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Though equine protozoal myeloencephalitis, or EPM, is not in the headlines as much as it was several years ago, this serious equine disease is still around. In fact, according to a study conducted by the National Animal Health Monitoring Systems, EPM is the most common neurologic disease in horses.

Caused by protozoa that invade the horse’s nervous system, EPM manifests as muscle weakness and uncoordinated movement. The horse might stumble, appear lame, have trouble backing up or negotiating slopes, be unable to chew and swallow easily, or have loss of muscle mass. Blood tests can show whether the animal, like about 50% of horses in the U.S., has been exposed to the protozoa. A spinal tap to collect and test cerebrospinal fluid can be done in order to determine whether protozoa have invaded the nervous system, in which case the horse can be diagnosed with an active infection.

Only about three in every 2,000 exposed horses will develop EPM, and of these, more than half will show significant improvement following treatment. Several oral medications can be used, and a veterinarian can advise which may be most appropriate for a particular horse. Depending on the product, treatment can take up to nine months or more to rid the horse’s body of all protozoa. Vitamin E (such as Nano-E®) is also often prescribed to support nerve health and function. To minimize damage to the nervous system, treatment should be started as soon as possible after a veterinarian determines that a horse may be infected with EPM.

EPM is somewhat more common in the eastern U.S. and is rarely found in the southwestern region of the country where the climate is very dry. Opossums are hosts of the protozoa, spreading the disease when horses ingest feed, hay, or water contaminated with opossum waste. Opossums prefer areas with access to water and are not found in arid regions. However, a recent survey by researchers at the University of California, Davis showed that horses in 42 states had developed antibodies to the protozoa that cause EPM. This finding suggests that any horse showing the neurologic signs consistent with EPM should be tested for the presence of these protozoa. EPM tests can also rule out the presence of the specific protozoa that cause EPM, allowing a veterinarian to eliminate the disease as a cause of the horse’s problem.

In areas where opossums are found, completely excluding them from pastures and barns is difficult if not impossible, but some steps can be taken to minimize their contact with feed and water that horses could ingest. Keep grain products in sealed containers, and clean up spilled feed promptly. Regularly clean stall feeders and water buckets. Fence horses away from natural water sources and install a watering system that can’t be accessed by opossums. Cover stored hay, and consider using a hay rack instead of feeding from the ground. Remove roadkill, as opossums feed on dead animals and may be attracted to this food source.

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