According to Bayer Animal Health, 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. The causative organism is a protozoal parasite named Sarcocystis neurona. The disease is not transmitted from horse to horse. Rather, the protozoa are spread by the definitive host the opossum. The infective stage of the organism, the sporocysts, are passed in the opossum's feces. The horse comes into contact with the infective sporocysts while grazing or eating contaminated feed or drinking water.
Once ingested, the sporocysts migrate from the intestinal tract into the bloodstream and cross the blood/brain barrier. There they begin to attack the horse's central nervous system. The onset of the disease may be slow or sudden. If left undiagnosed and untreated, EPM can cause
EPM is a serious and often fatal neurological disease of horses. Almost every part of the country has reported cases of EPM. However, the incidence of disease is much lower in the western United States especially in regions with small opossum populations. However, due to the transport of horses and feedstuffs from one part of the country to another, almost all horses are at risk.
Not all horses who are exposed to the protozoan Sarcocystis neurona will develop the disease and show clinical signs of EPM. Some horses seem to mount an effective immune response and are able to combat the disease before it gains a foothold. Other horses, especially those under stress, can succumb rapidly to the debilitating effects of EPM. Still others may harbor the organisms for months or years and then slowly or suddenly develop symptoms.