Study Links New Risk Factors to EPM Infection

Results from a recent study on equine protozoal myeloencephalitis (EPM)--a progressive neurological disease--identified three factors that increase a horse's risk of infection: living with cats, use for Western performance or racing, and age greater than 2 years. However, horse owners should consider the context of these findings before changing their horses' management in the hope of preventing EPM.

"Considering that the available treatments for EPM are expensive, not uniformly effective, and may be associated with side effects, the goal of this study was to identify factors that contribute to EPM and focus on ways to prevent this disease," explained Noah Cohen, VMD, MPH, PhD, Dipl. ACVIM, professor of medicine in Texas A&M's Department of Large Animal Clinical Science, and lead author on this study.

Data from 11 equine referral centers nationwide was included in this case-control study. The study documented 183 horses with EPM, 297 horses with neurological diseases not associated with EPM, and 168 horses with disease that was not neurologic. The data was collected between Sept. 1, 2001, and Aug. 31, 2003.

Results showed that living with cats, the horse's primary use/riding discipline, and age were all risk factors for EPM,. The presence of opossums, the definitive host for EPM, did not have any impact in this study.

While living with cats was a risk factor for EPM, horses in both the EPM and control groups resided with cats. Cohen explained that domestic cats are known to be an intermediate host for Sarcocystis neurona, the causative agent of EPM, so it is possible that cats play a role in the natural occurrence of EPM.

However, Cohen said these results need to be interpreted carefully by horse owners.

"It would be a great oversimplification to think that there is sufficient evidence to suggest that removing cats from barns would prevent EPM," emphasized Cohen, noting further research is needed.

It is possible that the reporting of opossum presence was not accurate as data were collected based on the owner's recollection of seeing opossums, which are nocturnal animals, the study authors noted. In addition, a 1-mile radius--the space in which owners noted whether they saw opossums in the study--might not be an appropriate indication of exposure.

Use of horse for Western performance or racing was also identified as an EPM risk factor compared to the non-neurologic control horses, but not when compared to the horses with neurologic diseases other than EPM.

The authors stated that it is unclear whether the horse's use could actually be a cause of EPM (perhaps transport or other stressors that might increase the risk of EPM are more common in performance horses), or if performance horses are simply more likely to be diagnosed with the disease because subtle signs of EPM are generally more obvious in athletic horses.

While age was a risk factor, researchers were not entirely surprised that horses less than 2-years-old were at a lower risk for EPM.

"Work from the University of California, Davis, demonstrated that exposure of foals and horses less than 2.5 years to S. neurona and Neospora hughesi--two parasites most often attributed to EPM infection--is rare," said Cohen.

Overall, these results contribute to the industry's body of knowledge concerning EPM, but controlling these three factors to prevent EPM is not realistic.

"Further research that clarifies the natural history of the disease and identifies risk factors that can be controlled to prevent EPM is essential," concluded Cohen.

This study, "A multi-center case-control study of risk factors for equine protozoal myeloencephalitis," was published in the Dec. 15, 2007, edition of the Journal of the American Veterinary Medical Association.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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