August 21st, 2010 - Looking at EPM

  • August 18, 2010
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Equine protozoal myeloencephalitis (EPM) is one of the most common causes of neurological problems in horses. The parasite that causes this disease is a one-celled protozoan that invades the brain and spinal cord and affects nerves that control the horse’s movements.

The first description of EPM in horses was published in 1970. It took several more years before research began to unravel some of the mysteries of this debilitating disease, and we are still learning about it. Most EPM cases in horses are caused by the protozoan Sarcocystis neurona, though some are caused by a protozoan called Neospora hughesi. S. neurona is found in North, Central, and South America and parasitizes several hosts including the cowbird, raccoon, cat, skunk, armadillo, sea otter, and opossum. The horse is an accidental host that picks up the parasite when eating feed contaminated with feces from an animal shedding oocysts. The most common mode of infection in Thoroughbreds is consumption of feed contaminated with opossum feces.

If the parasite gets past the horse’s natural defenses and invades the brain or spinal cord, the horse can show signs of EPM. If the parasite attacks the spinal cord, primary signs are ataxia (inability to control muscle movements), stumbling and incoordination, weak and wobbly gait, multi-limb lameness, and muscle atrophy.

If the parasite attacks the brain, which happens less commonly, signs may include depression, blindness, behavioral changes, seizures, and cranial nerve paralysis (affecting facial muscles, hindering ability to chew or swallow, with head-tilting and atrophy of various head muscles).

Dr. Nicola Pusterla, associate professor at the University of California-Davis, says about 10% of the neurological case load at their large animal hospital is diagnosed as EPM. “We also see a second organism that causes it. N. hughesi was once thought to be just a disease of the western part of the U.S., but we know now it is more widespread,” said Pusterla.

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