EPM Parasite Isolated from Healthy Horse's Blood

Scientists recently isolated the parasite that causes the neurological disease equine protozoal myeloencephalitis (EPM) from the blood of an immunocompetent horse--a normal, healthy horse. This research could lead to a better understanding of the way the single-celled protozoan parasite Sarcocystis neurona (which causes EPM) attacks a horse and the best ways to prevent and fight EPM.


An example of S. neurona merozoites growing in equine demal cell culture.

In 2002, a paper was published by M.T. Long and colleagues confirming the finding of S. neurona in the blood of an Arabian foal with severe combined immunodeficiency (SCID) after the administration of a single oral dose of S. neurona sporocysts. This was the first finding of the parasite, ever, in a horse's blood.  But this had not been done in a normal horse. The six yearling colts used in the current study were part of another study that was attempting to show if pyrantel tartrate (Strongid C) could be used effectively as a preventive drug for EPM (that article has been accepted for publication and is currently in press).

A Michigan State University student Mary G. Rossano, MS, PhD, who was finishing her PhD degree, was the lead author on the study. Rossano performed the study at Washington State University in Pullman, Wash., since opossums, which are responsible for infecting horses by shedding the S. neurona sporocysts, are extremely rare in that area of the country. Therefore, a very low percentage of horses have been exposed to S. neurona in that area and, thus, the investigators knew that the horses in the study received only the parasite under study.  This assured that the results would be valid.

In an attempt to recreate the natural infection scenario most horses likely encounter, the colts received small oral doses of S. neurona sporocysts each day. Linda Mansfield, VMD, PhD, head of the Enteric Diseases Laboratory at Michigan State University, president of the American Association of Veterinary Parasitologists, and Rossano's advisor, said, "We did as many studies as possible with this group of horses so we didn't have to experimentally infect so many horses."

Blood was taken regularly from the horses, and the researchers attempted to culture S. neurona in vitro (outside the horse's body) on cells grown in petri dishes in order to demonstrate the presence of the parasite. "In our study, we only saw the parasitemia in one horse, likely because the horses were immunocompetent," said Mansfield. The horse that cultured positive for EPM showed no apparent clinical signs despite careful observation.  He did have a positive western blot test result for the parasite in blood and a suspect result in cerebrospinal fluid. He was adopted later, so a post-mortem evaluation of his CNS was not performed.

" Our study showed that horses have some degree of resistance to S. neurona. Having said that, there's always a chance that we didn't recover the parasite in the sample because horses are large and they have a lot of blood, so it is possible that blood could have been drawn from a place where the microorganism was not directly circulating."  Mansfield said that it's extremely difficult to find microorganisms in blood. "If you think about a horse, they just have a huge amount of blood in relation to the amount of microorganisms that might be present at any one time," she said.

The researchers' long-term goal in performing the study was to produce better diagnostics and better preventive measures and treatments for EPM.

"This study, which was funded by Pfizer and conducted as a collaboration between veterinarians at Michigan State and Washington State Universities, and the previous study at Washington State (that found parasitemia in the SCID foal), are part of the strategy to understand what we call the pathogenesis of the disease.  Pathogenesis means how the parasite comes in and causes disease," Mansfield explained. "We have a lot of clues now that the immune system is very important in allowing the horse to control the parasite in order to prevent it from producing disease."

Many research groups are trying to find out which components of the immune system prevents infection with S. neurona. She explained, "Evidently, horses with reasonable immune systems can control the parasite. Once you understand what it is about the immune system that's doing the controlling, you can figure out a way to boost that immune response, and you would know exactly how to target your vaccine and treatment (to be more effective), and maybe develop a drug that could be administered as an effective preventative treatment."

Mansfield emphasized that horse owners can reduce the risk of having EPM diagnosed in their herd simply by eliminating opossums from the premises and cleaning up the barn (after spilling grain, etc.) to avoid attracting opossums and other vermin.

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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