ETIOLOGY

The cause is Sarcocystis neurona, an ampicomplexan protozoan.^1-3 Isolates of S. neurona can vary in their antigenic composition, such that variable responses are observed on immunoblot tests,⁴ but the importance of this variation in terms of pathogenicity of the organism is unknown. Neospora spp., including N. hughesi, cause myeloencephalitis rarely in horses.^5,6

EPIDEMIOLOGY

Equine protozoal myeloencephalitis (EPM) occurs in horses and ponies in Canada, United States, Central America, and Brazil. Reports of neurological disease in horses with antibodies to S. neurona in France have yet to be confirmed, but might represent cases of EPM in native horses outside of the Americas.⁷ The disease is reported in other countries in only horses imported from the Americas.⁸ Distribution of the disease appears to correlate with the range of the definitive host, Didelphis virginiana in North America, or the related species D. marsupialis and D. albiventris in South America.⁹ The disease has not been reported in donkeys and mules. Neurological disease associated with S. neurona has been reported in armadillos, sea otters, harbor seals, skunks, raccoons, zebra, lynxes, and cats.⁹

The disease usually occurs sporadically in endemic areas, although epidemics on individual farms are reported.¹⁰ The incidence of EPM is estimated to be 14 new cases per 10 000 horses per year.¹¹ The case fatality rate is approximately 7%, although up to 14% of horses are sold or given away because they are affected by EPM.¹¹ Approximately 40% of horses recover completely and another 37% improve but do not recover from the disease.¹¹ Another study reports that only 55% of horses with EPM examined at a referral hospital were alive a minimum of 3 years after diagnosis and treatment.¹²

Seroepidemiological studies, based on detection by Western immunoblot test of multiple antibodies to S. neurona in serum,¹³ indicates that 45–60% of horses in the United States are exposed to the agent but do not develop disease.^14-17

Vaccination with a product containing killed S. neurona induces a detectable antibody response in both serum and, in approximately 50% of horses, in the cerebrospinal fluid.¹⁸

PATHOGENESIS

Details of the pathogenesis of EPM are unknown. It is assumed that after infection, probably by ingestion, sporocysts excyst and release sporozoites which penetrate the gastrointestinal tract and enter endothelial cells. Subsequently, meronts (schizonts) develop which on maturation rupture and release merozoites. Schizonts are present in cells of the central nervous system, including neurons, glial cells, and intrathecal macrophages. Schizonts multiply in the infected cells, as evidenced by the presence of merozoites. Infection induces a non-suppurative inflammation, characterized by accumulations of lymphocytes, neutrophils, eosinophils, and gitter cells. Infection of neurons, and the associated inflammatory reaction, disrupt normal nervous function and contribute to the clinical signs of weakness, muscle atrophy, and deficits in proprioception.

Mechanisms permitting infection and proliferation of the organism have not been well defined. Horses with EPM have lesser cell-mediated immunity than do asymptomatic horses,³³ and the decrease in cell mediated immunity appears to be due to S. neurona suppressing immune responses to parasite-derived antigens.^34,35 However, foals with severe combined immunodeficiency administered S. neurona do not develop neurologic disease, despite prolonged parasitemia and infection of visceral organs by the organism, whereas immunocompetent horses do not have prolonged parasitemia but do develop neurologic disease.^36,37

CLINICAL FINDINGS

The incubation period after experimental infection of young horses ranges between 28 and 42 days,³ but is not known for the spontaneous disease. The clinical findings of EPM in horses are protean, and in endemic areas EPM should be considered as a diagnosis in any horse with clinical signs referable to the nervous system. S. neurona can infect any area of the brain and spinal cord, and may affect more than one site in an individual horse. Clinical signs of EPM range from barely perceptible changes in gait or behavior to recumbency, muscle atrophy, or seizures. The onset of signs may be insidious and gradual, or acute and rapidly progressive. Affected horses do not have increased temperature or heart rate, unless complications of the nervous disease occur.

Spinal ataxia, evident as weakness, hypometria or hypermetria, and defects in proprioception are common manifestations of EPM. Multifocal spinal or cervical disease causes all four limbs to be affected, while lesions caudal to the cervical intumescence cause signs in the rear limbs only. Signs of spinal ataxia range from subtle changes in gait which are difficult to differentiate from obscure lameness due to musculoskeletal disease, through obvious spinal ataxia evident as truncal sway, toe dragging, and circumduction of feet, to spontaneous falling and recumbency. Asymmetry of clinical signs, in which one limb is affected more than the contralateral limb, is highly suggestive of EPM, as cervical stenotic myelopathy and equine degenerative myelopathy usually cause symmetrical ataxia.

Lesions in the sacral cord cause signs of cauda equina syndrome, including tail paresis and urinary and fecal incontinence.

Lesions affecting spinal cord gray matter cause focal, asymmetric muscle atrophy, absent reflexes, or focal areas of sweating. Muscles frequently affected include the quadriceps, biceps femoris, epaxial muscles, and the supraspinatus/infraspinatus group. Equine protozoal myeloencephalitis can present as a brachial plexus injury evident as radial nerve paralysis.

Cranial nerve disease is a common manifestation of EPM. Common syndromes include:

EPM may also manifest as changes in personality and behavior, headshaking, and seizures.

CLINICAL PATHOLOGY

There are no characteristic changes in the hemogram or serum biochemical variables. Diagnosis has focused on the demonstration of antibodies to S. neurona in serum or CSF by western blot, indirect fluorescence testing, or IgM capture ELISA.^13,38-40

Interpretation of the results of western blot analysis of CSF for IgG antibodies to S. neurona is problematic because of the potential for blood contamination of the sample during collection,⁴¹ and the high sensitivity but low specificity of the test.¹³ Blood contamination of the sample is problematic in horses that are seropositive for antibodies to S. neurona and in which it is desired to know if antibodies are present in cerebrospinal fluid. Contamination of cerebrospinal fluid with blood can introduce antibodies from serum into the otherwise antibody free cerebrospinal fluid thereby causing a ‘false’ positive test.⁴¹ Contamination of cerebrospinal fluid with small quantities of blood with high concentrations of antibodies to S. neurona might not be detectable using red blood cell counts, albumin quotient, or immunoglobulin index, but could yield a positive result on western blot testing.⁴¹

The western blot test for detection of antibodies to S. neurona is sensitive (87%) but has poor specificity (44–60%).¹³ The utility of the test therefore depends on the pretest probability that the horse has EPM. The high sensitivity of the test means that the positive predictive value of a positive test in a horse with unequivocal signs of neurologic disease consistent with EPM, and from an area in which the disease is endemic, is very good.⁴² However, the positive predictive value of a positive test is very poor in horses with vague, or no, signs of nervous disease, or in horses from areas in which the disease is not endemic.⁴² The negative predictive value is good in either instance in that horses that test negative for the presence of antibodies are unlikely to have EPM.⁴² A negative result on a western blot analysis of serum or CSF, therefore, virtually eliminates the disease from the potential diagnoses, whereas a positive test in a horse with a high pretest probability of the disease contributes little to confirming the diagnosis. A positive test in a horse with a low probability of having the disease, such as presale testing of a clinically normal horse, does not mean the horse has the disease and is virtually useless in any assessment of the horse.

Foals of seropositive mares acquire antibodies, but not infection, by ingestion of colostrum from the dam.²⁹ These antibodies can be detected in both serum and cerebrospinal fluid of foals.^29,43 The mean time for foals to become seronegative for antibodies to S. neurona is 4.2 months.²⁹ Detection of antibodies to S. neurona in serum or cerebrospinal fluid of foals less than 4–6 months of age, even those with neurologic disease, should be interpreted with caution as the antibodies are likely derived from the dam.

An indirect fluorescent antibody test reliably detects antibodies to S. neurona in serum and cerebrospinal fluid of infected horses.^33,44 This test has the advantages of providing quantitative results, cheaper to perform, and is more accurate than immunoblots in the detection of antibodies. An IgM capture ELISA detects the presence of IgM antibodies to a S. neurona -specific antigen in serum and cerebrospinal fluid of naturally and experimentally infected horses, thereby providing evidence of recent infection.³⁹ Demonstration of recent infection in a horse with signs of neurologic disease increases the probability that the horse has EPM.

Examination of other variables in CSF is of limited use in the diagnosis of EPM, and measurement of creatine kinase activity in CSF has no diagnostic usefulness.⁴⁵ The use of the albumin quotient or IgG index to detect blood contamination of cerebrospinal fluid, or the intrathecal production of IgG is unreliable and not useful in the diagnosis of EPM.⁴⁶

NECROPSY

Lesions are limited to the spinal cord and brain, with the exception of neurogenic muscle atrophy. Gross lesions of hemorrhage and malacia may be visible in the central nervous system tissue. The lesions are asymmetrical, but may be more frequently encountered in the cervical and lumbar intumescences of the spinal cord. Histological examination reveals multifocal necrosis of the nervous tissue with an accompanying infiltration of macrophages, lymphocytes, neutrophils, and occasional eosinophils. This reaction is predominately non-suppurative and usually includes a degree of perivascular cuffing. Schizonts or free merozoites may be evident in tissues but are difficult to locate without immunohistochemical stains. The sensitivity of screening for the parasite in hematoxylin and eosin-stained sections of nervous tissue from cases with histologic changes suggestive of EPM was only 20%. The sensitivity improved to 51% when immunohistochemical staining of the tissue was employed.³² The same interpretative problems encountered when testing antemortem CSF samples apply when the fluid is collected at postmortem. Isolation in cell culture systems is possible but rarely attempted in diagnostic laboratories. PCR tests for these apicomplexan parasites can yield false negatives due to the random distribution of the parasite within CNS tissue.

TREATMENT

Specific treatment of EPM involves the administration of antiprotozoal drugs including ponazuril, diclazuril, nitazoxanide, or the combination of pyrimethamine and sulfadiazine. Ponazuril, an active metabolite of toltrazuril, is usually administered at a dosage of 5 mg/kg body weight orally once daily for 28 days. At this dosage, and at 10 mg/kg orally once daily for 28 days, administration of the drug results in resolution of clinical signs in approximately 60% of horses with EPM.⁴⁷ The initial dosage is 5 mg/kg q 24 h which is continued for 28 days if signs of improvement are evident after 14 days. If signs of improvement are not seen after 14 days, the dosage is increased to 10 mg/kg orally q 24 for 14 days.⁴⁸ Few adverse effects are noted, even at 30 mg/kg orally once daily for 28 days.⁴⁹

Nitazoxanide is administered at 25 mg/kg bodyweight orally for the first 5 days of treatment and then at 50 mg/kg orally for days 6–28 of treatment. Adverse effects noted include fever, anorexia, diarrhea, and worsening of clinical signs of neurologic disease.⁵⁰ Nitazoxanide is apparently effective in the treatment of EPM, based on a study of seven horses with the disease, but there are no reports of treatment of large numbers of horses.⁵⁰

Administration of the combination of sulfadiazine (or similar drug, 20 mg/kg, PO) and pyrimethamine (1–2 mg/kg, PO) every 24 hours given 1 hour before feeding is effective in approximately 60–70% of cases.⁹ This treatment is continued for at least 90 days if complete resolution of clinical abnormalities occurs, or longer if the signs of EPM do not resolve. Side-effects of the administration of a combination of a sulfonamide and pyrimethamine include enterocolitis, anemia, and abortion.¹⁰ Folic acid is often added to the diet of horses being treated for EPM, but this cannot be recommended because of its lack of efficacy in preventing anemia in treated horses,^10,51 and its ability to cause severe congenital abnormalities in foals born to treated mares⁵² and anemia and leucopenia in adult horses.⁵¹ Orally administered synthetic folates interfere with normal folate metabolism in horses being administered anti-folate drugs resulting, paradoxically, in folate deficiency.⁵¹ Adequate intake of folates in antiprotozoal-treated horses can be assured by feeding a diet containing good quality green foliage.

The decision to stop treatment in horses that do not completely recover is difficult. Some authorities recommend resampling CSF and continuing treatment until antibodies to S. neurona are no longer detectable. However, given that normal horses often have antibodies in their CSF, and that some treated horses never lose their positive western blot test, the decision to stop treatment should not be based entirely on this variable.

Some horses have a transient worsening of clinical signs in the first week of treatment. This is presumed to be due to the effect of the antiprotozoal agent causing death of protozoa with subsequent inflammation and further impairment of neurologic function. Relapse of the disease occurs in some horses when administration of antiprotozoal medication is stopped.

Supportive treatment of affected horses includes anti-inflammatory drugs (flunixin meglumine, 1 mg/kg IV, every 8–12 h; dimethyl sulfoxide, 1 g/kg as a 10% solution in isotonic saline IV, every 24 h for 3 days) and nutritional support for horses that cannot eat. Flunixin meglumine is often administered twice daily for the first 3–5 days of treatment with ponazuril or nitazoxanide, purportedly to reduce the inflammatory effects of death of protozoa in the central nervous system.

CONTROL

A vaccine composed of killed S. neurona is available in the United States. However, the efficacy of this vaccine has not been demonstrated. Furthermore, because the vaccine induces detectable antibody in serum and cerebrospinal fluid of vaccinated horses, there is concern that it can impair the diagnosis of EPM.¹⁸ Given the lack of demonstrated efficacy and potential for interference with diagnosis of EPM, use of the vaccine currently available in the United States (2005) is not recommended.

Sporocysts of S. neurona are resistant to usual concentrations of many of the conventional disinfectants including sodium hypochlorite (bleach), 2% chlorhexidine, 1% betadine, 5% benzyl chlorophenol, 13% phenol, 6% benzyl ammonium chloride, and 10% formalin.⁵³ The organism is killed by heating to 55°C for 15 min or 60°C for one minute.⁵³

Because protection of feed from contamination by opossums has been demonstrated to reduce the risk of horses developing EPM¹⁹ it is prudent to employ measures to reduce the exposure of animals and feed to opossum feces, and possibly feces of birds that might act as transport hosts.

There is interest in pharmacologic means of preventing infection of horses by S. neurona. Pyrantel pamoate has some efficacy against S. neurona in vitro but daily administration (2.6 mg/kg body weight in feed) does not prevent S. neurona infection of horses.⁵⁴

ETIOLOGY

Theileria parva is an apicomplex protozoan parasite of the class Sporozoasida and order Piroplasmida. There has been considerable naming and renaming of T. parva and the associated diseases in Africa.¹ ‘Classic’ East Coast Fever (ECF) occurs in East Africa and is associated with T. parva transmitted from cattle to cattle by the brown ear tick, Rhipicephalus appendiculatus. ECF also occurs either as Corridor disease in eastern and southern Africa or as January disease in central Africa. Corridor disease is transmitted from buffalo to cattle by either R. appendiculatus or R. zambeziensis and the agent responsible used to be called T. parva lawrencei. Close contact between buffalo, cattle, and ticks is essential. The disease is more acute than classical ECF, but after serial passage in cattle, it is indistinguishable from classical ECF.² January disease occurs mainly between January and March and the agent was named T. parva bovis. The disease is also more acute than classical ECF, death sometimes occurring within 4 days. These three clinical diseases are otherwise indistinguishable from one another, hence the causative agents are currently referred to simply as T. parva.

EPIDEMIOLOGY

PATHOGENESIS

Sporozoites of T. parva are injected into the bovine host by the tick in its saliva. Ticks must feed for 2–4 days before sporozoites in their salivary glands will mature and become infective to cattle. One tick can transmit sufficient sporozoites to cause a fatal infection in a susceptible animal. The sporozoites then enter lymphocytes and develop into schizonts in the lymph node draining the area of attachment of the tick, usually the parotid node. Infected lymphocytes are transformed to lymphoblasts which continue to divide synchronously with the schizonts so that each daughter cell is also infected. Eventually, infected lymphoblasts are disseminated throughout the lymphoid system and in nonlymphoid organs where they continue to proliferate. Later, some schizonts differentiate into merozoites, are released from the lymphoblasts and invade erythrocytes where they are referred to as piroplasms and are infective to ticks. Piroplasms ingested by ticks undergo several developmental stages and eventually form sporozoites in salivary glands, thus completing the cycle.

The dominating pathological lesion is generalized lymphoid proliferation resulting from uncontrolled proliferation of T-lymphocytes containing schizonts. This is followed later by necrosis of infected lymphoblasts induced by cytotoxic T-lymphocytes. The severe lymphocytolysis often leads to immunosuppression. Terminally, the animal develops severe pulmonary edema, probably due to release of vasoactive substances from lymphocytes disintegrating in the lungs. Erythrocytic indices are usually unchanged, but there may be terminal anemia in January disease.

CLINICAL FINDINGS

The basic syndrome associated with T. parva infection lasts for a few weeks. The incubation period is 1–3 weeks, depending on the virulence of the strain and the size of the infecting dose. Experimentally, the first clinical sign is enlargement of lymph nodes in the area draining the site of tick attachment (i.e. 8–16 days after attachment). One or 2 days later, there is fever, depression, anorexia, and a drop in milk in dairy animals. In later stages, there may be nasal and ocular discharges, dyspnea, generalized lymph node enlargement, and splenomegaly. In severe cases, diarrhea occurs, sometimes with dysentery, but usually only late in the course of the disease. Emaciation, weakness, and recumbency lead to death from asphyxia in 7–10 days. Terminally, there is often a frothy nasal discharge. Occasional cases of brain involvement occur and are characterized by circling, hence ‘turning sickness’ or cerebral theileriosis.

In southern Africa, cerebral theileriosis is associated with an aberrant form of T. taurotragi originating from the eland. There are localized nervous signs and convulsions, tremor, profuse salivation, and head pressing. Infection with the strain of T. parva (formerly T. parva lawrencei) responsible for Corridor disease causes a similar acute syndrome, with the additional lesion of keratitis and accompanying blepharospasm. ECF in Zimbabwe (formerly attributed to T. parva bovis) is generally slightly less virulent but is still frequently fatal.

CLINICAL PATHOLOGY

The parasites are evident as schizonts, sometimes in circulating lymphocytes, but mainly in biopsy smears of enlarged lymph nodes stained with Giemsa. Piroplasms are also easily visible in erythrocytes from day 16 after tick attachment and they increase in number until death. Over 30% of the red cells may be infected, but the level of intra-erythrocytic piroplasms is not correlated with the severity of the disease. T. parva piroplasms are difficult to differentiate from other piroplasms, hence the necessity to find schizonts. Blood counts will reveal a panleukopenia and thrombocytopenia with little or no anemia. The protozoa can be grown on a tissue culture of lymphoblastoid cells.

A range of serological tests is available, including indirect immunofluorescent antibody test (IFAT), complement fixation test, indirect hemagglutination test, and enzyme-linked immunosorbent assay (ELISA). The ELISA test is increasingly being used for seroepidemiological studies¹⁴ and the polymerase chain reaction (PCR) technology is available but IFAT is the most widely used test.³

NECROPSY FINDINGS

The most striking lesion is massive pulmonary edema, hyperemia and emphysema, along with hydrothorax and hydropericardium. Copious froth is present in the airways. The carcass is emaciated and hemorrhages are evident in a variety of tissues and organs. There is enlargement of the liver, lymph nodes and spleen, and ulceration of abomasum and intestines. Small lymphoid nodules (the so-called pseudo-infarcts) are present in liver, kidney, and alimentary track. In protracted cases, animals may have small, exhausted lymphoid organs.

Microscopic lesions are characterized by proliferating lymphoblastoid cells and varying amounts of necrosis in lymphoid organs, lungs, liver, kidneys, the gastrointestinal tract and other tissues, somewhat similar to a multicentric lymphoid tumor. Some lymphoblasts contain schizonts, which are better seen in impression smears stained with Giemsa stain. In cerebral theileriosis, infected lymphoblasts sequester in cerebral blood vessels and cause infarction.

Specimens to submit for pathology should include lymph nodes, lungs, kidneys, liver, and any other organ with gross lesions.

TREATMENT

Once an animal is manifesting clinical signs of ECF, treatment is generally considered to be either unsatisfactory or too expensive.¹⁵ Tetracyclines were the recommended treatment for many years, but they have only moderate efficacy, especially if the disease has been present for a few days. Two recently introduced drugs, halofuginone lactate and parvaquone, have had a much higher success rate, but recovered animals may become carriers unless the correct dose is used. Halofuginone lactate is an effective oral treatment for the acute syndrome at two doses, 1.2 mg/kg BW. Parvaquone (10 mg/kg BW, two doses 48 h apart) or the related buparvaquone (2.5 mg/kg BW, two doses 48 h apart) given IM is effective in most cases. In field trials, buparvaquone gives results comparable to those of parvaquone¹⁶ and cure rates are maximized by accurate diagnosis and prompt treatment of both ECF and intercurrent infections.¹⁷ Cure rates are even higher if the animals are also treated for pulmonary edema with dexamethasone¹⁸ or the diuretic, frusemide.^19,20

CONTROL

Until recently, the main method of control of ECF was to break the transmission cycle between cattle and ticks. This was achieved through widespread and strict application of acaricides at 3-, 5-, or 7-day intervals throughout the year (intensive dipping), adherence to legislation on cattle movements and quarantine, and good livestock and pasture management. With the ever rising costs of acaricides, their effect on the environment, the development of acaricide resistance, and frequent political problems in the affected regions, this strategy to control ECF and other tick-borne diseases in Africa has been revised.^21-23 Furthermore, it has been observed that indigenous cattle, constituting the majority of the herds in some of the affected countries, may lose their endemic stability with intensive dipping²⁴ and the process is not cost-effective. An integrated approach is now advocated involving the use of genetically resistant breeds, a judicious and selective application of acaricides at 3-week intervals (strategic dipping) or when there are at least 100 ticks per animal (tactical dipping), and the use of vaccines. It was reported that monthly applications of deltamethrin-based pour-on insecticide significantly reduced the incidence of ECF and other hemoparasitic diseases in smallholder dairy farms in Kenya.²⁵

The technique used for vaccination is immunotherapy or ‘infection-and-treatment method’. Initially, cryopreserved suspensions of T. parva sporozoites from ground-up infected ticks were injected into the patient. Now, sporozoites from cell culture are used. The infection they cause is controlled with long-acting oxytetracycline (20 mg/kg BW IM), or preferably parvaquone given at the same time, so that premunity is established. It is preferably to use a cocktail of different stocks of parasites. Vaccination, coupled with strategic dipping only when ticks are abundant, is usually successful and economically attractive,²³ provided local stocks of Theileria are included. Large-scale field trials of these vaccines are being carried out throughout East and Central Africa. Initial reports indicate that calves in high risk areas should be vaccinated at 1–2 months of age²⁶; that immunization campaigns are more efficient when concentrated in the period of low adult tick activity²⁷; and that immunization is of no benefit in herds under intensive tick control but is of high value when combined with strategic tick control.²⁸ Strategic control plus immunization can markedly reduce the risk of clinical ECF but immunized animals are carriers and all stages of R. appendiculatus can transmit infection from them to naïve animals.²⁹

Limited studies have indicated that cattle could be successfully immunized without concurrent tetracycline therapy by using low pathogenicity isolates as vaccines, for example, T. parva (Boleni),³⁰ or low infectivity sporozoite stabilates stored at –196°C for over 6 months.³¹ Because of the high cost of tetracyclines, this procedure would reduce the cost of vaccination by more than three-fold in the first year of field application. Furthermore, the T. parva (Boleni) isolate was reported to induce protection against a wide spectrum of Theileria stocks.

It needs to be stated that immunity is engendered so far only with live parasites that can establish an infection, but can also produce carriers. Hence, the risk inherent in the widespread use of such vaccines across national boundaries warrants further consideration. On the other hand, this process may accelerate progress to endemicity.³² The possibility of immunizing cattle with recombinant surface molecules from either the sporozoite (the p67 antigen) or the schizont, or a mixture of several antigens derived from both stages, is still being investigated.^33,34 Such a recombinant vaccine would avoid the breakdowns that occur with any immunotherapeutic technique and if the right antigens are found for the vaccine, the immunity engendered is likely to be broad, robust, and not parasite stock-specific.^33,35,36

ETIOLOGY

Theileria annulata is a member of the apicomplex group, like T. parva the cause of east coast fever. It is highly virulent for European dairy cattle. Infection in local zebu cattle is often subclinical.

EPIDEMIOLOGY

PATHOGENESIS

The life cycle of T. annulata is cattle-tick-cattle as for T. parva but unlike T. parva, the sporozoites of T. annulata invade and form schizonts mostly in macrophages/monocytes that express major histocompatibility (MHC) class II antigens, rather than in lymphocytes.⁴ Schizont infected cells multiply in the draining lymph nodes and disseminate rapidly throughout the lymphoid tissues and in non-lymphoid organs including the liver, kidney, lung, abomasum, and brain. Later, schizonts differentiate into merozoites and invade erythrocytes (as piroplasms). The pathogenesis therefore involves proliferation of lymphocytes and macrophages induced by schizonts and anemia with icterus induced mostly by the piroplasms. The lymphoproliferation is controlled by suppressor macrophages as a protective mechanism leading to recovery.⁵ Over 90% of erythrocytes may be parasitized, each by one or more merozoites. Immunosuppression may occur in the acute phase of lymphoproliferation, but is generally less marked than in ECF,³ probably because leukocyte numbers return to normal soon after the acute phase.

CLINICAL FINDINGS

In a stable endemic situation, there may be only mild or no clinical disease in local zebu cattle.⁴ Clinical signs are acute and severe in exotic cattle and less severe in crossbreeds, and are similar to those in ECF. However, the course is longer in tropical theileriosis and may last for weeks before death. Clinical signs include marked fever, swelling of superficial lymph nodes, inappetence, tachycardia, dyspnea, pale mucous membranes and icterus. Others are diarrhea, weight loss, convulsions, torticollis and other nervous signs.^6,7 In chronic cases, there may be small subcutaneous nodules from which schizonts can be demonstrated in smears.⁸

CLINICAL PATHOLOGY

As with ECF, examination of smears of blood and lymph node biopsy will reveal piroplasms in erythrocytes and schizonts in lymphocytes. Schizonts of T. annulata tend to be more common in the liver than in lymph node smears, but are otherwise indistinguishable from those of T. parva. Furthermore, the piroplasms are predominantly round and oval, as opposed to T. parva which has comma- and rod-shaped piroplasms. Anemia is a significant feature of tropical theileriosis, unlike in ECF, and is associated with bilirubinemia, hemoglobinuria, and bilirubinuria. The anemia results from destruction of erythrocytes containing piroplasms but other factors may include autoimmune hemolysis and poor bone marrow response. Reduction in white cell and platelet counts is less severe than in ECF, but animals dying from the disease show persistent and severe lymphocytopenia involving mainly T-lymphocytes.⁶

The most commonly used serological diagnostic technique is the indirect fluorescent antibody test. For surveys, an indirect enzyme-linked immunosorbent assay (ELISA) test using a recombinant T. annulata surface protein has been described.⁹ The ELISA tests provide higher sensitivity and specificity than IFAT.¹⁰ Carriers can be detected by the polymerase chain reaction (PCR),¹¹ a test that can also be used to detect infected ticks.

NECROPSY FINDINGS

Apart from pallor of mucous membranes and yellowish discoloration of tissues, the postmortem lesions in animals dying from tropical theileriosis are similar to those of ECF. Liver, spleen and lymph nodes should be submitted for laboratory examination to detect schizonts whereas merozoites are detected in blood smears.

TREATMENT

Buparvaquone is the most effective agent available, and the recommended dose is 2.5 mg/kg BW.¹² In calves, supportive treatment for anemia is indicated. Halofuginone at 1.2 mg/kg is also effective but tetracycline at 20 mg/kg is less so.

CONTROL

Indigenous cattle live with the disease and do not require any intensive tick control or treatment. For valuable exotic stock or their crossbreeds, vaccination and strategic tick control are recommended. Vaccines can be made from either the sporozoite or the schizont. The sporozoite vaccine is based on the infection-and-treatment method using schizont-infected cell lines and simultaneous tetracycline treatment as for T. parva. It has been suggested that the most economical way to control theileriosis in India is to vaccinate calves and to reserve buparvaquone for treating clinical cases.¹³ The schizont vaccine was formerly blood containing a mild strain of the parasite. The newer vaccines are prepared from live schizonts grown in lymphoid cell culture and attenuated by prolonged passage. They cause virtually no adverse reactions and vaccinated cattle show good resistance to the disease for at least 3.5 years. Therefore, it is necessary to revaccinate, preferably with a different cell line vaccine, if tick population is too low to establish endemic stability. The risk for spread of the vaccine strains in the field is very low.¹⁴

ETIOLOGY

Trypanosoma vivax, T. congolense, T. brucei brucei and T. simiae are the four main species responsible for African trypanosomoses affecting virtually all domestic mammals. T. vivax and T. congolense are the main pathogens of cattle. The four species are members of the Salivaria group of trypanosomes and are transmitted cyclically via the mouthparts of tsetse flies, hence the name salivarian trypanosomes. T. vivax is usually numerous in bovine blood, and can be identified by its very fast movement in wet films. In stained smears, it is a long, slender, monomorphic parasite with a rounded posterior end, a terminal kinetoplast, and a long free flagellum, but no prominent undulating membrane. T. congolense is smaller, sluggish in wet films, and often adheres to red blood cells by the anterior end. In stained smears, it is a short parasite with a marginal kinetoplast, no free flagellum, and no prominent undulating membrane. T. brucei is large like T. vivax, but its rapid movement is in confined areas of the wet film. In stained smears, it is pleomorphic and may occur as long and slender forms, intermediate forms, or short and stumpy forms. The slender and intermediate forms have a long free flagellum, pointed posterior end, subterminal kinetoplast and a prominent undulating membrane, whereas the stumpy forms resemble T. congolense, but are bigger and have a prominent undulating membrane. T. simiae is morphologically indistinguishable from T. congolense, but the organism will be swarming in the blood of pigs.

EPIDEMIOLOGY

PATHOGENESIS

Nagana in most species is a progressive, but not always fatal disease and the main features are anemia, tissue damage, and immunosuppression. Metacyclic trypanosomes are inoculated intradermally as the fly feeds. They multiply at this site provoking a local skin reaction (chancre), which is most pronounced in a fully susceptible host and may be slight or absent with some strains or species of trypanosomes. Within the chancre, metacyclic parasites change to trypomastigote form, enter the bloodstream directly or through the lymphatics, and initiate characteristic intermittent parasitemias. Their behavior thereafter depends largely on the species of trypanosome transmitted and the host. T. vivax usually multiplies rapidly in the blood of cattle, sheep and goats, and is evenly dispersed throughout the cardiovascular system, whereas T. congolense tends to be aggregated in small blood vessels and capillaries of the heart, brain, and skeletal muscle, and rarely causes heavy parasitemias in ruminants. Both species exert their effect mainly by causing severe anemia and mild to moderate organ damage. The anemia has a complex pathogenesis involving mainly increased erythrophagocytosis, some hemolysis, and dyshemopoiesis. Very acute infections with T. vivax in cattle or T. simiae in pigs result in fulminating parasitemias and disseminated intravascular coagulation with hemorrhages. Such syndromes resemble a septicemia, and anemia may not be severe.

T. brucei and, rarely, T. vivax, have the added capability of escaping from capillaries into interstitial tissues and serous cavities where they continue to multiply. Such infections result in more severe organ damage in horses, sheep, and goats, in addition to anemia. The cerebrospinal fluid is often invaded by T. brucei alone^12,13 or mixed with other species, or as a relapse after an apparently successful treatment.^12,14

Animals chronically infected with any pathogenic trypanosome may develop concurrent and even fatal bacterial, viral, and other protozoan infections as a result of immunosuppression. Pregnant animals may abort, and transplacental fetal infections occasionally occur. Trypanotolerant animals control parasitemias better and have less severe anemia and organ damage. They usually recover from the disease, but may act as carriers.

CLINICAL FINDINGS

There are no pathognomonic signs that would help in pinpointing a diagnosis. The general clinical picture is as follows but there are many variations determined by the level of tsetse challenge, the species and strain of the trypanosome, and the breed and management of the host. Acute episodes last for a few days to a few weeks from which the animal dies or lapses into a subacute to chronic stage, or the illness may be chronic from the beginning. Chronic cases may run a steady course, may be interrupted by periodic incidents of severe illness, or undergo spontaneous recovery.

The basic clinical syndrome appears after an incubation period of 8–20 days. There is fever, which is likely to be intermittent and to last for a long period. Affected animals are dull, anorexic, apathetic, have a watery ocular discharge, and lose condition. Superficial lymph nodes become visibly swollen, mucous membranes are pale, diarrhea occasionally occurs, and some animals have edema of the throat and underline. Estrus cycles become irregular, pregnant animals may abort, and semen quality progressively deteriorates.^15,16 The animal becomes very emaciated and cachectic and dies within 2–4 months or longer. Thin, rough-coated, anemic, lethargic cattle with generalized lymph node enlargement are said to have a ‘fly-struck’ appearance.¹⁷

In general, T. congolense is more pathogenic to cattle in eastern and southern Africa, whereas T. vivax produces a more serious disease in West Africa. However, severe outbreaks of T. vivax involving exotic dairy animals in East Africa occur; affected animals show mucosal petechiation, rhinorrhagia, dysentery, and death after an illness of only a few weeks. Mixed infections are common and are usually more severe. Furthermore, intercurrent bacterial, viral, or other parasitic infections may mask or complicate the basic clinical syndrome. Immune response to bacterial, and some viral, vaccines is also depressed but is restored if trypanocidal therapy is given at the time of vaccination.¹⁸

Clinical findings peculiar to the individual trypanosome are as follows:

CLINICAL PATHOLOGY

The anemia results in a progressive drop in packed cell volume, a non-specific but useful indicator in endemic areas. The classic method of confirming diagnosis is to demonstrate parasites in a wet blood film, and in a thin or thick blood smear stained with Giemsa. This is easiest in the early stages of the disease when parasitemic peaks correspond with fever. As the disease progresses, parasitemias become infrequent and the intervals between peaks grow longer, even though the animal is still sick. To increase the accuracy of parasitological diagnosis, it is now routine to concentrate the parasites in the buffy coat layer of a microhematocrit capillary tube. The buffy layer is then examined directly at low power (Woo’s method) or in a wet preparation with a dark ground/phase contrast microscope (Murray’s method). Both tests are simple, sensitive, and applicable to field use on individual animals as well as in herds. Blood should be examined fresh but may be refrigerated for up to 24 hours, beyond which most parasites will die and disappear from the sample.²²

In chronic cases, blood can also be inoculated into experimental animals, usually rodents, but this is cumbersome and is accurate for only T. brucei, and possibly T. congolense, but not T. vivax.

Another alternative is a series of standard serological tests to detect antitrypanosome antibodies in sera or other body fluids. The three tests used most often are the indirect immunofluorescent antibody test (IFAT), the capillary agglutination test (CAT), and the ELISA. These tests have the disadvantage that they indicate past as well as present infections, are difficult to standardize for different laboratories, and are not species-specific. The ELISA technique was modified to detect circulating trypanosome antigens (antigen-ELISA) using monoclonal antibodies that would distinguish between T. vivax, T. congolense, and T. brucei, and would detect only current or very recent infections.²³ Extensive field trials in Africa and the Caribbean have shown that the test is not sufficiently sensitive or specific and that it needs to be combined with other parasitological methods to provide reliable results.²⁴

The polymerase chain reaction (PCR) technique can be used to detect trypanosome DNA in tsetse tissues and in blood.²⁵ The test is sensitive and species specific and can be combined with ELISA.²⁶ Under experimental conditions, it can also be used to monitor therapy, especially relapses, since treated animals should become PCR negative 1–2 days after treatment.²⁷ It is also possible to do retrospective epidemiological survey using serum banks.²⁸ Dried blood spots on filter papers are also a useful source of DNA for the detection of T. congolense and T. brucei by PCR.²⁹ But the test is expensive and can only be done in specialized laboratories.

NECROPSY FINDINGS

TREATMENT

The number of trypanocidal drugs available for treating and preventing infections in endemic areas is limited, and about 6 million doses are administered yearly in Africa.³¹ The drugs have been in the market for over 30 years, their range of therapeutic safety is small, many of them cause severe local reactions especially in horses, and some may be fatal in high doses. Furthermore, as drugs are not always available and they are expensive, underdosing is common. These, plus the fact that some drugs are used both prophylactically and therapeutically, have led to cases of drug resistance.

Ideally, each country or region establishes a group of sanative drugs which are to be used only as a break in a course of one of the more common drugs. The sanative drug should provide moderate prophylaxis and avoid the development of resistance to the prime drug. These measures have not been well-executed in many countries and this may explain the increasing reports of multiple resistance to curative, sanative, and prophylactic drugs.^32,33 Nevertheless, the prevalence of drug-resistant trypanosomes in the field still remains unclear as there are no suitable methods for the detection of resistant field isolates.³⁴ For example, drug resistance has been reported to be very low in eastern Zambia³⁵ but widespread in Burkina Faso³⁶ and Ethiopia.³⁷

Strains are regarded as resistant when they fail to respond to the drug or when they relapse some time after an apparent cure. Relapses are more likely to occur if the commencement of treatment is delayed or the dose rate is inadequate. However, in field situations, there is hardly any regular monitoring of drug efficacy and animals may be reinfected with the same or other species of trypanosomes soon after an otherwise effective cure.

The common drugs in use against trypanosomes are set out below. The specific dose rates vary with animal species, the specific trypanosome, and the specific purpose (curative, prophylactic, or sanative).^31,38

CONTROL

The control of trypanosomosis in enzootic countries involves control of tsetse fly population, prophylactic treatment, good husbandry of animals at risk, and use of trypanotolerant animals. There is no vaccine against the disease and, in spite of intensive research, none appears likely in the near future because of the ability of trypanosomes to readily change their glycoprotein surface coat through a process called antigenic variation.

Control of tsetse has been successfully attempted in some African countries, but reinvasion is frequent if the land is not properly utilized. The earliest methods involved bush clearing and elimination of game animals on which tsetse feed. These methods were effective in eradicating or controlling tsetse in some parts of the continent, especially in southern Africa, but they resulted in destroying valuable plant and animal resources, and also led to soil erosion. More recent methods involved the use of insecticides, especially DDT and endosulfan, applied strategically in the form of ground and aerial spraying over large expanses of land. As tsetse are sensitive to insecticides and no resistance has developed, considerable successes were achieved in some countries. However, spraying insecticides is costly and harmful to the environment. These harmful effects are considerably reduced if the insecticides, for example, synthetic pyrethroids, are applied directly on the animal in the form of spray or pour-on formulation. The latter offers great promise⁴⁰ and also reduces tick infestation in treated animals.

Other effective methods involve targets impregnated with insecticides and traps that attract and catch tsetse. These are simple and cheap and can be constructed and maintained by local communities.⁴¹ Furthermore, they do not pollute the environment and are suitable for both small- and large-scale farming. They have been used to reduce tsetse fly population by over 97% within 7 months in a community in the Congo⁴² and to greatly reduce the incidence of trypanosomosis in another community in south-western Kenya over a 12-year period.⁴³ Another method is the sterile male technique. Since the female tsetse only mates once in a lifetime, this technique is theoretically able to eradicate a targeted tsetse species in areas where other methods have been used to reduce its density. But it is expensive. Finally, it should be stated that development of the land for agriculture, industries, highways, etc. will effectively destroy the habitat for tsetse flies. This has occurred significantly in Nigeria where there were rapid economic activities and expanding human population in the 1970s and 1980s.⁴⁴

Attempts at trypanosomosis control have also been directed to prophylactic dosing with chemicals such as suramin, prothridium, and isometamidium (Samorin). Prophylaxis is used along with other methods in areas where there is a heavy tsetse challenge. The prophylactic effect is supplemented by the development of antibodies, and the total period of protection may be as long as 5 months. However, it is customary to give four or five treatments per year. The productivity response to this pattern of treatment is good if general husbandry is also adequate. The downside of this approach is that it has reportedly led to drug resistance in many countries.

Trypanotolerant animals are being used to establish ranches in areas where tsetse challenge is not too heavy, but they have not been readily accepted in some countries, supposedly because they are smaller in size and they produce less milk than other indigenous breeds and crosses with exotic breeds.

For effective control of trypanosomosis in Africa and in Central and South America, an integrated approach will mostly likely produce the desired results in each region. In the absence of a vaccine, control methods must combine reduced exposure to the vectors (large scale tsetse trapping and pour-on applications) with strategic treatment of exposed animals (chemotherapy and chemoprophylaxis) along with use of trypanotolerant animals when feasible.

ETIOLOGY

Trypanosoma evansi, the first pathogenic trypanosome to be identified in 1880 in India, belongs to the brucei group (subgenus Trypanozoon) but is not capable of cyclical development in tsetse Glossina spp. In blood smears, T. evansi is morphologically indistinguishable from T. brucei, but at the molecular level, the structure of the kinetoplast DNA of T. evansi is different.¹ T. equinum in South America is now accepted as a dyskinetoplastic variant of T. evansi rather than a separate species.

EPIDEMIOLOGY

PATHOGENESIS

Trypanosomes are inoculated into the host from the contaminated mouthparts of biting insects or the saliva of vampire bats. Parasite multiplication in the blood and body fluids causes inflammatory changes and anemia just like T. brucei.⁶ The parasite frequently localizes extravascularly in tissues including the central nervous system where it is less exposed to chemotherapeutic agents.

CLINICAL FINDINGS

The main clinical findings are intermittent fever, progressive anemia, edema of dependent parts of the body, dullness, listlessness, loss of body condition despite a good appetite, nasal and ocular discharge, and terminal nervous signs, including paraplegia, paralysis, delirium, and convulsion.^1-4 Surra is invariably fatal in camels and horses, death occurring within a few days or a few months, but camels may exhibit chronic signs for years. These signs include a reduction in milk yield and capacity for work, and a high abortion rate in pregnant females.⁷ Cattle and buffalo in endemic areas usually have mild infections which may be exacerbated by stress from adverse climatic conditions, work, or intercurrent disease. Signs may include irregular estrus, abortion, and stillbirth in cows, and poor semen quality in bulls. Outbreaks of a more severe disease in indigenous zebu cattle have been reported in Thailand, characterized by nervous signs and high mortality rate; the signs included circling, excitation, jumping, aggressive behavior, lateral recumbency, convulsion, and finally death.⁸

CLINICAL PATHOLOGY

As with T. brucei, parasite detection including the buffy coat method is easier in the acute phase. In the chronic phase, repeated sampling for some days may be required. In addition, suspected blood samples may be inoculated into rats or mice both of which are highly susceptible. A number of non-specific serological tests can also be used but are more reliable in areas where other forms of trypanosomes do not exist. These include mercuric chloride, formol gel, or stilbamidine test for increased serum protein levels. Specific antibody detection tests are also available, but await large-scale evaluation and standardization.⁹ They include direct card agglutination test (CAAT) for antibodies, the latex agglutination test (Suratex) for circulating antigens, the indirect fluorescent antibody test (IFAT), and the enzyme-linked immunosorbent assay (ELISA). In a study in Kenya, CAAT and Suratex were reported to be more sensitive than parasitological methods in revealing the true extent of surra in camel herds,¹⁰ contrary to an earlier study from Chad.¹¹ Techniques involving the polymerase chain reaction (PCR) have been described; they are more sensitive and specific, but too expensive for routine use.^4,12

NECROPSY FINDINGS

The carcass is emaciated and pale and may be icteric but, as in T. brucei infections, there are no pathognomonic gross and microscopic lesions. However, a lymphoplasmacytic infiltrate of various organs, including the brain and spinal cord, is characteristic. Parasites are detectable in body fluids if the carcass is fresh. Tissues for laboratory examination should include blood, brain, lymph nodes, spleen, and liver.

TREATMENT

Drugs used for treating nagana could be used for surra, but the outcome is less favorable owing to their low trypanocidal activity against T. evansi and their specific toxicity for camels and horses. Furthermore, the drugs are not able to cross the blood–brain barrier to reach parasites in the cerebrospinal fluid and nervous tissue. As a result, drug resistance readily occurs, especially if the accurate dose is not given.

Quinapyramine sulfate (quintrycide) is used curatively for camels, and diminazene aceturate (Berenil) for horses and camels. On the other hand, quinapyramine prosalt (trypacide), suramin (naganol) and isometamedium chloride (samorin or trypamidium) are used both curatively and prophylactically. The new drug is a water-soluble arsenical, RM110 or Cymerlasan (melarsomine), given SC at 0.3–0.6 mg/kg BW. It is as effective as berenil in infected camels.^7,13

CONTROL

Unlike in nagana, control measures are aimed primarily at the host rather than the vector, which is abundant. The measures include detection and treatment of infected animals, prophylactic treatment of susceptible animals, and their protection from biting flies and bats, where possible. As in nagana, there is no vaccine.

ETIOLOGY

Trypanosoma equiperdum belongs to the brucei group, subgenus Trypanozoon, but occurs only as long, slender, and monomorphic forms and is morphologically indistinguishable from T. evansi. It is the only pathogenic trypanosome that does not require an arthropod vector for its transmission and it resides more in extravascular tissue fluid than in blood.

EPIDEMIOLOGY

PATHOGENESIS

T. equiperdum shows a remarkable tropism for the mucosa of genital organs, the subcutaneous tissues, and the peripheral and central nervous systems. Trypanosomes deposited during coitus penetrate the intact genital mucosa, multiply locally in the extracellular tissue space, and produce an edematous swelling which may later undergo fibrosis. Subsequent systemic invasion occurs and localization in other tissues causes vascular injury and edema, manifested clinically by subcutaneous edema. Invasion of the peripheral nervous system and the spinal cord leads to incoordination and paralysis.

CLINICAL FINDINGS

The severity of the clinical syndrome varies depending on the strain of the trypanosome and the general health of the horse population. The disease in Africa and Asia is much more chronic than in South America or Europe and may persist for many years, often without clinical signs, although these may develop when the animals’ resistance is lowered by other disease or malnutrition.

The incubation period varies between 1 and 4 weeks, but could extend to more than 3 months in some animals. Initial signs may not be recognized until the breeding season. The ensuing disease will manifest genital signs in the primary stage, cutaneous signs in the secondary stage, and nervous signs in the tertiary stage.⁵

In the stallion, the initial signs are swelling and edema of the penis, scrotum, prepuce, and surrounding skin, extending as far forward as the chest. Paraphimosis may occur and inguinal lymph nodes are swollen. There is a moderate mucopurulent urethral discharge. In mares, the edema commences in the vulva and is accompanied by a profuse fluid discharge, hyperemia, and sometimes ulceration of the vaginal mucosa. The edema spreads to the perineum, udder, and abdominal floor. In Europe, the disease is more severe, genital tract involvement often being accompanied by sexual excitement and more severe swelling.

In the secondary stage, cutaneous urticaria-like plaques, 2–5 cm in diameter, develop on the body and neck and disappear within a few hours up to a few days. These so-called ‘silver dollar spots’ are pathognomonic for dourine but are not always present, and are uncommon in endemic areas. Succeeding crops of plaques may result in persistence of the cutaneous involvement for several weeks.

The tertiary stage is characterized by progressive anemia, emaciation, weakness, and nervous signs appearing at a variable time after genital involvement. Stiffness and weakness of the limbs are evident and incoordination develops, progressing terminally to ataxia and paralysis. Marked atrophy of the hindquarters is common and in all animals there is loss of condition, in some to the point where extreme emaciation necessitates destruction. Lack of coordination of the hind legs, swelling of the external genitalia, and emaciation were the most common clinical signs in horses suspected to have dourine in Ethiopia.³

CLINICAL PATHOLOGY

Trypanosome detection is difficult, but should be attempted in edema fluid, subcutaneous plaques, and vaginal or urethral washings or blood in early stages. Inoculation of blood into laboratory rodents is not as helpful as with other members of the brucei group. An efficient complement fixation test (CFT) is available and was the basis for a successful eradication program in Canada, but there are discrepancies in some of the CFT results.⁶ Other serological tests that can be used include the indirect fluorescent antibody test (IFAT), the capillary agglutination test for trypanosomes (CATT), and the enzyme linked immunosorbent assay (ELISA), but the CFT remains the most reliable.^5,7 Serological tests do not distinguish between members of the brucei group and hence they are of limited value in areas where T. brucei or T. evansi is endemic, even when monoclonal antibodies are used. The polymerase chain reaction (PCR) has been used to detect trypanosome DNA and is an indication of an active infection, unlike the CFT which detects past and current infections. But the PCR test cannot yet distinguish T. equiperdum from T. evansi. Furthermore, it has not been possible to isolate new strains of T. equiperdum from clinical cases that have appeared in various parts of the world since 1982.^6,8 Therefore, new internationally recognized tests for the diagnosis of dourine are needed urgently.

NECROPSY FINDINGS

Emaciation, anemia, and subcutaneous edema are always present, and edema of the external genitalia may be evident or the external genitalia may have healed, leaving the characteristic depigmented scars of permanent leukodermic patches.⁵ Lymph nodes are enlarged and there is softening of the spinal cord in the lumbosacral region.

Histological lesions consist of lymphoplasmacytic infiltration in the spinal nerves, ganglia, and meninges of the lumbar and sacral regions, and in affected skin and mucosa. Trypanosomes can be found in sections of the skin and genital mucosa during the primary and secondary phases of the infection. Affected lymph nodes show non-specific lymphoid hyperplasia.

TREATMENT

Many trypanocidal drugs have been used in the treatment of dourine, but results are variable, chronic cases in particular being unresponsive to treatment. Berenil (diminazene) at 7 mg/kg BW as a 5% solution injected IM, with a second injection of half the dose 24 hours later, or suramin (10 mg/kg IV for two to three treatments at weekly intervals), or quinapyramine sulfate (3–5 mg/kg in divided doses injected SC) have been tried. The main drawback is that treated animals may remain inapparent carriers.

CONTROL

In dourine-free countries, an embargo should be placed on the importation of horses from countries where the disease is endemic, unless the animals have been properly tested and found negative. Eradication on an area or herd basis is by the application of the CFT, along with strict control of breeding and movement of horses. Positive reactors are disposed of, and two negative tests not less than a month apart can be accepted as evidence that the disease is no longer present. Castration or neutering of infected animals is not adequate because mating can still occur.