“Why should we be concerned about EPM (equine protozoal myeloencephalitis)?” asked William J.A. Saville, DVM, Dipl. ACVIM, PhD, of The Ohio State University (OSU) at the 2002 American Association of Equine Practitioners (AAEP) convention. “Because it is still an important equine disease, although we have a low incidence of the disease, and regardless of therapies available to treat EPM, it still results in neurological deficits.”

Saville presented a summary of what is known about EPM, and he believes that with better understanding of the disease, wildlife management, risk-factor manipulation, prophylactic medications, and possible vaccination, that EPM can be prevented.

EPM has received a lot of press, partially because of its devastating neurological effects on the horse, and also because in recent years researchers were able to complete the life cycle of the single-celled protozoan parasite that causes the disease, Sarcocystis neurona. “Up until the past two years, we didn’t know how the organism was maintained in Nature,” said Saville. “And if we didn’t know how it was maintained, how could we control it? The pathogen is poorly understood, and it has been a difficult task."

The Sarcocystis spp. life cycle consists of a scavenger (definitive host), carrion (intermediate host), and occasionally an aberrant host (such as the horse). The definitive host in the S. neurona life cycle is the opossum, which will eat just about anything. Early EPM research demonstrated that there were remains of domestic cats, skunks, and raccoons in the stomach contents of opossums, and the presumption was the animals like to eat road kill when other, more preferred types of food are not available. The horse apparently gets EPM from ingestion of opossum feces containing sporocysts.

“We knew the definitive host was the opossum, but very little was known about the intermediate host,” said Saville. Since then, researchers have determined that the domestic cat, the nine-banded armadillo, the skunk, the raccoon and the sea otter can all serve as intermediate hosts in the EPM life cycle. Intermediate hosts can be fed S. neurona sporocysts, and their resulting sarcocyst-infected muscle can be fed to laboratory-raised opossums. Then these opposums excrete S. neurona sporocysts, which can then be fed to experimental mice and horses—resulting in both species developing antibodies against S. neurona and displaying signs of neurologic disease.
The role of the intermediate host is more obvious in some cases than others. The opossum is unlikely to attack and kill anything—it is the dead intermediate host that promotes the cycle. Saville warned that horse owners should be reminded that “the cat does likely play a role in the EPM cycle in nature, but it’s the dead cat, not the live one, that fits the cycle,” explained Saville. So live barn cats need not be removed from premises.

According to OSU research, risk factors for EPM include:

  • Age and occupation of the horse (racehorses and show horses especially, and those who undergo transport);
  • Season of the year;
  • Presence of woods on the premises;
  • Presence of opossums;
  • Lack of feed security;
  • Health events before diagnosis (which would make the horse more vulnerable to infection); and
  • Previous cases of EPM diagnosed on the farm.

The following prevention methods are suggested to address the risk factors:

  • Improvement of the immune status while in transit;
  • Preventing the access of opossums to property, or at least the horse feed (removing the woods, and thus the habitat of the opossum, would not likely solve the problem, as the opossum has learned to adapt very well).
  • Studies at OSU and other places showed that there was an increased risk for EPM in the fall, but this was attributed to major horse competitions and thereby the increased transit. “But we know that the opossum does not usually start to eat much carrion until the late summer, early fall, and into the winter, and this can be tremendously important to how we manage our horses,” said Saville. Therefore, it is especially important to try and keep opossums away from the horses’ feed source and living environment.

There is an EPM vaccine available from Fort Dodge Animal Health. According to Saville, currently there is little data available that indicates the vaccine is efficacious. There is a study in progress involving several universities and private practices on the efficacy of the vaccine.

In terms of preventive drugs, Saville said that recent research has demonstrated that diclazuril, a triazene derivative medication and herbacide, will prevent S. neurona infection in knock-out mice (KO, mice specifically bred for this research). The medication is used in several species in other countries in poultry and swine. He said that these medications and similar compounds might be developed as a preventive therapy in the future.

One billion sporocysts can come out of one opossum, and according to Saville, they can last in the environment for more than a year. However, sporocysts in the environment can be killed under some circumstances. Research completed by scientists at OSU, the USDA, the University of Sao Paulo, and the Virginia-Maryland Regional College of Veterinary Medicine, published in the December, 2002 issue of The Journal of Parasitology showed that the most effective way to kill sporocysts in the barn is steam cleaning. Sporocysts heated to 50˚C (122˚F) for 60 minutes and 55˚C (131˚F) for five minutes were still infective to KO mice, while sporocysts heated to 55˚C (131˚F) for 15 minutes and 60˚C (140˚F) for more than one minute were unable to infect the mice. According to the researchers, none of the following disinfectants were effective in killing sporocysts: bleach (Clorox at 10, 20, and 100%), 2% chlorhexidine (Novalsan), 1% betadine (Betadine), 5% o-benzyl-p-chlorophenol (TB plus), 12.56% phenol (Wexcide), 6% benzyl ammonium chloride (NPD), and 10% formalin. Treatment with undiluted ammonium hydroxide (29.5% ammonia) for 1 hour killed sporocysts; however, when the ammonia was diluted, it did not kill sporocysts. “Results of the present study indicate that to prevent tracking of sporocysts between barns or stalls, it is necessary to change boots or use disposable boot covers because the common practice of using disinfectant footbaths to prevent cross-contamination will not kill sporocysts,” said the researchers. “Steam cleaning of equine facilities will likely be useful to kill the sporocysts in between animal use,” they continued.

Until we learn more about EPM, Saville said, all we can do to help reduce incidence of EPM is reduce risk factors and kill as many sporocysts in the barns as possible.

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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