Currently, the only method of control of EPM is early detection of neurologic abnormalities, diagnosis of the disease, and appropriate treatment. There are several approaches to the diagnosis of EPM; (1) clinical signs, (2) response to treatment, and (3) analysis of cerebrospinal fluid for the presence of anti-S. neurona antibodies, as well as cytologic analysis. Diagnosis by clinical signs alone is difficult, because clinical signs are commonly indistinguishable from other causes of spinal ataxia, weakness and lameness, including cervical stenotic myelopathy, equine degenerative myelopathy, equine herpes virus myelitis and equine motor neuron disease. Clinical signs should be useful in providing a level of suspicion for EPM, particularly when the case presentation is classic. The three "A's" of asymmetric ataxia and focal muscle atrophy are highly suggestive of EPM, and unlikely to be caused by other disease. However, most EPM cases are impossible to distinguish from other diseases, and therefore require further diagnostic tests.
About one third of horses with EPM show some response to treatment with pyrimethamine-sulfadiazine within the first 10-14 days of treatment. Therefore, response to treatment may be used as an initial approach to the diagnosis of EPM, but if the horse fails to respond within two weeks, additional diagnostics should be sought. Additionally, since the treatment is expensive, and usually lasts over six months, it is usually best to positively identify the horses that should be treated.
Cerebrospinal fluid (CSF) analysis is essential in diagnosis of most cases of EPM. The CSF may be taken from either the atlanto-occipital (A-O) space (behind the ears) or lumbosacral (L-S) space (in the back), but the L-S space is preferred. The CSF flows in a caudal direction (towards the tail) and most horses with EPM will be infected caudal to the A-O space. Therefore, sampling of CSF from the L-S space will be more likely to reflect inflammation and anti-S. neurona antibodies in horses with EPM. Cytologic analysis of CSF in EPM classically reveals increased total protein concentrations and increased white blood cell counts. However, the infection is often localized and small, and therefore is not reflected by abnormaities of the CSF. In a study of horses that died or were euthanatized because of severity of signs, only 71% of horses had abnormalities detectable in the CSF. Most horses with EPM have mild clinical signs, and would be unlikely to have abnormal CSF analyses. Production of antibodies against S. neurona, on the other hand, presumably will occur regardless of the size or location of the focus of infection.
Antibodies are detected by the Immunoblot test that was originally developed by Dr. Dave Granstrom at the University of Kentucky, and further improved and modified by Dr. Clara Fenger, formerly of Neogen Corporation. Antibodies may be present in the CSF either by leakage across the blood-brain barrier or by intrathecal production. Peripheral blood lymphocytes circulate through the CSF and are retained in the central nervous system if the antigen that they recognize is present. The presence of protozoa stimulates circulating lymphocytes that are specific for S. neurona to remain in the central nervous system, and produce antibodies locally. Identification of these antibodies in the CSF in the absence of blood contamination or breakdown of the blood-brain barrier is indicative of protozoa in the central nervous system. The sensitivity and specificity of this test, as originally developed, are about 89%.
Antibodies can also be detected by FIAX, a modified immunoflourescent antibody test. This test is performed using Sarcocystis cruzi bradyzoite antigen, and relies on cross- reacting antibodies. No relationship has been found between the FIAX test and the Western blot and most experts agree that the Western blot is the most accurate method of diagnosing EPM.
The modifications employed by Dr. Fenger to improve the test include using a different gel system that provides greater repeatability, and therefore accuracy of the test. Additionally, semi- quantitation has been added to the testing method to improve repeatability of the test. Analyses of the relative improvement of antibody quantity with treatment is also available now. This testing is available only at Neogen Corporation.
A DNA test was also developed for detecting possible protozoal infections in the central nervous system of horses. This test was developed by Dr. C. Fenger, and co-investigators, and this test detects the presence of parasite DNA in horse CSF. The advantage of the DNA polymerase chain reaction (PCR) test is that the analysis does not rely upon detection of the horse's immune response to the disease, but rather upon direct detection of the parasite in the CSF. This method appears to be more sensitive than the immunoblot in very early EPM infections, before the onset of a detectable immune response and in very chronic cases, where the immune response has waned. However, the immunoblot appears to be a better test in most other cases, because parasite DNA is difficult to detect during an aggressive immune response. In general, the PCR test is NOT valuable in the diagnosis of EPM (Fenger et al., in press, c). This test is only recommended when the clinical signs exhibited by the horse have been present for 3 weeks or less. It is probably more valuable to wait 2-3 weeks before testing the horse. More recently, studies have suggested that both false positive and false negative tests occur with the PCR test, which renders this test even less meaningful.