1. Most horses in the United States will be exposed to the primary EPM-causing protozoa at some point in their lives.

“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.

2. Don’t rely on serology (blood test results) in a neurologically normal horse to diagnose EPM.

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.

3. Accurate diagnosis is challenging.

To get a definite EPM diagnosis, veterinarians must confirm:

• Neurologic signs consistent with EPM. “Think of the three ‘As’: asymmetry, atrophy (muscle wasting), and ataxia (incoordination),” Bianco said.
• Elimination of other differential diagnoses. These are plentiful, ranging from infectious diseases (EEE/WEE, WNV, EHV-1 myeloencephalitis, rabies) to lameness, Wobbler syndrome, and myopathies (muscle diseases).
• Evidence of intrathecal (within the cerebrospinal fluid, or CSF) antibody production against neurona and/or Neospora hughesi (another causative yet less common protozoa). Veterinarians can conduct two tests on the fluid: a SAG-2,3,4 ELISA and an IFAT.

4. We have effective, FDA-approved drugs to treat EPM.

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.

5. Final thoughts: EPM is likely overdiagnosed and overtreated.

“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.”