By Sarah Netherton/University of Illinois College of Veterinary Medicine
Equine protozoal myeloencephalitis is a neurological condition of horses that is caused by a parasite, says 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 with 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 Austin, about half of the horses in North America have been exposed to this parasite, but less than 1 percent of exposed horses are sickened by the disease.
EPM affects young horses and work or race horses more commonly than others because 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," Austin said.
Signs that a horse may have EPM include lameness that does not respond to therapy, toe dragging, stumbling or an otherwise abnormal gait. The 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 affecting multiple areas.
A blood test for exposure to the parasite is available. However, because 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. Austin said recovery might 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," Austin said.
Keeping opossums out of areas where food is stored and where horses eat and drink is essential in stopping the spread of this disease. Austin also advises horse owners not to feed cats in the barn because this will attract opossums.
An archive of pet columns from the UI College of Veterinary Medicine is available at http://vetmed.illinois.edu/petcolumns/. Requests for reprints of this article may be directed to Chris Beuoy, firstname.lastname@example.org.