"Equine Protozoal Myeloencephalitis (EPM) was first recognized in the United States in the early to mid-1970s. Initially, investigators believed that the parasite responsible for causing EPM was Toxoplasma. In the early 1990s researchers isolated the causative organism and named it Sarcocystis neurona. It was soon recognized as a major cause of disease of the nervous system in horses. EPM can affect the horse’s brain, brainstem, spinal cord, or any combination of these three areas of the central nervous system. Clinical signs may suggest focal or multifocal disease, which means the disease may affect a very small (focal) part of the central nervous system (CNS) or many parts of the CNS (multifocal). Therefore, the disease may present itself with a variety of different clinical signs, dependent on the location of the damage caused by the organism within the CNS."

EPM is caused by Sarcocystis neurona. This small parasite needs 2 hosts (a definitive and an intermediary host) to complete its development cycle. Since 1995 we know that the definitive host is the North American opossum (Didelphis virginiana) and that it is the source of infection for horses. The natural intermediate host is still unknown; however, intermediate stages of S. neurona have been identified in laboratory raised cats, armadillos, skunks, raccoons and sea otters. As the research progresses, other species may be found to be intermediary hosts. The horse is a dead-end, aberrant host, as infectious forms of the parasite are not passed from horse to horse or from horse to the definitive or true intermediate hosts. Infections in the horse most likely come from pasture, hay, grain, and water contaminated with opossum feces containing sporocysts of Sarcocystis neurona.

For more information on EPM please visit the following websites.

Equine Protozoal Myeloencephalitis - Ohio State University

Equine Protozoal Myeloencephalitis - Wikipedia