Equine Protozoal Myeloencephalitis (EPM): Neurological Welfare

EPM is a debilitating neurological disease of horses caused by protozoal parasites, producing progressive incoordination and muscle weakness that profoundly affects welfare.

Key Facts

EPM is caused primarily by Sarcocystis neurona, a protozoan parasite spread through opossum faeces.
Opossums are the definitive host; horses are dead-end hosts that cannot spread the disease further.
Exposure is common in North America — up to 50% of horses test seropositive — but clinical disease is less common.
Clinical signs include asymmetric ataxia, muscle atrophy, weakness, and abnormal sweating patterns.
Severity ranges from subtle gait irregularities to severe incoordination and inability to rise.
Diagnosis combines clinical signs, serum and CSF antibody testing, and ruling out other neurological conditions.
Treatment with antiprotozoal drugs (ponazuril, diclazuril, pyrimethamine/sulphadiazine) is effective in many cases.
Recovery can be incomplete, leaving horses with residual neurological deficits.

Welfare Considerations

EPM causes progressive deterioration of the horse's ability to control its body — a distressing experience for an animal whose identity is bound to movement and physical capability. Severely affected horses may fall when turning, struggle to rise, and experience pain from muscle atrophy. The unpredictability of progression creates anxiety for horses and owners alike. Treatment is expensive and prolonged; horses that do not respond adequately may face euthanasia as their quality of life declines below acceptable thresholds. Prevention through feed and water protection from opossum contamination, prompt diagnosis, and early treatment initiation all improve welfare outcomes. Horses with residual deficits require careful management to prevent falls and injuries.

What You Can Do

Store feed in sealed containers that opossums cannot access and fence around water sources.
Know the signs of neurological disease in horses and seek prompt veterinary assessment.
Begin treatment early — outcomes are better when EPM is caught before severe deficits develop.
Work with your vet to monitor treatment response and adjust protocols as needed.
Assess residually affected horses for safe rideability and quality of life with professional input.
Support research into EPM prevention and treatment through equine health organisations.