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EPM Remains a Threat to Horse HealthBy Kentucky Equine Research Staff · July 10, 2014

Equine protozoal myeloencephalitis, or EPM, might not be in the news as much as it was a few years ago, but it’s still a threat to horses. In fact, EPM is one of the most common neurologic diseases affecting equines in North America. Horses can develop EPM after ingesting hay, grass, or grain that has been contaminated with microscopic protozoa found in opossum waste. The natural range of opossums includes the U.S. and parts of Canada, though the risk equine of exposure is lower in dry, treeless areas that are not favored by these marsupials.

EPM affects the horse’s nervous system. Because nerve signals control muscle function, the muscular system is also affected. Signs may be mild, such as a drooping ear or a slight decrease in performance. Owners might not notice these signs or could attribute them to different conditions. In other horses, EPM can manifest as lameness, poor coordination, difficulty swallowing, or an inability to stand. In general, horses with less severe signs have a better chance of recovery than those that are more seriously affected. Early diagnosis and treatment are important in maximizing the horse’s chances for recovery.

Because the signs of EPM may be somewhat vague and are similar to those of other neurologic problems, diagnosis requires laboratory tests of the horse’s blood or cerebrospinal fluid. No single test is 100% accurate, so more than one analysis is sometimes done. However, if signs are consistent with EPM and other neurologic diseases have been ruled out as much as possible, treatment may be started before test results are known.

Several medications in paste, pellet, and oral suspension form are available to fight the protozoal infection. Treatment ranges from one to several months of daily medication, with success rates of around 60 to 70%. Veterinary management can also include nonsteroidal anti-inflammatory drugs to reduce inflammation and vitamin E to support nerve function.

There is currently no USDA-approved vaccine to guard horses against EPM, so property management is the only way owners can minimize the chance of exposure for their horses. Because opossums favor wooded areas near water, horses should be pastured in areas that don’t adjoin overgrown or swampy areas. In barn areas, spilled grain should be cleaned up, and feed and garbage should be tightly covered. Loosely covering stacked hay may reduce the chance that opossums will contaminate it. Owners can also support their horses’ immune systems by providing optimal nutrition and limiting stress as much as possible.