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EPM can look like many other neurologic or musculoskeletal diseases. In general, gait abnormalities associated with neurologic diseases such as EPM will be irregularly irregular.¹ Regularly irregular gait abnormalities are more suggestive of a primary musculoskeletal issue. The signs seen with EPM will depend on where the protozoal damage has occurred. Multifocal disease involving brain, brainstem and/or spinal cord; hindlimb deficits with no forelimb deficits; and asymmetrical signs (especially acute muscle atrophy) can all increase the index of suspicion for EPM but appropriate testing is still recommended to confirm the diagnosis.¹

1. Reed SM, Furr M, Howe DK, et. al., Equine Protozoal Myeloencephalitis: An Updated Consensus Statement with a Focus on Parasite Biology, Diagnosis, Treatment, and Prevention. J Vet Intern Med 2016;30:491–502.

To make the most accurate antemortem diagnosis, the following steps should be performed. First, a complete neurologic examination should show clinical signs consistent with EPM. Next, other potential causes of the signs should be ruled out (e.g. cervical radiographs). Finally, immunodiagnostic testing of serum and CSF should be performed to show intrathecal antbody production. Several independent studies have evaluated the sensitivity and specificity of the various EPM diagnostic tests. The SnSAG2,4/3 ELISA serum:CSF ratio (Equine Diagnostic Solutions, Lexington KY) has been shown to have the highest sensitivity while the IFAT (UC Davis) has the highest specificity. Both of those laboratories also offer Neospora hughesi testing. The SAG 1,5,6 offered by Pathogenes does not have any published sensitivity or specificity values.

Serum testing only shows presence of exposure to either Sarcocystis neurona or Neospora hughesi, not active infection of the central nervous system. The seroprevalence of these organisms in equine populations around the United States varies and does affect the positive predictive value of a positive serum test. In general, horses with true EPM infections will have a positive serum titer while negative serum titers make EPM very unlikely. In peracute cases, however, serum may initially be negative for antibodies so the diagnosis should not be ruled out.¹ EPM serology should only be performed on neurologically abnormal horses as interpreting titers in normal horses is not possible. At this time, comparison of antibody titers in the cerebrospinal fluid to those in serum remains the best ante-mortem diagnostic test. See EPM Diagnostic Flowchart.

1. Reed SM, Furr M, Howe DK, et. al., Equine Protozoal Myeloencephalitis: An Updated Consensus Statement with a Focus on Parasite Biology, Diagnosis, Treatment, and Prevention. J Vet Intern Med 2016;30:491–502.

This can be a difficult situation. As discussed, presence of antibodies in the blood stream does not indicate active infection of the central nervous system nor does it provide any predictive evidence that that horse will develop neurological signs in the future.

To best confirm an ante-mortem diagnosis of EPM (or many other neurologic diseases), cerebrospinal fluid collection is necessary. Fortunately there are three sites at which CSF collection is possible depending on the situation. Aspirates of the lumbosacral space or ultrasound-guided aspirates of the C1-C2 space are possible in the standing horse, while CSF can be collected from the atlanto-occipital space in the anesthetized, recumbent horse.

Spinal fluid should be submitted for cytology as well as specific EPM testing. Cytology will allow interpretation of red blood cell contamination which could alter interpretation of antibody testing. Most immunodiagnostic labs will also calculate the C-value or antigen-specific antibody test if there is any question as to intrathecal antibody production versus CSF contamination. Cytologic abnormalities are uncommon with EPM but are possible.¹ If EPM testing is negative, any cytologic abnormalities seen will help guide further differential diagnoses.

1. Reed SM, Neurology Is Not a Euphemism for Necropsy:A Review of Selected Neurological Diseases Affecting Horses. AAEP Proceedings MILNE Lecture, Vol. 54 2008;78-109.

Complete blood count and chemistry can be performed to diagnose or eliminate concurrent illnesses but do not show consistent abnormalities with EPM alone.

At any point in the diagnostic workup, consultation within a board certified large animal internal medicine specialist may be helpful. If you don’t know one in your region, you can use www.vetspecialists.com or call your local veterinary school. Documenting your neurologic exam including any pictures or videos can be helpful. If obtaining video, don’t walk with the camera and be sure to continue recording any moving exams until the horse comes to a complete stop. Be sure to have a complete vaccination history and any other relevant medical records. It is generally most helpful for the veterinarian to establish initial contact with the specialist before the horse owner makes an appointment.

There are three different FDA approved drugs currently available to treat EPM, Marquis (15% w/w ponazuril) Antiprotozal Oral Paste, Boehringer Ingelheim Animal Health USA Inc., Duluth, GA, Protazil® (1.56% diclazuril), Merck Animal Health, Madison, NJ, and ReBalance® (sulfadiazine/pyrimethamine Oral Suspension), PRN Pharmacal, Pensacola, FL. There are multiple non-FDA approved drugs which have not been through the same safety and efficacy studies as the three listed above. Ancillary treatment with anti-inflammatories, immune modulators, and supportive care may also be recommended.