Skunk Revealed As EPM Intermediate Host

Researchers have discovered that the striped skunk serves as an intermediate host for Sarcocystis neurona, the single-celled protozoan parasite that causes the neurological disease equine protozoal myeloencephalitis (EPM) in horses. The striped skunk’s range of habitat encompasses much of the United States, Canada, and northern Mexico, so this discovery better explains the wide geographic distribution of EPM. The results of this project follow a previous one that identified the nine-banded armadillo as a natural intermediate host for EPM.

This study involved the collaboration of various researchers from different institutions. Ellis Greiner, PhD, Andy Cheadle, Charles Yowell, Pamela Ginn, DVM, Robert MacKay, BVSc, PhD, Dipl. ACVIM, and John B. Dame, PhD, worked from the University of Florida. Debra Sellon, DVM, PhD, Dipl. ACVIM, and Melissa Hines, DVM, PhD, Dipl. ACVIM, worked on the study from Washington State University, and Antoinette E. Marsh, PhD, contributed from the University of Missouri. Greiner, who is a professor in the Department of Pathobiology at the UF’s College of Veterinary Medicine said, “We were looking at a variety of aspects of (S. neurona), and how it interacts with the horse, skunk, and opossums. Major research contributions are made this day and age by teams—I don’t think any of us could do this type of thing alone.”

In this study, striped skunks initially negative for antibodies to S. neurona were inoculated with the parasite collected from a naturally infected opossum. Skunks developed antibodies to S. neurona, sarcocysts developed in their muscles, and the muscles were fed to laboratory-reared opossums that then shed sporocysts in their feces. A pony foal and laboratory mice were inoculated with the sporocysts. The foal developed antibodies to S. neurona in the central nervous system fluid. The mice all developed antibodies and central nervous system disease and died or had to be euthanized.

“The beauty of it is that this fills out the rest of S. neurona’s range for North America and explains how opossums could be infected where we don’t have armadillos,” explained Greiner. The domestic cat also has been identified as an intermediate host in the laboratory, although it hasn’t been proven that the cat serves as a natural intermediate host.

“The difference between the skunk study and the cat study is that in order to get a really good infection in the cat, researchers had to immunosuppress the cat (depress its immune system) before the parasite could be observed in the cat muscles,” explained Cheadle. “We didn’t do anything to the skunks and they became heavily infected with sarcocysts (when the sporocysts were administered). The skunk has been reported be naturally infected with sarcocysts, therefore it is highly likely to be a natural intermediate host. Reports of naturally infected skunks are needed to provide definitive data implicating them as an intermediate host in nature.”

Research groups around the country are investigating other possible hosts. “I have an inkling that raccoons might be intermediate hosts too, based on some similar data,” said Cheadle. “They are very similar to skunks in that they actually have a Sarcocystis species inside their muscles that is very similar in structure to those we found in the skunk,” he added.

Next the research group will attempt to confirm that the cycle involving the skunk can be completed in nature as well as in the laboratory. Researchers are optimistic because S. neurona readily develops sarcocysts in the skunks. “We have a very limited skunk population in Florida, so we’re going to collaborate with another university (with a more endemic skunk population than in Florida) and they’re going to collect skunks and we will do the tissue work, as well as confirm the identity of the parasite,” he explained.

In the meantime, elimination of skunks isn’t the answer to keeping EPM out of your stable area. “A hundred skunks in a pasture with a horse won’t do a thing to that horse—the opossum is (still) the obvious source of infection,” said Greiner.

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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