Clinical Diagnosis of Equine Protozoal Myeloencephalitis (EPM) *

Equine protozoal myeloencephalitis (EPM) has been widely described in the veterinary literature. Even in appropriately treated horses it can be a progressive, debilitating neurological disease. Either of the known causative agents, Sarcocystis neurona (common) and Neospora hughesi (rare), can produce signs of focal or multifocal central nervous system disease. Although spinal ataxia and weakness appear to be the most common presentation of EPM, signs are variable among affected horses and can mimic any other equine neurological disease. As a result, EPM is inherently a difficult diagnosis to establish definitively, and the diagnosis must always be considered tentative in the living horse. It is not surprising, therefore, that confusion exists among veterinarians attempting to diagnose this disease and interpret ancillary test results. The following consensus opinion is intended to serve as an aid to equine clinicians attempting to establish a diagnosis of EPM in horses presented for evaluation of neurological disease.