Equine protozoal myeloencephalitis (EPM) is a neurologic disease of horses that occurs in endemic form in the Americas and sporadically in other countries. The endemic form is caused by ingesting sporocysts of the protozoan Sarcocystis neurona in contaminated feed or water. Sporadic disease occurs worldwide and is caused by Neospora hughesi.
The protozoa can infect any part of the central nervous system, so almost any neurologic sign can develop. Signs of spinal cord involvement are more common than signs of brain infection. The disease usually begins subtly but can be sudden and severe at the onset. Infection of the spinal cord causes weakness, loss of coordination, and muscle wasting in the legs and torso. When the sacrocaudal spinal cord is involved, there are signs of cauda equina syndrome. Other spinal cord lesions may result in areas of spontaneous sweating or loss of reflexes and cutaneous sensation. When the disease infects the brain, signs may be depression, abnormal head tilt, facial paralysis, vision problems, behavioral abnormalities, and seizures.
The disease is difficult to diagnose, because the signs are similar to those for other diseases. EPM can be treated effectively with antiprotozoal drugs. These are applied as a paste, and the treatment lasts from one to several months, depending on the particular treatment used. Without treatment, equine protozoal myeloencephalitis is often fatal, but the severe signs, particularly recumbency, may not occur for years after infection. The disease can progress steadily or in a stop-start fashion.
Antiprotozoal drugs may also be useful for prevention, although there is no scientific consensus.
The likeliest source of the protozoa that causes endemic EPM is opossum feces, so horse owners should attempt to keep opossums away from horse feeding areas. Horse feed or pet food should not be left out, and open feed bags should be kept in closed containers. Bird feeders, garbage, and fallen fruit should be removed from the area.