HealthWatch: Update on EPM

If it looks like equine protozoal myoencephalitis (EPM) and smells like EPM…it probably isn’t, say five experts in a feature article in The Horse’s upcoming June issue.

EPM remains a rare cause of neurologic disease in horses—less than 0.5% of horses are diagnosed with EPM annually in the United States. However, if you do the math, 0.5% of all the horses residing in the United States still amounts to several thousand horses diagnosed with EPM each year.

Most horses have been exposed to the parasites that cause EPM and have antibodies against them, said Dr. William Saville of The Ohio State University’s College of Veterinary Medicine.

“I have seen a lot of horses with EPM in my career,” stated Dr. Steve Reed of Rood & Riddle Equine Hospital in Lexington. However, he noted, EPM is likely quite rare in the subset of horses that have no overt neurologic signs and only a history of “poor performance.”

“There are a lot of horses on racetracks, for example, that are on EPM medications,” said Reed. “Do they all have EPM? No. But, the trainers are adamant that the horses race better when they are being treated with ponazuril (the drug used most often to rid a horse of the causative parasite).”

Saville added, “It just makes no sense to treat horses for poor performance with EPM medications without performing a full work-up.”

Experts currently recommend a horse only be diagnosed with EPM if:
• He has clinical signs consistent with the disease such as incoordination, and asymmetric muscle wasting;
• The veterinarian has ruled out all other neurologic diseases that are similar to EPM; and
• There is a positive EPM test result.

Reed said a veterinarian should rule out other neurologic diseases by performing a neurologic examination, localizing the disease to the spinal cord (especially to the neck region), and taking radiographs of the neck to make sure the horse isn’t a wobbler (caused by compression of the spinal cord).

Currently, four tests are available for diagnosing EPM, which is caused by the protozoan parasites Sarcocystis neurona or Neospora hughesi:

Western blot (Immunoblot) This detects antibodies to S. neurona in either blood (serum) or cerebrospinal fluid (CSF). A positive Western blot merely means the horse has been exposed to the parasite and has developed antibodies to S. neurona. A positive test does not necessarily mean the horse is actively infected. In contrast, a negative test suggests the horse is not infected with S. neurona.

Serum indirect fluorescent antibody test (IFAT) This test measures the level of antibodies (i.e., a titer) against either S. neurona or N. hughesi in the serum of horses. As in the Western blot test, a positive result simply indicates exposure to the parasite, but it does not necessarily mean the horse is actively infected.

Enzyme-linked immunosorbent assay (ELISA or SAG1 test) This assay measures antibodies to the primary surface antigen (SAG1) of S. neurona. (An antigen is a disease-causing substance that stimulates the immune system.) Titers of 1:100 and greater indicate exposure to S. neurona and (according to the laboratory) possibly active disease if associated with clinical signs of EPM.

Polymerase chain reaction (PCR) Veterinarians perform the PCR assay on cerebrospinal fluid and neural tissue to identify DNA fragments of S. neurona. The assay is run on CSF samples only with a concurrent positive ELISA result. A negative result does not necessarily mean the horse does not have EPM, as the horse might have antibodies to another surface antigen.

“From my perspective, I think that the most pressing need with respect to EPM is a test that accurately detects active infection and disease,” concluded Dr. Noah Cohen of Texas A&M University.

Excerpted from The Horse: Your Guide to Equine Health Care. Free weekly newsletters at