Opossums: The Scourge of a Horse Owner’s Existence?

Q. Can you explain more about opossums and equine protozoal myeloencephalitis (EPM)? At one time we were all told that opossums are a host of EPM and help spread the causative parasite, but with the prevalence of opossums and horses in Kentucky, shouldn’t it be an epidemic? Can you share insight into EPM, how it is contracted, and if opossums are, indeed, the scourge of a horse owner’s existence? Is there any truth to the hypothesis that cats can spread it, too? And do we absolutely have to hate and wage war on opossums lest they cross our horses’ fields and infect them? I know they have some benefits to our world since they eat so many ticks and almost never carry rabies due to their low body temp. I am tired of everyone I meet hating opossums and wanting to kill each one they encounter. Any information pro or con would be appreciated; maybe I am not as anti-opossum as I need to be.

A. You are correct that opossums (not cats) are the host animal that transmits Sarcocystis neurona, the protozoan parasite that causes EPM. You are also correct that opossums are everywhere in the Bluegrass region, and they’re not very selective in where they defecate (the mode of parasite transmission). So, it’s not too surprising that many horses in this area (>75%) have been infected by S. neurona, as revealed by the presence of antibodies against the parasite circulating in their blood.

The disease is not an epidemic, however, because most horses are apparently able to control the infection. It would seem that there is a small subpopulation of horses that are susceptible to the disease. Unfortunately, we don’t understand what factor(s) make horses more likely to progress to full-blown EPM when exposed to the parasite. It could be something intrinsic in the horse (for instance, its genetics), or it could be some external factor that causes the horse to be more susceptible (environment, nutrition, co-infection with a virus, etc.) Logically, a more complete understanding of this aspect of the disease would help us better control it, either through preventive measures (change in environment/nutrition), prophylactic treatment with a drug, or (ideally) with a logically designed vaccine.

Bottom line: Yes, opossums are the culprit that transmits the causative parasite of EPM to horses. While efforts to remove opossums will reduce the risk of infection on a farm, “nature abhors a vacuum,” as they say, and new opossums are bound to migrate into areas where the population has been reduced. Thus, I don’t think it’s feasible to completely stop horses from getting infected. I can acquire several different pathogens from my dog and cat, but I don’t intend to get rid of them because of it (although the cat does get on my nerves sometimes!). I do, however, keep the cat’s litter box far from where food is stored, and I think the same logic applies to opossums. Make an effort to keep them out of the barn and away from the food and water supplies. You won’t completely prevent a horse from being exposed to the parasite, but perhaps less frequent exposure will also reduce the likelihood of EPM occurring.

—Daniel K. Howe, PhD, UK Gluck Equine Research Center
University of Kentucky

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