Mysterious EPM

While rare, is this disease still something to be concerned about?

Much has been learned about Equine Protozoal Myeloencephalitis (EPM) in the last 25 years. And yet many questions still remain, and there continues to be controversy even among the experts.

The biggest challenge seems to be in the area of diagnostics. While EPM is a commonly diagnosed neurological disorder, it is actually rare and very difficult to get a definitive diagnosis. But before we get into that, it is helpful to review what we do know about the potentially fatal EPM.


In EPM, protozoa (single-cell organisms) infect the brain and spinal cord. The majority of cases are caused by Sarcocystis neurona, but some are due to an infection of Neospora hughesi. These parasites are found in North, South and Central America, so EPM is only seen in countries beyond the Americas when horses have been exported. Fortunately, the parasites cannot be transmitted from horse to horse. They are passed in the feces of opossums that contaminate feed. When the horse ingests the organism, an immune response is mounted which usually takes care of it. However, in a small number of cases, the parasite attacks the spinal cord or brain, and EPM develops.

According to an article written by Martin Furr, DVM, PhD, Dipl ACVIM, on behalf of the Equine Research Coordination Group, symptoms of EPM in the spinal cord include a stumbling, weak or weaving gait and muscle atrophy. When EPM strikes the brain, which is less common, symptoms may be blindness, inability to chew or swallow, head tilt, seizures and muscle atrophy in the head.


Researchers agree this is the area that needs the most work. EPM is very difficult to diagnose because it looks like many other neurological disorders. Furr lists the three criteria that must be met for proper diagnosis:

  1. clinical signs that are consistent with EPM
  2. ruling out other diseases of the nervous system that have similar symptoms
  3. laboratory tests that show an immune response to the organism.

Performing diagnostics such as radiographs can be used to rule out other issues, but not to diagnose EPM. Some veterinarians do not want to, or are not able to, go to the diagnostic lengths necessary to rule out other issues. Rather, they treat the horse for EPM and monitor the response. In some cases it may be up to the horse owner to pursue vets who will work toward a complete diagnosis.

There are four tests available for EPM. They are:

  1. Western blot test—A positive result here means that the horse has been exposed to the parasite and has developed antibodies. It does not necessarily mean the horse is actively infected.
  2. Enzyme-linked immunosorbent assay (ELISA) test—This test measures the level of antibodies. Titers of 1:100 and up indicate exposure, and possible active infection if the horse also shows symptoms.
  3. Polymerase chain reaction (PCR) test—The PCR test is used to identify DNA fragments of S. neurona from neural tissue. It is typically run after other positive tests to confirm the diagnosis.
  4. Serum indirect fluorescent antibody test (IFAT)—This measures the antibody levels and also indicates exposure, but not necessarily an active infection. The IFAT test is one that can check for both organisms that cause EPM.

No test alone can confirm the disease, and unfortunately with any of the tests you can get a false positive. That makes it even more frustrating. One option that helps is the “EPM panel” offered by the laboratory at UC Davis Veterinary Medical Teaching Hospital. They will run both the Western blot and IFAT tests at the same time for one price.

But help might be on the way. There continues to be research on developing a test that accurately diagnoses an active infection and disease. William Saville, DVM, PhD, Dipl. ACVIM from Ohio State University’s College of Veterinary Medicine, has developed an IgM test that has worked well experimentally but has yet to go through trials.


Treatment is one area that has seen significant advancement. Medications are available that effectively treat EPM, but the earlier started, the better. Ponazuril (marketed as Marquis) was the first drug available, but Nitazoxanide (Navigator) and trimethoprim-sulfadiazine (Re-Balance) are also FDA-approved medications. While these treatments are only 60 to 65 percent effective, methods such as acupuncture, chiropractic and herbal remedies currently have no scientific support.


Ideally, prevention would occur through a vaccine against S. neurona. While there has been a lot of dedicated research in this area, no effective vaccine has been developed. Until that happens, good management practices are the best weapons we have. Keeping food containers closed and not accessible to wildlife is one such step.

Stressful events such as shipping can cause EPM to flare up. Pretreating with ponazuril has been shown to decrease risk, so you could pretreat prior to and during a stressful event. Remember, though, that most horses have been exposed to S. neurona, and the majority of them do not develop EPM.






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