Equine Protozoal Myeloencephalitis

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Last updated on
5 min read

Key takeaways

Equine protozoal myeloencephalitis (EPM) is a serious neurologic disease in horses caused by the infectious protozoan organism, Sarcocystic neurona (and rarely, Neospora hughesi)

  • Horses become infected with the protozoan by ingesting food or water contaminated with opossum feces containing infective sporocysts
  • While the onset of the disease can be sudden or insidious, symptoms typically include uncoordinated gait, lameness, muscle atrophy, weakness, difficulty swallowing, and seizures
  • Diagnosis is via physical and neurologic examination, bloodwork, and cerebrospinal fluid (CSF) analysis to rule out other diseases with similar, overlapping symptoms
  • Antiprotozoal treatment is required for treatment in addition to other supportive care (anti-inflammatories and antioxidants)
  • EPM can cause long term neurological effects if left untreated
  • Although recovery can be lengthy and relapse is possible, with prompt and aggressive diagnosis and treatment, prognosis is fair to good
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A closer look: Equine Protozoal Myeloencephalitis

Equine protozoal myeloencephalitis (EPM) is a very serious disease that can cause life-long debilitating neurological defects, and can be fatal if left untreated. EPM is one of the most common causes of neurologic diseases in horses. It is most common in areas where opossums reside, as they are responsible for carrying the EPM-causing parasite, S. neurona. With proper diagnosis and aggressive treatment, most horses can recover from the disease. Signs of neurologic disease always require emergency veterinary attention.

Risk factors

Several horses can become exposed to the parasite and combat the infection before clinical disease develops. Horses that undergo a stressful event following infection are more likely to succumb to clinical disease. Symptoms can either be sudden, or more insidious in onset. Symptoms can also vary depending on the area of central nervous system damage. For example, damage to the spinal cord may result in ataxia and gait abnormalities, whereas brain involvement may present as lethargy, facial paralysis, and seizures.

Possible causes

The cause of EPM is the protozoan parasite Sarcocystis neurona. Less commonly, the protozoan parasite Neospora hughesi can also result in a similar disease. Horses become infected with the parasite by ingesting food or water contaminated with sporocysts (egg-like infective stage) that are shed in opossum feces. Once ingested, the parasite migrates through the body and causes damage to the central nervous system, which results in the observable symptoms.

Other animals (cats, raccoons, skunks) can also carry the infectious sporocysts in their feces.

Main symptoms

Testing and diagnosis

There is no specific diagnostic test available for EPM in live horses; the only definitive test for EPM is by testing nervous tissues post-mortem. Horses that present with neurological disease undergo a full physical and neurologic examination. Additional testing to support the diagnosis includes:

  • Bloodwork
  • Cerebrospinal fluid analysis
  • Diagnostic imaging, such as X-rays
  • Testing for antibodies against the parasite in the blood

Steps to Recovery

Treatment includes the use of antiprotozoal drugs, as well as other supportive medications (anti-inflammatories and antioxidants such as vitamin E) to alleviate symptoms. Treatment is often lengthy and costly, and approximately 10-20% of horses can relapse. Euthanasia may be recommended for horses that quickly deteriorate or do not respond to treatment.

Some horses can be infected with the organism for months to years before symptoms are visible and a diagnosis is made. Following diagnosis, treatment can last between 1-6 months depending on the medications used and response to treatment.

The prognosis with treatment ultimately depends on the length of infection prior to treatment, the severity of symptoms, and the location of the central nervous system affected. Prompt treatment is key for the best prognosis. Most horses recover with aggressive treatment, although lasting neurological deficits following treatment and relapse are still possible in a small percentage of horses.


Broadly, EPM can be prevented by keeping animals out of food and water storage. Examples of these preventative measures include:

  • Closing feed rooms and containers
  • Cleaning up dropped grain
  • Feeding heat-treated, processed foods
  • Providing clean, fresh water to horses
  • Cleaning water tanks/buckets
  • Disposing of opossum and other animal carcasses away from the horse pasture

Maintaining optimal health in horses is also key as stressed horses are more likely to become symptomatic following exposure to S. neurona. Ways to maximize health include:

  • Regular veterinary exams
  • Proper nutrition
  • Regular exercise
  • Routine deworming and vaccinations
  • Minimizing stress (stressful events typically include shipping, showing, training, and pregnancy)

A vaccine against S. neurona is available, although its efficacy is unknown. Currently, other preventative measures are more strongly encouraged over vaccination.

EPM is not contagious between horses.

Is Equine Protozoal Myeloencephalitis common?

EPM is uncommon, but it is the most common cause for neurologic disease in horses.

Typical Treatment

  • Anti-protozoal
  • Anti-inflammatories
  • Antioxidants (vitamin E)


- Writing for American Association of Equine Practitioners
Amy Young - Writing for UC Davis Veterinary Medicine Center for Equine Health
Marcia King - Writing for The Horse
Courtnee Morton, DVM - Writing for PetMD

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