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 (and in some areas as high as 90%) of horses in the United States may have been exposed to the organism that causes EPM. The primary causative organism is a protozoal parasite called Sarcocystis neurona, and with much less frequency, another protozoal parasite—Neospora hughesi can also cause this condition Fortunately, only a very small percentage of horses which have been exposed to either of these organisms will ever develop the disease we call EPM.The disease is not transmitted from horse to horse. Rather, the organism Sarcocystis neurona is spread by the definitive host, the opossum, which acquires the organism from cats, raccoons, skunks and armadillos and sea otters. The infective stage of the organism (the sporocysts) is passed in the opossum's feces. The horse comes into contact with the infective sporocysts while grazing or eating contaminated feed or drinking water. The definitive or intermediate hosts for Neospora hughesi have not yet been identified.
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 devastating and lasting neurological damage.
