Stumbling, tripping, and lameness or gait abnormalities can be early signs of EPM.||| Photo: iStock

This serious neurologic disease of horses primarily caused by Sarcocystis neurona, a protozoan parasite that invades the brain, brainstem, and spinal cord. Another protozoan parasite, Neospora hughesi, is a less common cause of EPM.

One study indicates that, overall, 78% of U.S. horses have antibodies against S. neurona and 34% have antibodies against N. hughesi. However, less than 1% exposed to EPM will develop clinical signs.

S. neurona is spread to horses from a definitive host, in this case, an opossum. Horses become infected with EPM through contact with opossum feces through grazing or contaminated feed. The definitive life cycle of N. hughesi is not yet fully understood