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Equine protozoal myeloencephalitis remains challenging to definitively diagnose. Here, a practitioner offers an update about the disease.

Posted by Lucile Vigouroux | Oct 12, 2021 | Diagnostics and Technology, Diseases and Conditions, Equine Protozoal Myeloencephalitis (EPM), Lameness, Neurologic Disease, Vet and Professional, Welfare and Industry

“If you took a blood sample from a random, healthy horse in the U.S., you’d be more likely to get a positive EPM result than a negative one,” Bianco said. And that’s exactly what researchers did, on a large scale. In a 2017 study James et al. found that 78% of 5,250 healthy horses nationwide had antibodies against the primary EPM-causing protozoa, Sarcocystis neurona, in their blood.

Luckily, the presence of antibodies does not equal disease. In most cases, exposure to S. neurona happens without escalating to EPM. “We truly don’t know how many of these seropositive horses actually develop EPM,” Bianco said. Veterinarians are also unsure what causes certain horses to develop the disease while most others appear unaffected. Suspected risk factors include preexisting immunosuppression or stress, increased exposure to opossum feces, and varying pathogenicity (ability to cause disease) of different S. neurona strains. With up to 84% of the U.S. equine population being seropositive but mostly unaffected by S. neurona exposure, Bianco emphasized serology’s inadequacy at diagnosing EPM. Rather, she recommended submitting blood from a neurologically normal horse only to rule out the protozoal disease.

To get a definite EPM diagnosis, veterinarians must confirm:

The goal of EPM treatment is to stop the progression of clinical disease. Veterinarians can eliminate the protozoa using one of the three FDA-approved drugs: Marquis (ponazuril), Protazil (diclazuril), and ReBalance (sulfadiazine and pyrimethamine). “All three products have comparable efficacy,” Bianco said. “But the challenge with any drug is penetrating the blood-brain barrier enough to reach therapeutic concentrations in the CSF.” In terms of supportive care, veterinarians can administer flunixin meglumine (Banamine) for its anti-inflammatory properties and vitamin E for neurogenic healing. Bianco said she also considers administering corticosteroids in cases of severe ataxia.

“A definite diagnosis is necessary before prescribing treatment,” Bianco said. In her professional opinion, EPM is likely overdiagnosed. She attributes this to owner demand for treatment and to practitioners’ lack of comfort conducting CSF taps in the field and consequent reliance on serology. “In addition to being financially costly, empiric (relying on practical experience) EPM treatment can lead to protozoal resistance and delay proper treatment if the true diagnosis is being missed,” Bianco said. “Getting to the root of the problem ensures that the horse receives the proper treatment in a timely manner, increasing its chances of a better outcome.”

Lucile Vigouroux holds a master’s degree in Equine Performance, Health, and Welfare from Nottingham Trent University (UK) and an equine veterinary assistant certification from AAEVT. She is a New-York-based freelance author with a passion for equine health and veterinary care. A Magnawave-certified practitioner, Lucile also runs a small equine PEMF therapy business. Her lifelong love of horses motivated her to adopt her college care horse, Claire, upon graduation.

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