Horse Equine Protozoal Myeloencephalitis

EPM is a serious neurological disease in horses caused by protozoal parasites. Rapid diagnosis and treatment prevents permanent neurological damage.

Key Facts

Equine Protozoal Myeloencephalitis is caused by Sarcocystis neurona, with opossums as the definitive host
EPM is endemic in the Americas — horses in North and South America are at risk
Signs include ataxia, muscle atrophy, asymmetric weakness, and behavioral changes
Diagnosis involves CSF testing and serum antibody titers — neither is completely definitive
FDA-approved treatments including ponazuril, diclazuril, and sulfadiazine-pyrimethamine achieve good outcomes when started early
Relapse occurs in approximately 10-20% of treated horses — monitoring after treatment is important

Welfare Considerations

EPM welfare concerns center on the neurological suffering of affected horses and the management urgency required to prevent permanent damage. Horses with severe EPM experience ataxia that impairs their ability to move normally, find footing, and maintain balance — conditions that cause both the horse distress and significant fall and injury risk. The suffering of neurological disease is compounded by the frustration of a normally active animal unable to coordinate movement. Treatment started early in the disease course achieves good outcomes with minimal permanent deficits; treatment delayed until severe disease develops may leave horses with residual neurological impairment that permanently reduces welfare and precludes return to work. The welfare imperative for rapid recognition and treatment is clear.

What You Can Do

Seek immediate veterinary assessment for any horse showing gait abnormalities, asymmetric weakness, or behavioral changes
Implement feed management practices that prevent opossum access to horse feed and water sources
Begin approved treatment promptly when EPM diagnosis is established or strongly suspected
Monitor treated horses for relapse during and after treatment tapering periods
Provide supportive care including safe footing and reduced exercise during acute neurological episodes
Work with your vet on realistic return-to-work assessment after EPM treatment completion