Pet Columns, Office of Public Engagement, College of Veterinary Medicine at the University of Illinois at Urbana-Champaign

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Equine Protozoal Myelitis: A Complex and Evasive Disease


Pet Column for the week of November 26, 2001


Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
Jennifer Stone
Information Specialist

Equine Protozoal Myelitis is caused by a one-celled protozoan called Sarcocystis neurona, which normally infects the opossum, its natural host. When this organism infects horses, however, it migrates to the brain, brainstem, and spinal cord, causing a variety of neurological problems.

Dr. Kathy Thomas, a veterinarian completing an equine residency at the University of Illinois Veterinary Teaching Hospital in Urbana, says, "Opossums like to hang out in barns, especially when cat food is stored there. Horses are infected through ingestion of the protozoan left behind in the feces of the opossum."

Because the organism is widespread, many horses-up to 50 percent in some areas of the country-have been exposed to it and will test positive for antibodies against the organism. A positive blood test does not mean that the horse has the disease, but rather that the horse has encountered the organism and created antibodies against it. For this reason, blood tests cannot determine the presence of disease, so testing horses that show no sign of disease is not advisable. It is not known why few of the horses exposed to the organism actually get the disease.

Dr. Thomas says, "The signs caused by this organism are highly variable. Every case is different because the presentation of the disease depends on where the organism is located in the central nervous system." Signs of this disease can include circling to one side, choking, inability to swallow, lameness, muscle atrophy, and even seizures. "The large number of possible signs is what makes this disease so frustrating," says Dr. Thomas. "However, if you are seeing neurological signs, then you should consider EPM a possible cause."

The signs of EPM can occur suddenly or develop slowly over time. It can be difficult to differentiate the effects of EPM from a mechanical defect. Often the first sign of the disease is an intermittent lameness that does not respond to treatment. The best way to diagnose EPM is to have a veterinarian conduct a complete physical and neurological examination and do diagnostic testing. Diagnostic tests should only be performed on horses with neurologic problems.

Treatment for this disease must last at least 30 days, but sometimes treatment can last as long as six months, depending on the severity and persistence of the organism. Treatments for the disease vary widely in price, ranging from $120 to $1,000 for a 30-day treatment. Many horses respond to the less expensive conventional therapy, while some may require more aggressive therapies. The newer drugs are more effective and easier to administer, but they may be prohibitively expensive for some people.

Many horses that are treated for this disease recover with no complications; however, some that do recover experience a relapse at a later time. Dr. Thomas says, "A horse that comes down with the disease suddenly and responds well to immediate treatment has a better chance of recovering than do horses with a the slower-developing form of the disease." This may be because the organism has a longer time to cause permanent neurological damage within the central nervous system.

As a horse owner, you can try to protect your horse from this disease by providing good sanitation, keeping all feed storage containers covered, and taking measures to keep opossums away from your stable by periodic trapping and relocation. If you have a horse that is having neurological or lameness problems or you have any other questions regarding EPM, please contact your local equine veterinarian.