The Latest on EPM

Is it possible that the horse is a natural intermediate host in the life cycle of the parasite that causes equine protozoal myeloencephalitis (EPM), meaning the parasite can develop to a reproductive state in the horse? Are there other parasites that cause EPM? Can Strongid C 2X daily dewormer prevent infection in horses with Sarcocystis neurona? Does shipping a horse twice increase his risk of developing clinical signs of EPM?

The annual American Association of Veterinary Parasitologists gathering was held July 19-22 and included presentations on several equine-related topics. The EPM Society held a roundtable during that meeting after presentations from several top EPM researchers.

Life Cycle Includes Horse?

The life cycle of S. neurona involves the definitive host (opossum) that feeds on muscles of dead intermediate hosts (such as the striped skunk, raccoon, nine-banded armadillo, and cat). The parasite must advance to the sarcocyst stage of its life cycle in the muscle of an intermediate host for a carnivore or omnivore (such as the opossum) to eat it and allow the parasite to continue its life cycle.

 Linda Mansfield, VMD, PhD, head of the Emerging Parasitic Diseases Laboratory at Michigan State University, reported on a 4-month-old cross-bred colt euthanized due to severe neurologic problems from EPM. Mansfield found mature parasites in the brain and spinal cord of this colt and mature sarcocysts (contained in muscle cycts) in the tongue.

"The finding of mature, intact S. neurona schizonts and sarcocysts in the tissues of this single horse is suggestive that horses have the potential to act as intermediate hosts," Mansfield noted, adding that further studies are needed to see if this is found in other horses, and whether the parasite can be passed from horses to opossums.

It is not thought that the horse sheds the parasite in its feces and spreads it to another horse, or an infected horse can "give" the parasite to a healthy horse.

Other Causes of EPM?

Researchers are now tempering their comments on EPM by saying some horses show clinical signs consistent with the disease, but live in areas that are devoid of opossums. In other words, S. neurona is still the parasite primarily causing clinical disease, but there are other potential protozoal parasites that could cause similar clinical signs.

Can Strongid C Daily Dewormer Prevent EPM?

One paper from Elizabeth A. Kruttlin, DVM, a Masters student at Michigan State University (MSU), noted that have shown that pyrantel tartrate (the active ingredient in Strongid C daily dewormer) is effective in killing the sporozoite stage (infectious stage) of the parasite in the laboratory. However, Mary Rossano, a PhD student at MSU, did an experiment in weanlings showing that daily administration of pyrantel tartrate does not prevent infection with S. neurona in horses when given according to label directions.

Transporting Twice, Worse EPM?

Bill Saville, DVM, MS, PhD, of The Ohio State University (OSU), had developed a transport-stress model where horses infected experimentally with S. neurona on the day they arrived after transport developed clinical signs of the disease in about a week. His theory was that a second transport would cause more severe clinical signs.

His double transport study demonstrated consistent, significant clinical signs in all horses at approximately the same time period post-inoculation, but the signs were most severe in horses which did not experience a second transport. He said that didn’t make sense due to the supposed immunosuppressive effects of transportation.

One factor he thought might have confounded the experiment results was the difference in housing and care. The weanlings in the experiment which were only shipped once were housed in a facility in open steel pens with different personnel feeding and handling the foals each day. The other groups were transported to OSU, where mostly female veterinary students cared for them every day.

"They were pampered on a daily basis, and I think that made a difference," said Saville.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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