CEPS/Veterinary Extension
2938 Vet. Med. Basic Sciences Bldg.
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907

By Joseph Hahn
Information Specialist
University of Illinois
College of Veterinary Medicine

Equine protozoal myelitis (EPM) is a disease that is being diagnosed with increasing frequency in recent years. While more is being learned every day, there are still some aspects of this disease that remain a mystery.

"The parasite that causes EPM is now thought to be Sarcocystis neurona," says Dr. Jonathan Foreman, an equine veterinarian specializing in internal medicine at the University of Illinois College of Veterinary Medicine Teaching Hospital at Urbana. "It is believed that transmission of this parasite is through the food or water sources, but that has yet to be proven."

Possums are thought to be important in the transmission of EPM as inapparent carriers of S. neurona. These possums urinate and defecate in the water or food supply, leading to transmission of the parasite. The disease is not thought to be contagious from one horse to another.

Horses with EPM generally show signs of a neurological disorder, taking funny steps, tripping and stumbling. They may be dangerous to ride. "The most common cause of neurologic signs in our hospital is EPM," says Dr. Foreman.

There are two ways to diagnose EPM. The first is with a blood test, looking for antibodies to the parasite. The problem with this method is that greater than 50 percent of teenage horses will have a positive blood test, but not have the clinical disease. This test indicates exposure to the parasite instead of an active infection. The second, and more definitive, method is to test spinal fluid for antibodies to the organism (called a Western Blot test) or actual DNA from the parasite (called a PCR test).

"While the blood test does not indicate the presence of the disease, a negative test is very suggestive (greater than 90%) that the horse does not have EPM," says Dr. Foreman. "A positive blood test may indicate the need for a spinal tap, but does not definitively indicate EPM is present."

"There are two drugs that have been used to combat EPM," says Dr. Foreman. "They are Trimethoprim Sulfa and Pyrimethamine. Side effects with drug treatment are uncommon. A slight anemia may develop, but is usually only a concern for pregnant mares. Foals born to these mares should be watched closely after birth and appropriate medical therapy for anemia should be instituted if necessary."

Dr. Foreman estimates the cost of treating this disease at approximately $150 to $300 per month, depending on the size of the horse and the drug formulations used. Horses are usually treated for 30 days to determine a positive response to treatment, and then treated for an additional 60 to 90 days if the treatment is positive. A second spinal fluid analysis should be performed before discontinuing treatment.

Dr. Foreman notes that this disease is now diagnosed more commonly because of the relatively new spinal tap tests. "Don't worry too much if your horse is diagnosed with EPM," he says. "Although it is a serious condition, most EPM-positive horses respond to treatment and do not have a relapse. They return to work and do just fine. Some, however, are never normal despite treatment."

If you would like further information, contact your local equine veterinarian.