Twelve Years of EPM Research: Are We Any Smarter?

"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.

Research using the life cycle model has led to Saville and others to a better understanding of how many sporocysts (the infective stage of the parasite) it takes to infect an opossum in the laboratory, which in turn lets researchers know more about how many sporocysts it takes to infect the horse with S. neurona in the research barn. Scientists now understand how quickly the parasite gets into the horse’s central nervous system and causes clinical signs. "We know what we're giving and what we're getting," said Saville of the dose of sporocysts vs. clinical signs in the horse.

During the life cycle studies, Saville learned how stress and heightened cortisol levels lead to clinical signs of EPM in exposed horses. Since then, Saville and others have been using a travel stress model to induce EPM clinical signs in infected research horses. To their surprise, it was found that horses shipped once were more likely to develop signs than those shipped twice (those shipped twice presumably had more compromised immune systems). This is likely due to different handling systems at the two study sites.

Saville explained, "I think that EPM is immune-mediated, and a horse needs an intact immune system to develop clinical signs and lesions" when experimentally infected with EPM. Reinforcing that idea, clinical signs of EPM were never seen in an immunocompromised foal (in which S. neurona was found in the blood). He said that future research emphasis should be placed on the immunity aspect of the disease.

All of these life cycle studies and stress model studies are helping Saville and others to better understand EPM and how to prevent it and treat it in the horse. Saville currently is completing studies on the EPM vaccine, but cannot discuss the results.

"We need to look further into developing diagnostic tests and preventive methods," he added. "So, going back to the question, are we smarter about EPM? I think we are, but I still think we have a long way to go."

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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