduction in milk production lags behind the reduction in energy intake. The incidence of type I ketosis tends to occur 3 to 6 weeks postpartum as milk production peaks, in contrast to type II, which is seen immediately after calving.²⁵⁴ Physical findings include normal vital signs; firm, dry feces; moderate depression; and sometimes reluctance to move. Ruminal motility may be decreased if the animal has been anorectic for several days. Occasionally pica is seen. Often the odor of ketones can be detected on the breath and in the milk. Clinical signs may spontaneously disappear without treatment when an equilibrium between milk yield and dietary intake is reached. Transient nervous signs such as staggering and blindness may occur for short periods of time. Displaced abomasum (particularly left displacement or LDA), metritis, mastitis, and peritonitis (particularly traumatic reticuloperitonitis) are common primary disease entities leading to secondary ketosis. Although LDA has traditionally been thought to result in ketosis, recent work has demonstrated that elevated blood NEFAs and ketones precede LDA by 2 days and increase the risk of the condition by as much as eightfold.²⁷¹ Ketosis decreases immunoresponsiveness, leaving affected cows more vulnerable to concurrent infections. This is attributed to the hypoglycemia and suppressive effects of NEFAs.²⁷¹ Less common causes of secondary ketosis are subclinical hypocalcemia, mild ruminal overload and laminitis, lameness caused by sole abscesses and ulcers, pyelonephritis, and musculoskeletal injuries after calving.

In the nervous form of ketosis, there is an acute onset of bizarre neurologic signs, including circling, proprioceptive deficits, head pressing, apparent blindness, wandering, excessive grooming behavior, pica, and excessive salivation. These animals may show hyperesthesia, bellowing, moderate tremors, and tetany.²⁷² They may behave aggressively toward people or inanimate objects and appear ataxic when ambulating. Episodes of nervous signs last 1 to 2 hours and recur at 8- to 10-hour intervals.²⁶⁰ Diseases such as listeriosis, rabies, lactation tetany, acute lead poisoning, and Claviceps paspali poisoning should be considered as possible differential diagnoses.

A very thorough physical examination must be performed to differentiate primary from secondary ketosis. A mild fever and increased heart rate are often associated with primary diseases that are inflammatory in nature. Moderate to severe ketonuria may be seen. During the first 2 weeks after calving, cows may require treatment for concurrent diseases. If appetite does not return after standard therapy for the primary disease entity, further therapy for ketosis may be necessary. The tendency for ketosis to recur necessitates careful reassessment by repeating the physical examination to detect primary diseases causing secondary ketosis.

Clinical Pathology and Laboratory Aids

Ketone bodies may be detected in urine, plasma, and milk (Table 41-8).^260,273-280 The literature is reported in the International System of Units (SI units) (mmol/L) and conventional units (mg/dL), which makes interpretation difficult. The vagueness of clinical signs has made it difficult to determine precise definitions of ketone bodies in clinical ketosis. As a general guideline, animals with clinical ketosis will have blood glucose concentrations of 20 to 40 mg/dL, total blood ketones >30 mg/dL, total urine ketones >84 mg/dL, and total milk ketones >10 mg/dL. Individuals with subclinical ketosis are those that have no clinical signs of ketosis but have low-normal blood glucose, total blood ketones of 10 to 30 mg/dL, and total milk ketones of 2 mg/dL. Recent studies have statistically determined BHB to give the best correlation for subclinical ketosis with a threshold of 1400 μmol/L (14.4 mg/dL).²⁸¹ Secondary ketosis tends to fall between the ranges for clinical and subclinical ketosis, depending on the duration of the primary disease process.²⁵⁹

Table 41-8 Blood, Urine, and Milk Analysis in Ketosis

Commercially available tests have changed substantially in the past few years, with greatly improved diagnostic capability. Urine ketones may give a positive result in otherwise normal cows, because ketones are concentrated to 2 to 20 times the blood ketone level. Ketone bodies in the milk more closely reflect ketone blood levels, making milk ketones a better indicator of ketosis (approximately 50% of blood concentration).^279,280 Of the ketone bodies, AcAc tends to be the most unstable and difficult to detect in samples. Various products that detect the Ac, AcAc, and/or BHB are available. A recent review of available tests found that two products, one detecting AcAc in urine at the “small” reading (Ketostix, Bayer, Elkhart, Ind.) and the other detecting BHB in milk (KetoTest, Sanwa Kagaku Kenkyusho, Nagoya, Japan), had acceptable sensitivity and specificity for screening herds for subclinical ketosis.²⁸² In a second study, Pink test liquid (www.profs-products.com, Germany) and Ketolac (Hoescht, Germany) were also found to be satisfactory for diagnosis of subclinical ketosis.²⁸³ Blood levels of volatile fatty acids (VFAs), FFAs, and NEFAs are increased in ketosis. New test kits have been developed that are capable of assessing NEFAs on site (VDx Diagnostic Analyzer, Newburg, Wis.). NEFA levels of >0.5 mEq/L were satisfactory in detecting herd problems with subclinical ketosis.^284,285 Problems exist with diurnal variation in blood levels of VFAs, FFAs, and NEFAs, making timing of blood sampling difficult.^280,286,287 These tests can be used to monitor dairy herds for negative energy balance and the risk of subclinical and clinical ketosis and associated disease problems.²⁸⁵

The central role of the liver in energy metabolism of ruminants associates the onset of clinical ketosis with elevations of liver enzymes and abnormal liver function test results. Serum aspartate aminotransferase (AST) and sorbitol dehydrogenase (SDH) may be increased in severe cases. The degree of liver dysfunction is mild compared with that of cows with fatty liver syndrome. The sulfobromophthalein (BSP) clearance test was the classical evaluation of liver function particularly in overconditioned cows at risk for fatty liver syndrome development (see Chapter 33) but is currently unavailable. Serum bile acid levels have had variable results and may not be useful in differentiating mild, moderate, and severe fatty infiltration of the liver, but they do correlate with severe hepatic damage.^288,289 The BSP clearance test also did not differentiate between mild and moderate fatty infiltration of the liver. Liver biopsy and determination of triglyceride or triacylglycerol (TAG) are the gold standards for accurately assessing the degree of hepatic lipidosis and are relatively simple to perform in cattle.²⁹⁰

White cell counts vary and may reflect stress or the primary disease process that the animal is experiencing. Studies vary in the reported effect of elevated ketone bodies and decreased blood glucose on lymphocyte proliferation and immunoglobulin production.²⁷¹ Serum calcium and magnesium levels may be slightly decreased in animals that are anorectic. Cortisol levels are usually within the normal range, and plasma insulin is elevated initially but depressed as the feed intake is decreased.²⁵⁴

Pathophysiology

Ruminants are exquisitely programmed to use forages in the production of energy for growth, maintenance, pregnancy, and lactation. The control of energy in the ruminant is under the hormonal control of mainly insulin and glucagon. Corticosteroids, GH, catecholamines, and leptins have also been shown to have important roles in the fine-tuning required at the critical times of transition between late pregnancy and early lactation. It is important to keep in mind that ketone bodies are an integral part of normal energy metabolism in ruminants, and it is only when the negative energy balance overwhelms the ability of the cow to use the available ketone bodies that a disease condition occurs.

Control of blood glucose in ruminants is mainly under the control of insulin, which favors cellular uptake of glucose, lipogenesis, and glycogen synthesis while decreasing lipolysis and hepatic gluconeogenesis.^290,291 Ruminants are considered to be relatively insulin resistant, but during early lactation low insulin concentrations are accompanied by high tissue insulin sensitivity.²⁹² Glucagon counteracts insulin by increasing lipolysis and hepatic gluconeogenesis and decreasing lipogenesis. Catecholamines modulate energy metabolism by favoring lipolysis and decreasing lipogenesis. GH levels are normally high in early lactation and inhibit lipogenesis in adipose while increasing gluconeogenesis in the liver. Adipocytes have been of increasing interest in all species, as they have been shown to produce a variety of endocrine factors (leptin, resistin, IL-6, TNF-α, and adiponectin).^293,294 Leptin has potential for influencing feed intake and resistance to insulin and increasing energy expenditure and has been documented to be increased in obesity. The extent and importance of these interactions have not been completely investigated in ruminants.

In the normal lactating cow, energy is presented to the liver in the form of VFAs, bacterial protein, and a small amount of glucose and protein that escapes degradation in the rumen (Fig. 41-17).^255,260,261 The principal VFAs are acetate, propionate, and butyrate, which are produced in an approximate 70:20:10 ratio, respectively, in the rumen. Acetate is mainly used in fat synthesis, although there is some evidence to suggest that it may be a minor glucose source by entering at the acetyl coenzyme A (CoA) level. Butyrate is condensed into acetoacetyl CoA, which can be partially oxidized to ketone bodies or transformed into acetyl CoA that can enter the tricarboxylic acid (TCA) cycle (no net gain of glucose). Propionate enters the TCA cycle directly at the level of succinyl CoA (equivalent to 30% to 50% of glucose production in the ruminant).²⁹⁵ Therefore acetate and butyrate are ketogenic, and propionate is glycogenic. The normal ratio of production in the rumen is four ketogenic to one glycogenic VFA. Significant production of ketone bodies occurs in the ruminal epithelium and mammary gland, as well as in the major production site, the liver.^259,295 Ketone bodies are normally used by the TCA cycle in the heart, kidney, skeletal muscles, and mammary gland through the acetyl CoA pathway.

Fig. 41-17 Hepatocyte cellular metabolism under negative energy balance in the presence (left side) or absence (right side) of adequate glucose or glucose precursors.

Efficient oxidation of acetyl CoA depends on an adequate supply of oxaloacetate, which is generated from gluconeogenic precursors, mainly propionate (from the rumen) and lactate and pyruvate (from anaerobic metabolism of glucose).²⁵⁵ Skeletal muscle mass also provides amino acids for gluconeogenesis. Lactating cows preempt a large quantity of propionate and lactate for milk production in the form of lactose.²⁹¹ Supplies of oxaloacetate are reduced, slowing down the TCA cycle and the use of acetyl CoA. The backlog of acetyl CoA is diverted into the formation of ketone bodies.

Adipose tissue stores energy in the form of triglycerides, which can be mobilized to provide NEFAs that either enter the TCA cycle through acetyl CoA, adding to the formation of ketone bodies, or are reesterified into triglycerides or TAG (see Fig. 41-17). The ruminant liver is inefficient in partitioning the increased NEFAs into triglycerides and secreting them into the circulation as lipoproteins. Apoprotein B is required to form VLDLs. Deficiency of apoprotein B will result in accumulations of TAG or fatty infiltration of the liver and abnormally elevated ketone bodies. The negative energy balance that occurs in postpartum dairy cows further reduces available carbohydrate and accelerates fat mobilization and ketone body formation. The ketone bodies in clinical ketosis are mainly produced from NEFAs in the liver, which shift in response to low carbohydrate supplies from the pathways of esterification and complete oxidation of acetyl CoA to CO₂ to partial oxidation of acetoacetyl CoA to ketone bodies.^259,291,295 The net results are ketonemia, ketonuria, ketolactia, hypoglycemia, and low levels of hepatic glycogen.

Clinical ketosis in ruminants occurs when the demand for glucose by the mammary gland or fetus exceeds the energy resources available from the diet and fat mobilization, resulting in hypoglycemia. The daily glucose requirements of a dairy cow increase above normal maintenance by 30% in late gestation and by 75% with the onset of lactation.²⁹¹ The average energy requirements of a 1000-lb lactating cow have been estimated to be 50 g of glucose per hour.²⁹⁶ Only 10% of the glucose requirement is available in the form of glucose.

In late pregnancy and early lactation it is common for a negative energy balance to occur, which results in subclinical ketosis. Any additional nutritional, metabolic, or other disorder resulting in decreased feed intake will make the subclinical ketosis clinical.

Epidemiology

The morbidity of clinical ketosis is extremely variable and difficult to measure. Management and nutrition are influential factors that vary widely from farm to farm and area to area. Prevalence rates (number of cases at a point in time) have been reported to be 13.1% (range 3% to 22%, depending on lactation) for clinical ketosis and 33.8% (range 31% to 41%) for subclinical ketosis.^297,298 Incidence rates (number of cases that occur in 1 year) have been reported to range from 1.87% to 13% for clinical ketosis and 7.3% to 12.1% for subclinical ketosis.^297-301 Studies using newer tests for ketone bodies have reported subclinical ketosis incidence rates of 59% and 43% using cutoff points of 1400 and 1200 μmol/L, respectively.³⁰² Most cases occur in the first 6 weeks after calving, with the peak incidence at 3 to 4 weeks after calving.^298,303,304 Breed differences have been found, as well as genetic tendencies within bull-daughter families.^299,300 The incidence of clinical ketosis increased with parity, peaking at the fifth to sixth lactation.^298,299 Cows that were diagnosed with clinical ketosis once had increased risk of ketosis at subsequent calvings.^301,303 Recurrence of ketosis in individuals may reflect digestive capacity and metabolic efficiency, as well as milk production, which are likely to have a multifactorial mode of inheritance.

Generally, clinical and subclinical cows are high producers and are overconditioned at calving.^304,305 Fat cows have been shown to have a 25% reduction in dry matter intake (DMI) and higher turnover rates of fatty acids because of more fat mobilization.³⁰⁶ Environmental factors that affect the incidence of clinical ketosis include season (increased during midwinter), climate, stabling (increased in stabled vs. loose housing), and feeding regimen (increased with number of feedstuffs used and fewer numbers of feedings per day).^301,304,307 Diets that are less than 8% protein on a dry matter basis before calving or that have high protein levels (greater than 20% DM) after calving have been associated with high incidences of herd ketosis.³⁰⁸ One study reported an association of increased incidence with a high standard of management and decreased incidence if the wife, versus other family members or paid workers, assisted with chores.³⁰⁷

Thirty percent to 40% of cases are complicated by concurrent diseases such as metritis, traumatic reticuloperitonitis, and abomasal displacements.³⁰⁸ These cows are classified as having secondary ketosis. A diagnosis of parturient paresis, alone or in combination with retained placenta, increased the risk of clinical ketosis.^301,304 Cows with metritis are more likely to be diagnosed as having subclinical ketosis.²⁹⁷ Displaced abomasum and lameness in a previous lactation were associated with clinical ketosis.³⁰⁴ One study reported that cows with elevated NEFA concentrations prepartum and elevated BHB postpartum had an increased risk of developing an LDA and ketosis.²⁷⁰ Cystic ovaries, increased calving to first service interval, and increased calving to last service interval have been associated with subclinical ketosis.^297,304,305 Others reported no association between ketosis and the calving interval and number of inseminations.³⁰⁹ Most genetic studies are based on the lactation previous to the diagnosis of ketosis and have found either no difference or a tendency toward higher production in cows with ketosis.^299,309-312 One study demonstrated a genetic correlation between milk production and ketosis using only first lactation records.³¹³ Subclinical ketosis was found to be associated with losses of 1 to 1.4 kg of milk per day or 4.4% to 6% of the mean daily production.²⁹⁷ Other authors³¹² found an association between diagnosis and treatment of clinical ketosis, with an increase of 2.5% in production over the entire lactation. A negative correlation of ketosis with culling rate indicated that high-producing cows with ketosis were less likely to be culled than low-producing cows.^301,314

Necropsy Findings

The mortality rate from primary ketosis is extremely low. In animals that die with clinical ketosis, a fatty liver is likely to be the only pathologic finding. In secondary ketosis lesions are associated with the primary disease condition.

Treatment and Prognosis

Treatment of secondary ketosis requires correction of the primary condition while ensuring the provision of an adequate diet. In ketosis secondary to the butyrate content of silage, the diet can be manipulated to eliminate or dilute the silage.²⁵⁸

Many treatments have been used for primary clinical ketosis. The goal of treatment is to limit the mobilization of fat by increasing the availability of glucose or glucose precursors and promoting uptake of glucose by cells.³¹⁵ The pathophysiologic reasoning for specific treatments of ketosis has been reviewed.³¹⁶ Clinical response to traditional treatment with intravenous glucose, oral propylene glycol, corticosteroids, and insulin has been well documented. In addition, many nontraditional treatments and feed supplements have shown variable success in treating and preventing ketosis.

Traditional therapy with intravenous injections of 100 to 500 mL of 50% glucose (dextrose) gives marked clinical improvement. A transient hyperglycemia is produced; return to preinjection levels occurs in 2 hours. Blood ketones drop immediately, clinical signs disappear, and milk production increases by 5 to l0 lb for at least one milking.³¹⁷ Nervous signs may reappear in l2 to 24 hours, and milk production drops again over 2 to 3 days. There are fewer relapses if the glucose injections are repeated frequently. Solutions containing a mixture of 25% dextrose and 25% fructose have been used in an attempt to prolong the hyperglycemic action. Ideally a continuous intravenous glucose infusion at 0.5 g/min should be administered until milk ketone test results are negative.^296,317 In my opinion a slow infusion of 20 L of 2.5% glucose (with half normal saline) over 24 hours improves the clinical response. This solution allows fast enough drip rates for ease of catheter maintenance without the danger of causing osmotic diuresis and excessive water loading. Urine is monitored by dipsticks for negative glucose and decreasing levels of ketone bodies several times daily. Daily monitoring of the blood glucose (once or twice daily) measures the adequacy of dextrose administration. Careful monitoring for hypoglycemia is required when intravenous glucose is discontinued. This may be impractical for field situations but is worthwhile in clinical settings with valuable individuals.

Glucose precursors may be given orally in the feed or as a drench and provide a source for gluconeogenesis. These include a propylene glycol drench at 225 g (8 oz) bid for 2 days, followed by 110 g (4 oz) once daily for 2 days or glycerol at 500 g bid for up to 10 days.³¹⁸ Overuse of propylene glycol may have a deleterious effect on ruminal flora, decrease ruminal motility, and cause diarrhea, necessitating its discontinuation and the institution of ruminal transfaunations. In one study, adding propylene glycol to the diet of postpartum cows decreased NEFA and BHB levels but did not significantly change milk production, health, or fertility and therefore was not economically beneficial.³¹⁹ Glycerol drenches at 1 to 2 L orally per cow alleviated symptoms of ketosis, but including glycerol in the transition feed did not.^320,321 Sodium propionate (125 to 250 g bid orally), ammonium lactate (120 g bid orally), and sodium lactate (360 g bid orally) have also been used as feed additives to provide alternate glucose sources. All of these tend to lower the butterfat test result and may cause digestive disturbances if prolonged treatment is used.³¹⁷

Glucocorticoids are often used to prolong the hyperglycemic effect by decreasing tissue uptake of glucose and reducing milk production for up to 3 days. Dexamethasone (0.04 mg/kg) and betamethasone are most commonly used. In one study a single treatment of dexamethasone (0.04 mg/kg) significantly increased blood glucose for 6 to 9 days and decreased milk production for 1 to 7 days.³²² Caution must be used, because overdosing may reduce feed intake and exacerbate the condition of cows with fatty liver syndrome.³²³ Anabolic steroids such as trienbolone acetate have resulted in decreases in blood levels of ketone bodies without depression of milk production but are prohibited in the United States.³²⁴

Low doses of long-acting insulin (200 IU of protamine zinc insulin subcutaneously [SC] once every 48 hours) have been used as an adjunct to intravenous glucose and glucocorticoid therapy. Use of the newer slow-release insulin (Humilin, Ultralente human insulin rDNA, Eli Lilly, Indianapolis, Ind.) at a dose of 0.14/IU/kg of body weight IM provided an insulin peak by 12 hours postinjection, with return to preinjection levels by 24 hours.³²⁵ Blood glucose lowered by 21% at 6 to 12 hours and returned to preinjection levels within 24 hours. Pancreatic secretion of insulin is reduced in ketotic cows in response to intravenous infusions of glucose.³²⁶ Insulin assists in suppressing fatty acid mobilization and increasing tissue uptake of glucose while stimulating hepatic glycolysis. Intravenous glucose combined with insulin when administered over several days has been shown to decrease NEFAs and liver triglycerides and increase hepatic glycogen.³²⁷

There has been interest in using glucagon injections to stimulate gluconeogenesis and limiting lipolysis, which decreases triglyceride accumulation in the liver during early lactation. Recent studies reported glucagon had no adverse effects but had only minor effects on the lipid transport in early lactation.³²⁸ Glucagon is not commercially available.

Lipotrophic agents such as choline (25 to 50 g daily PO), cysteamine (750 mg IV every 2 to 3 days), and L-methionine have been suggested as feed additives or treatments that increase mobilization of fat in the liver.^329-331 Choline (25 g daily) may also be given subcutaneously but should not be given intravenously because it acts as a neuromuscular blocker. Therapy with lipotrophic agents has not been proven effective in controlled trials and may even be harmful in cases of severe liver damage.³³⁰

Cobalt deficiency and therefore vitamin B₁₂ deficiency have been implicated as a cause of ketosis.²⁶³ Vitamin B₁₂ is an essential cofactor in the metabolism of propionate as it enters into the TCA cycle.²⁶² Blood and liver levels of vitamin B₁₂ are reduced in the postparturient cow.²⁶⁶ Although vitamin B₁₂ and cobalt may be added to the diet, the effectiveness of vitamin B₁₂ has not been proven.³³²

Supplementation of prepartum and early lactation diets with chromium decreased serum NEFAs but had no effect on milk production or milk components.³³³ Decreased NEFA was greatest at 1 week postpartum. Chromium may potentiate the action of insulin and has a role in the activation of thyroid hormone.

Nicotinic acid (niacin) and nicotinamide have been used with variable effects.²⁶² Nicotinamide coenzymes in mammary tissue were reduced in ketotic cows compared with normal cows.³³⁴ The suggested dose of nicotinic acid is 6 g orally once daily for up to 10 weeks after calving. Milk production was slightly increased, and blood levels of ketone bodies and FFAs were lower in niacin-treated cows.³³⁵ Niacin decreases blood ketones and FFAs and increases blood glucose. Niacin has also been used in combination with propylene glycol or monensin, but no significant differences were found between treatment and control groups.^336,337 Biotin is a coenzyme in the process of gluconeogenesis and has been studied in the peripartum period.³³⁸ Although NEFAs and hepatic TAG were lower, biotin did not decrease BHB.

Ionophores increase the ratio of propionate formation in the rumen and have been documented to decrease the incidence of clinical ketosis.²⁸¹ As of October 2004 monensin (Rumensin, Elanco, Greenfield, Ind.) was cleared for inclusion in feed for dry and lactating dairy cattle in the United States and is valuable as a tool for preventing clinical and subclinical ketosis.

Chloral hydrate is a traditional treatment that increases the breakdown of starch in the rumen and influences the ruminal production of propionate. The initial oral dose is 30 g, followed by 7 g bid for several days.³¹⁷ Chloral hydrate may be particularly helpful for its sedative effects in treating cows with recurring nervous ketosis.

As in all metabolic diseases, nursing care is important. Supportive therapy may include ruminal transfaunations, provision of a variety of palatable feeds, and exercise.

Prevention and Control

Because the underlying mechanism of clinical and subclinical ketosis is one of negative energy balance during the first 8 weeks of lactation, prevention and control can be addressed in three steps. The feeding and management of cows during late lactation and the dry period should promote good body condition at calving (see Chapter 9 for BCS system). Optimum intake of lactating rations at the commencement of lactation must be encouraged by introducing the ration in a stepwise fashion. The ideal ration during early lactation is highly palatable and of an appropriate energy density.^263,285

A moderate amount of body fat should be available for mobilization and milk production at parturition. The body fat that is lost in early lactation must be stored in the previous late lactation by feeding to National Research Council (NRC) recommendations.²⁶³ Body condition of the dry cow must be maintained, and fetal growth provided for. It is essential that cows do not become too fat before parturition.^261,305,306 There is evidence that fatty infiltration of the liver begins before parturition, particularly in individuals with fat cow syndrome.²⁸⁸

The introduction of the lactating ration should be made as smoothly as possible to encourage maximum intake and minimize digestive upsets. Feeding of the lactation ration in limited amounts may begin as early as 4 to 5 weeks before parturition so that typically 8 to 9 lb of concentrates are being fed at parturition. After calving, incremental increases of a few pounds per day are made until ad lib levels are reached (at approximately 2 to 4 weeks).²⁶⁰ Ensuring adequate bunk space and minimizing pen changes at the time of parturition will have a critical, positive effect on DMI immediately.²⁸⁵

The ideal early lactation ration is highly palatable and meets NRC recommendations. It is beyond the scope of this text to outline detailed ration balancing. The key aspects are maintaining high energy density and optimum levels of fiber and protein without compromising the DMI. Calculations of the energy requirements of a lactating cow are expressed as total net energy of lactation and are obtained from NRC charts.²⁶³ Recommendations for fiber content of the diet are based on the acid detergent fiber (ADF) and NDF. NDF is a good estimate of the bulk of a diet, and the DMI of a ration depends on the NDF (1.2% of body weight). Not all forage analysis laboratories report the NDF, but it can be approximated by dividing the total digestible nutrients by 100. Protein should be provided as both rumen degradable (soluble and nonprotein nitrogen) and rumen undegradable (bypass). Excess protein and fat should be avoided.²⁸⁵ Specific instructions for formulating these ingredients into a ration are provided by multiple ration-balancing programs.²⁶³

The diet must also be balanced in its minerals. Cobalt may be added if there is an indication of inadequate levels. Nicotinic acid is recommended as a feed additive at 6 to 12 g/head/day in early lactation rations.³³⁵ Inclusion of chromium in peripartum diets may also be beneficial.³³³

Problems that arise from silage with high butyrate concentrations may require substitution or dilution of the affected silage with other feeds for the cows in early lactation. High-butyrate diets are often tolerated because they encourage a higher milk butterfat content.^258,317 The addition of protected fats in the form of the calcium salts of long-chain fatty acids or a high proportion of saturated long-chain fatty acids (palmitic and stearic acids) increases the energy density of the ration without reducing the fiber content.³³⁹ These compounds are not degraded in the rumen but are digested in the abomasum and small intestine. Feeding protected fats results in increased milk production, slightly decreased DMI, and increased FFAs, as well as stabilized blood ketones and weight loss through a glucose-sparing effect. However, there is some evidence that added fat does not help in the periparturient period and may interfere with other treatments.³⁴⁰ Glucogenic precursors such as propylene glycol and sodium propionate have been incorporated into early lactation rations for many years. Both of these compounds are not palatable to dairy cattle and are better reserved for treatment of individual cases of ketosis.

Subclinical and clinical ketosis should be detected and treated as early as possible to prevent deleterious effects on health and production. This may be accomplished by encouraging clients to use ketone tests routinely on milk or urine during the first 50 to 60 days after parturition. Several companies are developing commercial automated sampling systems that can routinely test milk for ketone bodies.^341-343 Cattle with positive test results should have a thorough physical examination. Institution of supportive therapy for subclinical ketosis should include administration of oral propylene glycol. A high prevalence rate of clinical and/or subclinical ketosis would necessitate investigation of the feeding program.