Horse EPM: Welfare Management of Equine Protozoal Myeloencephalitis

Equine protozoal myeloencephalitis (EPM) is a neurological disease in horses caused by protozoan parasites. Early treatment and attentive welfare management are key to meaningful recovery.

Key Facts

EPM is caused most commonly by Sarcocystis neurona, transmitted through opossum feces contaminating feed and water
Approximately 50-60% of horses in North America have been exposed to S. neurona, but only a small proportion develop clinical disease
Clinical signs range from subtle asymmetry and ataxia to severe incoordination, muscle atrophy, and inability to rise
Diagnosis is challenging - CSF testing and IFAT serology support diagnosis but no test is definitive without post-mortem examination
FDA-approved treatments include ponazuril (Marquis), diclazuril (Protazil), and sulfadiazine/pyrimethamine combination
Treatment course is typically 28 days with ponazuril or diclazuril; many horses require longer courses for significant improvement
Approximately 60-70% of treated horses show improvement; complete recovery depends on severity and duration before treatment
Relapse occurs in 10-20% of treated horses, requiring repeat treatment courses

Welfare Considerations

EPM causes neurological impairment that significantly affects horse welfare through incoordination, muscle weakness, pain, and inability to perform normal activities. The welfare challenge lies in the variability of clinical presentation - subtle ataxia may be dismissed as lameness or behavioral issues for weeks before EPM is considered, allowing disease to progress. Once diagnosed, treatment response is variable and the improvement timeline uncertain, creating ongoing welfare monitoring challenges. Horses with severe disease that cannot rise without assistance require intensive supportive care to prevent secondary injuries from falls. The welfare decision about whether to continue treatment and supportive care, or to pursue euthanasia, in severely affected horses requires honest veterinary assessment of recovery probability versus current suffering. Prevention through opossum exclusion from feed storage is highly effective.

What You Can Do

Store hay and grain in opossum-proof containers or buildings - opossums are the definitive host and their exclusion prevents exposure
Investigate any unexplained ataxia, asymmetric weakness, or muscle atrophy promptly - early EPM treatment improves outcomes
Seek neurological assessment from a specialist if EPM is suspected - diagnosis requires careful clinical examination and testing
Begin FDA-approved treatment promptly once EPM is suspected - waiting for definitive diagnosis may cost recovery time
Implement physical therapy and supportive care during treatment - maintaining muscle mass and balance aids recovery
Provide safe footing and confined, padded areas for horses with significant ataxia to prevent falling injuries
Monitor for relapse signs over the 12 months following treatment completion
Make honest welfare assessments in severely affected horses - prolonged suffering without realistic recovery prospects should prompt euthanasia discussions