“I think every time we find out something about EPM (equine protozoal myeloencephalitis), it turns up more questions,” said Bill Saville, DVM, PhD, Dipl. ACVIM, an associate professor in the Department of Veterinary Preventive Medicine at The Ohio State University’s College of Veterinary Medicine. On March 22 at the University of Kentucky’s Gluck Center in Lexington, Saville presented information from 12 years of EPM research, mainly focusing on work completed in the past five years.

EPM is caused by the single-celled protozoan parasite Sarcocystis neurona. Saville said that by the mid-1990s, researchers learned that the opossum was a definitive host for S. neurona, meaning the scavenger is needed for the perpetuation of the parasite (it reproduces in the opossum). They also knew that the horse was an aberrant host, meaning it is infected by accident through the ingestion of opossum feces and isn’t a part of the S. neurona life cycle.

An interesting note that Saville made about the life cycle was that as early as the 1940s, it was reported that cat, skunk, and raccoon muscle were found in the stomachs of road-killed opossums, revealing the opossum’s carnivorous scavenger status. “Had we been thinking about scavengers earlier, we might have found the intermediate hosts sooner.” (Intermediate hosts essentially act as vectors for the parasite–opossums become infected by eating S. neurona-infected muscle of dead intermediate hosts.)

Beginning in 2000, researchers at several institutions discovered that the domestic cat, skunk, nine-banded armadillo, raccoon, and sea otter, could be EPM intermediate hosts in a laboratory setting. Saville added that the only one that has not been validated as a natural intermediate host is the skunk and that many other intermediate hosts might exist. He emphasized that the domestic cat does not appear to play a large role in the cycle, but that the armadillo is a “leading player.” Studies have shown that a large percentage of raccoons in some states have been exposed to S. neurona, which strengthens its role as an intermediate host