Equine Protozoal Myeloencephalitis (EPM): Neurological Disease in Horses

EPM is a serious neurological disease in horses caused by protozoal parasites. It causes progressive neurological deficits and requires prompt diagnosis and treatment for welfare.

Key Facts

EPM is caused primarily by Sarcocystis neurona, a protozoan parasite transmitted via opossum feces contaminating horse feed and water.
EPM is most prevalent in North and South America, where the Virginia opossum (the definitive host) is native.
Clinical signs range from subtle gait abnormalities to severe incoordination, muscle atrophy, dysphagia, and facial nerve deficits.
Diagnosis is challenging — serum and CSF antibody tests are supportive but not definitive; clinical signs and response to treatment are important.
Treatment with antiprotozoal drugs (ponazuril, sulfadiazine/pyrimethamine) is effective if started early; neurological damage may persist.
Horses with severe ataxia are at risk of self-injury from falls — management during treatment requires careful housing and movement restriction.
Prevention focuses on eliminating opossum access to feed and water storage areas.

Welfare Considerations

EPM causes real suffering through progressive neurological impairment. Ataxic horses cannot move safely, are at risk of falling and self-injury, and may experience frustration from their inability to perform normal activities. Horses with advanced EPM may be unable to eat or drink normally if cranial nerve deficits affect swallowing. The disease can progress rapidly without treatment, making prompt veterinary evaluation critical. Treatment response is often good when initiated early, with many horses recovering significantly. However, horses with severe neurological damage may not recover to a degree compatible with quality of life or athletic use, requiring honest welfare assessment and potentially humane euthanasia. Pain management and supportive care are important during the treatment period.

What You Can Do

Store feed and water in sealed containers inaccessible to opossums to reduce EPM transmission risk.
Seek prompt veterinary assessment if your horse shows any signs of neurological dysfunction — early treatment gives the best prognosis.
Follow the prescribed antiprotozoal treatment course in full — premature cessation is associated with relapse.
Provide safe, restricted housing during treatment to prevent falls and self-injury in ataxic horses.
Ask your vet about anti-inflammatory support alongside antiprotozoal treatment to reduce neurological inflammation.
Monitor recovery carefully and be realistic about prognosis — some horses with severe damage will not recover to acceptable function.