Equine protozoal myeloencephalitis (EPM) continues to frustrate North American horse owners and veterinarians as one of the most common neurologic diseases in horses–and one of the most challenging to diagnose and treat. At the 2011 Western Veterinary Conference, held Feb. 20-24 in Las Vegas, Nev., Steve Reed, DVM, Dipl. ACVIM, of Rood & Riddle Equine Hospital in Lexington, Ky., delivered an overview of the disorder and discussed diagnostic and treatment options.

Background

Reed explained that EPM was first noted in Kentucky and Pennsylvania in the 1970s, and the causative organism (a protozoon called Sarcocystis neurona) was first identified in the 1980s. Later, researchers determined that S. neurona has a two-host life cycle. 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 these sarcocysts 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