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Some Lameness Caused by Equine Protozoal Myeloencephalitis


Pet Column for the week of September 30, 2013

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[Dr. Scott Austin]
Dr. Scott Austin, an equine veterinarian at the University of Illinois Veterinary Teaching Hospital in Urbana, says EPM affects young horses and work or race horses more commonly than other horses.

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“Equine protozoal myeloencephalitis, or EPM, is a neurological condition of horses that is caused by a parasite,” says Dr. Scott Austin, an equine veterinarian at the University of Illinois Veterinary Teaching Hospital in Urbana. He explains how this disease infects horses and how it is treated.

The parasite, Sarcocysitis neurona, is carried by opossums. Horses can become infected this disease by ingesting grain contaminated with opossum feces, which contain the parasite eggs. Grain is contaminated when opossums find their way into grain storage areas.

According to Dr. Austin, about half of horses in North America have been exposed to this parasite, but less than 1% of exposed horses are sickened by the disease.

EPM affects young horses and work or race horses more commonly than others, since these groups are under more stress and their immune systems are less able to combat the disease.

“The disease attacks the central nervous system and leads to weakness and incoordination,” says Dr. Austin.

Signs that a horse may have EPM include lameness that does not respond to therapy, toe dragging, stumbling, or an otherwise abnormal gait. This disease can cause neurologic problems that are asymmetrical, affecting the left and right sides of the body differently.

To make a diagnosis, a veterinarian will perform a thorough neurologic examination, looking for a cause of disease that is multifocal, or affecting multiple areas.

A blood test for exposure to the parasite is available. However, since half of the horse population has been exposed, a positive result will not yield a definitive diagnosis, though a negative test probably means that a horse is not infected.

A veterinarian will consider clinical signs such as lameness along with test results to rule out other possible causes. X-rays can be taken and the diet examined to ensure there are no vitamin deficiencies. A spinal tap may be recommended to support a definitive diagnosis of EPM.

Anti-protozoal drugs are prescribed to treat EPM. These are given for 30 to 60 days, and horses are re-evaluated a month after starting treatment. Dr. Austin says that it can recover may take up to a year.

Treatment is considered successful if the horse improves by one neurologic grade, based on a scale of 1 to 5, with 1 being a normal horse and 5 being a horse unable to stand.

“It is important to realize that a horse can relapse with EPM or can be infected again,” says Dr. Austin.

Keeping opossums out of areas where food is stored and where horses eat and drink is essential in stopping the spread of this disease. Dr. Austin also advises horse owners not to feed cats in the barn, since this will attract opossums.

For more information about equine protozoal myeloencephalitis, speak with your local equine veterinarian.