Equine protozoal myeloencephalitis 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. Infection of the spinal cord is particularly common and causes weakness, loss of coordination, and muscle wasting in the legs and torso. When the disease infects other parts of the central nervous system, signs can include spontaneous sweating, loss of reflexes, diminished sensitivity to touch, dullness, abnormal head tilt, facial paralysis, vision problems, behavioral abnormalities, and seizures.

The disease is difficult to diagnose, but it can be treated effectively with antiprotozoal drugs. These are applied as a paste, and the treatment lasts from 28 days to 6 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 equine protozoal myeloencephalitis 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.

Last full review/revision July 2011 by William B. Thomas, DVM, MS, DACVIM (Neurology); Daniela Bedenice, DrVetMed, DACVIM, DACVECC; Kyle G. Braund, BVSc, MVSc, PhD, FRCVS, DACVIM (Neurology); Cheryl L. Chrisman, DVM, MS, EDS, DACVIM (Neurology); Caroline N. Hahn, DVM, MSc, PhD, DECEIM, DECVN, MRCVS; Charles M. Hendrix, DVM, PhD; Maureen T. Long, DVM, PhD, DACVIM; Robert J. Mackay, BVSc, PhD; Karen R. Munana, DVM, MS, DACVIM (Neurology); Charles E. Rupprecht, VMD, MS, PhD; Josie L. Traub-Dargatz, DVM, MS, DACVIM; Susan L. White, DVM, MS, DACVIM