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By Deb Eldredge
Equine protozoal myelitis, or EPM as it is commonly referred to, has frustrated and challenged horse owners, veterinarians and researchers for years. It’s only been in the last 20 years or so that the agent causing this disease, the primary host and the life cycle have been identified. About 14 new cases of EPM per 10,000 horses show up each year. That may not seem like many unless your horse is one of those 14. EPM is caused by protozoa—with virtually all cases caused by Sarcocystis neurona and a few by Neospora hughesi. The primary host, who must be present for certain stages, is the opossum. Intermediate hosts include cats, armadillos, raccoons and skunks. Horses contract the disease by eating feed, drinking water or grazing areas contaminated by opossum feces. Not every opossum carries the protozoa. One study showed about 18 to 25% of opossums have this protozoa. In addition, not every horse exposed to the protozoa will become ill. In fact, out of all exposed horses, less than 1% will show up with clinical EPM. Illness requires the protozoa to cross the blood/brain barrier and many horses have immune systems that catch this invader long before that point. Still, S. neurona can, and does, reach the spinal cord and/or brain in some horses.