Equine protozoal myeloencephalitis (EPM) is one of the most discussed diseases in the horse health industry. Many owners are familiar with the clinical signs horses display once affected, but how much do they know about Sarcocystis neurona, one of the disorder's causative agents?

During a presentation at the Advances in Equine Neurological Diseases Symposium, held Dec. 6 in Lexington, Ky., Daniel K. Howe, PhD, a molecular parasitologist at the University of Kentucky Gluck Equine Research Center, provided extensive insight into the parasite and its relationship to EPM.

Lifecycle and Infection

Howe first discussed S. neurona's two-host lifecycle–something many horse owners are familiar with. To complete its life cycle, this organism requires a definitive host (the opossum), which feeds on the muscles of a dead intermediate host (such as a raccoon, skunk, cat, or armadillo) containing S. neurona sarcocysts. Once ingested by the opossum, the parasites mature to their infective stage (sporocysts), which the opossum passes in its feces.

Horses, which are generally considered "dead-end hosts" (meaning they typically can't pass the protozoa on to other animals), contract the disease by ingesting infected matter, often grass or hay contaminated with opossum feces containing S. neurona sporocysts.

Once the horse ingests the sporocysts, the parasite essentially makes itself at home in the horse's body. At this point the horse becomes infected with (or exposed to) the organism. Studies suggest 30-50% of horses in the United States have been infected with S. neurona; for