Equine protozoal myeloencephalitis epm is a master of disguise this serious disease can be difficult to diagnose because its signs often mimic other health problems in the horse and signs can range from mild to severe more than 50 percent of all horses in the united states may have been exposed to the organism that causes epm. Equine protozoal myeloencephalitis epm is a devastating disease that can leave horses severely disabled and may result in death diagnosis is often difficult because the onset is very similar to other diseases especially chagas chagas and epm are caused by different protozoa and require different treatment. Equine protozoal myeloencephalitis epm is caused by the protozoal parasites sarcocystis neurona and neospora hughesi although most cases are due to s neurona the definitive host for s neurona is the opossum and horses become infected when they ingest feed or water contaminated with opossum feces. Understanding the epm disease equine protozoal myeloencephalitis epm is a master of disguise this serious disease can be difficult to diagnose because its signs often mimic other health problems in the horse and signs can range from mild to severe more than 50 percent of all horses in the united states may have been exposed to the organism