Equine protozoal myeloencephalitis mimics other horse health issues, so detecting the disease can take some detective work. The disease results from exposure to the protozoa Sarcocystis neurona, most often carried by opossums and excreted in their feces. Horses pick up the parasite by grazing in areas with infected feces or consuming otherwise contaminated water or feed. That's just one reason it's important to keep opossums and other wildlife out of your feed rooms and hay storage areas.
Signs and Symptoms
Early signs include gait abnormalities, muscle wasting and periodic incoordination, most notable when going up and down hills. As the infection progresses, the horse may experience seizures, facial paralysis, head tilt and an inability to hold himself upright without relying on a fence or stall wall for balance. As the Merck Veterinary Manual notes, "Almost any neurologic sign is possible." The horse may exhibit behavioral changes along with his neurological issues. You might notice him constantly falling asleep, often in odd positions. In a worst-case scenario, he collapses and dies.
While any horse might develop EPM, certain equines appear more vulnerable. Horses with compromised immune systems are less able to fight off the infection, as are horses in stressful situations. For a horse, stress can be caused by long-distance shipping, a new owner or barn, and even long-term bad weather. In the West, where fewer opossums live, the incidence of EPM is lower.
While a blood test might reveal exposure to the protozoa, many exposed horses never become symptomatic. There's no specific test available for EPM, but your vet can collect cerebrospinal fluid from your horse, then test the sample for the presence of antibodies.
Unfortunately, only about 25 percent of horses recover completely from EPM, becoming able to return to their previous level of work. Horses with EPM often have relapses, and that's especially true if they did not receive the full complement of antibiotic therapy.