EPM (Equine Protozoal Myeloencephalitis)
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EPM (Equine Protozoal Myeloencephalitis)

by Laura Waitt, DVM, DACVIM (LAIM)
Feb 23, 2023

What is EPM?

Equine Protozoal Myeloencephalitis (EPM) is a challenging and frustrating neurologic disease of horses. Everything from clinical signs, to diagnosis, to treatment is variable. If you ask two people about EPM, you’ll get eight opinions and once you understand the disease this makes sense why. Many horses with suspected EPM are referred to an internal medicine specialist for initial diagnosis and treatment recommendations.


EPM is a confusing disease because many horses are exposed to the parasites Sarcocystis neurona and Neospora hughesi, which cause EPM, but not all horses get sick. The parasites are eaten and travel throughout the body before coming to rest in muscle or neurologic tissue. As such, the clinical signs seen depend on where the parasite ends up in the body. Signs vary from a sudden onset with rapid and severe progression to more gradual presentations with waxing and waning signs. EPM has been commonly associated with abnormalities in the nerves supplying the face causing an inability to swallow, muscle wasting, seizures, spinal ataxia (not knowing where feet are in space), but other presentations are also possible. The signs can be localized to one area of the neurologic system or can affect multiple areas such as both the brain and spinal cord. The signs are often asymmetric, which can help differentiate it from other commonly encountered neurologic diseases.

The source of the parasite is contamination of horse feed or water by infected opossum feces. A recent study of 5,000 healthy horses across the United States showed that 78% of the horses were seropositive to S. neurona, which means they were exposed to the parasite but had no clinical signs. Only 34% of those horses were positive for N. hughesi. It has not been determined why, if so many horses are exposed, so few develop neurologic disease. Stress is implicated but not conclusively proven. In America, the annual incidence of EPM is less than 1% demonstrating that exposure does not equal clinical disease.


Diagnosing EPM is complicated because there is not only one correct way to diagnose. The most accurate way involves comparing titers to the parasites that cause EPM in the cerebral spinal fluid and the blood and proving that antibody production is coming from the neurologic system. Blood titers alone, while less expensive and less invasive, can be positive if a horse is exposed, but EPM might not be the cause of their clinical issues. Additionally, it is important to rule out other causes of neurologic disease that can cause similar signs to EPM. It is vital, with this complicated disease, to get an accurate diagnosis before pursuing treatment.


Treatment can be frustrating because just over half of the horses treated for EPM respond to treatment. When they do respond, often, it is only a partial response. Additionally, in a smaller percentage of horses, neurologic signs recur again and again no matter what they are treated with. Making treatment even more difficult, there are only three FDA-approved medications for EPM treatment and they are expensive. Some of the newer drugs developed to treat EPM show some promise and with time may reach FDA approval.  The internist evaluating your horse, together with your primary care veterinarian, will discuss which of these medications is best suited for your horse and may suggest other ancillary treatment.


Currently, no vaccination is available to prevent EPM. The only vaccine developed was removed from the market because it demonstrated no efficacy. Research is underway into the periodic use of medications to “deworm” prone individuals to prevent the parasite from getting into the neurologic system.

EPM is a complicated and frustrating disease that does not have a clear prognosis with treatment. Work closely with your veterinarian to get your questions answered and have them reach out to their local internal medicine specialist for the most recent recommendations in the literature. 

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