Equine Protozoal Myeloencephalitis (EPM) – NAVLE Study Guide

Overview and Clinical Importance

Equine Protozoal Myeloencephalitis (EPM) is the most common infectious neurological disease of horses in the Americas. It is caused by infection of the central nervous system (CNS) with the apicomplexan protozoan parasites Sarcocystis neurona (approximately 95% of cases) and, less commonly, Neospora hughesi. The disease can affect any region of the CNS, from the cerebrum to the caudal spinal cord, resulting in highly variable clinical presentations.

EPM is characterized by asymmetric neurological deficits, which is a key distinguishing feature from other equine neurological diseases. While approximately 50-90% of horses in endemic areas have been exposed to S. neurona (as evidenced by seropositive status), fewer than 1% of exposed horses develop clinical disease.

Etiology and Life Cycle

Causative Agents

EPM is caused by two apicomplexan protozoan parasites: Sarcocystis neurona (responsible for approximately 95% of cases) and Neospora hughesi (a minority of cases). Both organisms are obligate intracellular parasites that target neural tissue.