Understanding EPM in Horses

A decade or two ago, horse owners heard a lot about equine protozoal myeloencephalitis, also known as EPM or “that opossum disease.” First identified in the United States in 1964, EPM wasn’t widely known until several years had passed and the disease became the subject of articles in scientific journals and the popular equine press. Though a number of cases are still being diagnosed, EPM has fallen from prominence as horse owners have become more concerned about newer threats such as Hendra virus, piroplasmosis, and equine herpesvirus.

Horses are exposed to EPM when they drink water or eat grass, hay, or grain that has been contaminated with the feces of opossums infected with the protozoal parasite Sarcocystis neurona. In areas where opossums are found, as many as 50 to 60% of horses have been exposed to the disease, though only about 1% of exposed horses will develop clinical signs of neurologic disease.

No one knows why some horses become ill with EPM while most others seem unaffected. It is suspected that horses with challenged immune systems may be at increased risk. Pregnant mares and racehorses in training seem to be affected somewhat more frequently than other horses, while foals and weanlings are rarely affected.

Signs of EPM include incoordination, weakness in one or more limbs, muscle wasting, asymmetric appearance of body muscling or facial features, and difficulty eating or drinking. Some horses show only one sign while others show several. EPM is difficult to diagnose based on clinical signs because these problems can be caused by several neurologic conditions. The best method of diagnosis is an examination of cerebrospinal fluid. Horses with EPM have a high concentration of antibodies in this fluid and are also positive with blood. Results are about 90% accurate in deciding whether or not a horse has EPM.

To treat EPM, horses are given drugs to reduce inflammation and reduce the protozoal population. Side effects include diarrhea, anemia, lowered fertility in stallions, and risks to the fetus in pregnant mares. Depending on which treatment is prescribed, horses are treated for one to three months or longer. Treatment is generally more successful if it is started early in the course of the disease.

No disease prevention program is entirely effective. However, management steps can be taken to lower the risk of horses ingesting contaminated food or water. These include storing grain in covered bins or other containers; keeping feed room doors closed; covering stored hay loosely with tarps or plastic sheets; and promptly cleaning up spilled grain. To keep immune levels as high as possible, follow good nutrition and exercise plans, and schedule regular vaccinations, dewormings, and veterinary examinations.