08/11/2026
EPM, Equine Protozoal Myeloencephalitis, is one of the most recognized neurologic diseases in horses⦠and one of the most daunting diagnoses for horse owners across the country.
Horse owners, trainers, and breeders have become increasingly proactive when it comes to recognizing and managing EPM. Exposure to Sarcocystis neurona, the organism responsible for most cases, is incredibly common. Depending on geographic location, studies suggest a large percentage of U.S. horses have antibodies indicating exposure, yet only a very small percentage of exposed horses ever develop clinical EPM.
That distinction matters:
Exposure does not equal disease.
EPM is a potentially progressive and debilitating neurologic disease, but it is treatable, and early recognition and treatment can significantly improve the chance of a favorable outcome.
The opossum is the definitive host of S. neurona, which is why the geographic distribution of EPM largely follows areas where opossums are found. Opossums shed infective sporocysts in their f***s, contaminating feed, hay, pasture, or water. Horses become infected when they ingest those sporocysts.
In the small percentage of exposed horses that develop EPM, the organism ultimately reaches the central nervous system, where inflammation and damage can produce neurologic abnormalities.
EPM is not contagious from horse to horse.
Signs can vary tremendously depending on where the nervous system is affected. They may include:
⢠Asymmetric muscle loss or weakness
⢠Incoordination or ataxia
⢠Stumbling or dragging toes
⢠Changes in gait or performance
⢠Difficulty backing or navigating inclines
⢠Weakness that may be more noticeable on one side
⢠Changes in balance
⢠Less commonly, abnormalities involving the face, swallowing, or other cranial nerves
One of the frustrating things about EPM is that there isnāt one simple test that can definitively diagnose every case in a living horse.
Blood testing can demonstrate antibodies to S. neurona or Neospora hughesi, but because exposure is so common, a positive blood test alone does not mean a horse has EPM.
Diagnosis is generally based on a thorough neurologic examination, ruling out other causes of neurologic disease, and antibody testing of serum and/or cerebrospinal fluid (CSF). Comparing antibody levels between serum and CSF can provide stronger evidence that antibodies are being produced within the central nervous system.
Once EPM is suspected or diagnosed, veterinarians have several treatment options. FDA-approved antiprotozoal medications include ponazuril, diclazuril, and sulfadiazine/pyrimethamine. The medication and duration of treatment should be determined by the veterinarian based on the individual horse and clinical findings.
Additional supportive therapies may also be recommended depending on the case. Vitamin E is commonly incorporated into neurologic cases because of its important antioxidant role in nervous and muscular tissue, particularly when a horseās dietary intake or blood concentration is inadequate.
You may also hear about additional or alternative protocols being used in the field, but not every medication marketed or discussed for EPM has the same level of evidence or FDA approval. That is why treatment, and especially āpreventive treatmentā of an otherwise normal horse, is something worth discussing with your veterinarian rather than automatically putting every horse on the same protocol.
So what CAN we do proactively?
Start with the basics.
Support the immune system through appropriate nutrition, adequate Vitamin E, quality forage, parasite control, good biosecurity and management, and minimizing unnecessary stress where possible. Protect feed and water sources from wildlife contamination, keep feed securely stored, and discourage opossums from accessing barns, feeders, hay, and water.
But one of our biggest recommendations costs absolutely nothing:
Know your horseās normal.
Know how they normally walk.
Know how they back.
Know how they turn.
Know how they place their feet.
Know what their muscle symmetry looks like.
Know what their ānormalā responses to neurological tests.
Learning a few simple neurologic observations you can perform at home can help you recognize subtle changes sooner, not just with EPM, but with neurologic disease or injury in general.
Because with neurologic cases, the little things matter.
And noticing that something isnāt quite right may be the first step toward getting your veterinarian involved early and ensuring your horse does not have the lasting effects of EPM. We will be linking one of the best videos to show how you can check your horses if you want to know their normal. If you suspect anything neurological with your horses, get an appointment with your vet to further work through diagnosis and next steps.