EPM Testing, Treatment Options Reviewed

EPM Testing, Treatment Options Reviewed

Beard recommended veterinarians perform a thorough neurologic examination and rule out as many differential diagnoses as possible prior to testing for EPM.

Photo: Stephanie L. Church, Editor-in-Chief

One of the most common equine neurologic diseases—equine protozoal myeloencephalitis, or EPM—is also one of the most difficult to diagnose. Further, with only three FDA-approved treatment options, treating EPM can be a challenge as well.

At the 2013 Western Veterinary Conference, held Feb. 17-21 in Las Vegas, Nev., Laurie Beard, DVM, MS, Dipl. ACVIM, associate clinical professor at Kansas State University's College of Veterinary Medicine, reviewed the current diagnostic and treatment options for veterinary attendees.

Beard said that EPM is the most common equine neurologic disease in North America, and it affects horses of all ages, breeds, and disciplines. It is a progressive (increasing in extent and severity) and potentially fatal neurologic disease caused by protozoal (single cell) microorganisms—most commonly Sarcocystis neurona and less commonly Neospora hughesi—that cause inflammation in the brain and/or spinal cord. Clinical signs of disease vary widely, she said, and include:

  • Ataxia (incoordination), ranging from mild to severe, depending on disease status;
  • Muscle atrophy;
  • Lameness, ranging from mild to severe;
  • Head tilting; and
  • In severe cases, recumbency (the inability to stand or rise).

In many cases, EPM clinical signs are asymmetric, meaning one side of the horse's body is more severely affected than the other.

Beard also noted that disease prevalence generally correlates with opossum (S. neurona's definitive host and the animal that passes EPM-causing organisms on to horses) populations in specific geographic regions.

Current Diagnostic Options

When it comes do diagnosing EPM, Beard noted a few key points that veterinarians should consider:

  • First, she said, perform a thorough neurologic examination and rule out as many differential diagnoses as possible prior to testing.
  • She recommended only testing horses that have clinical signs of neurologic disease. "No test is 100% accurate," she said. "When you test a group of animals with a very low prevalence of disease (i.e., normal horses), the positive predictive value—meaning that a positive test is really a true positive—is very low. However, if you test a group of animals with a higher prevalence of disease (i.e., horses with neurologic disease) the positive predictive value (the positive test result is really a true positive) of that test increases."
  • Consider performing multiple tests; Beard says this will increase the sensitivity of the test (or increase the chance of finding a positive test result) but can decrease specificity (an increased chances of a false positive).

Commonly used EPM diagnostic test options include:

  • Western blot: The first commercially available EPM test was the Western blot. It's still used today and essentially gives practitioners a yes or no answer as to whether a horse has developed antibodies against EPM's causative agents. Beard said when the Western blot test is run on cerebrospinal fluid, it's sensitive to blood contamination, potentially leading to false positive results. This test can be (and often is) performed with a blood sample, as well, she said.
  • IFAT: A newer and commonly used diagnostic test is the immunofluorescent antibody test, or IFAT. The IFAT identifies the immune response to S. neurona's and N. hughesi's surface antigens (SAG) and produces a quantifying number (referred to as a quantitative test), or titer, that expresses the concentration of antibodies circulating in the horse’s blood. This test is most successful when used on a blood sample, Beard said. "If all you have is a blood sample, it is the test I would pick," she said. "This test is still useful for CSF (cerebrospinal fluid) as well."
  • SAG-1 ELISA: Another quantitative test uses an enzyme-linked immunosorbent assay format (ELISA) to measure the antibody response to the surface antigen SAG-1. Some strains of the S. neurona organism do not contain this surface antigen, generating false negative results.
  • SAG-2, 3, and 4 ELISA: Finally, Beard discussed the newest EPM diagnostic test, which measures antibodies to the S. neurona surface antigens SAG-2, SAG-3, and SAG-4 in blood and CSF. This test focuses on the ratio of titers in blood compared to CSF and is generally considered positive when the ratio (as calculated by the laboratory that carries out the test) totals less than 100, she said. This test appears to have a good sensitivity and specificity, Beard said.

"There's no perfect test—there will always be false positives and negatives," Beard said. Thus, she recommended veterinarians retest horses if they're not satisfied with initial results.

Treatment Options

The three FDA-approved EPM treatments—ponazuril, diclazuril, and sulfadiazine and pyrimethamine—all report 60%-70% success rates, said Beard.

  • Ponzauril (marketed as Marquis) is available in paste form, she said. The recommended dose is 5 milligrams per kilogram (mg/kg) of body weight daily for a minimum of 28 days. Some veterinarians believe that combining ponazuril with a small oral dose of dimethyl sulfoxide (commonly known as DMSO) could help increase ponazuril's bioavailability (the amount of drug that actually reaches systemic circulation), Beard said.
  • Diclazuril (marketed as Protazil) is sold as an alfalfa pellet; the recommended dose is 1 mg/kg daily for a minimum of 28 days.
  • Sulfadiazine and pyrimethamine (marketed as Re-Balance) has recently been made available again after a span of being commercially unavailable. The recommended dose of the oral suspension is 15 mg/kg of sulfadiazine and 1 mg/kg of pyrimethamine once daily for a minimum of three to six months.

Beard said some veterinarians use adjunct therapeutic options including non-steroidal anti-inflammatory drugs, natural vitamin E, and immunostimulators in addition the FDA-approved treatment options.


The severity and progression of clinical signs are the most important prognostic indicators, Beard said.

"Horses that present with severe ataxia (and have a) rapid progression to recumbency have a very poor prognosis," she said. "The prognosis in horses with mild to moderate clinical signs is usually considered good, with the majority of horses making a significant improvement in clinical signs."

She cautioned that a relapse of clinical signs can be an ongoing problem in some cases.

Take-Home Message

EPM remains a diagnostic challenge for veterinarians due to its extremely variable clinical signs and the current diagnostic tests available. Despite that, many mildly to moderately affected horses recover with treatment.

About the Author

Erica Larson, News Editor

Erica Larson, News Editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in three-day eventing with her OTTB, Dorado, and enjoys photography in her spare time.

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