The Latest on EPM Research

The annual American Association of Veterinary Parasitologists met July 19-22 with presentations on many equine-related topics. The EPM Society held a roundtable during that meeting after presentations from top EPM (equine protozoal myeloencephalitis) researchers, discussing topics such as whether horses can be a natural intermediate hosts in the Sarcocystis neurona life cycle, and whether other parasites can cause EPM.

Life Cycle Includes Horse?

The S. neurona life cycle 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 an intermediate host's muscle for an omnivore (such as the opossum) to eat it and continue the 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 cysts) in the tongue.

Mansfield said this finding "is suggestive that horses have the potential to act as intermediate hosts." She added that further studies are needed to see if this might be 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 that an infected horse can "give" the parasite to a healthy horse.

Other Causes of EPM?

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

Transporting Twice, Worse EPM?

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

His double transport study demonstrated consistent, significant clinical signs in all horses in about 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.

He said one factor that might have confounded the results was the difference in housing and care. The weanlings which were only shipped once were housed in a facility with open steel pens and 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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