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What is EPM?

Know the signs of EPM when conducting a prepurchase exam on a potential new horse.

From AQHA Corporate Partner Merial and Dr. Sarah Reuss


My trainer told me I should ask for an EPM test as part of the prepurchase exam on a horse I’m considering. I don’t know what EPM is or if that is a good idea?


Equine protozoal myeloencephalitis is a neurologic disease in horses in North and South America. The disease happens when the protozoa Sarcocystis neurona or Neospora hughesi infect the central nervous system. It may result in a variety of signs, including tripping, stumbling, incoordination, muscle loss, difficulty swallowing or blinking, or inability to stand.

Looking at horses around the country, upwards of 90% of horses have been exposed to these protozoa. Fortunately, very few horses (less than 1%) of exposed horses will develop infection of the central nervous system and signs of disease. Blood tests in normal horses are not helpful when it comes to EPM. When exposed to any infectious organism (bacteria, virus or protozoa), the horse’s immune system makes antibodies against that organism. Those antibodies are what is measured by the various EPM serologic tests. For EPM, you may have heard of either an ELISA or IFAT test. When run on blood, a positive serologic test only tells you that the horse was exposed to the protozoa.

If the horse has the actual disease, it will have neurologic abnormalities that should be apparent during the routine prepurchase exam performed by your veterinarian. If tripping, incoordination or other neurologic abnormalities are observed, you will have to seriously consider whether you want to purchase that horse. To confirm a diagnosis of EPM in a horse with neurologic abnormalities, the best test is to take a sample of the cerebrospinal fluid (CSF) and look for antibodies in that fluid. Antibodies in the CSF will be high if the protozoa has truly infected the central nervous system. I strongly advise against blood testing in neurologically normal horses as the test result is not an indication of whether the horse has had or is likely to develop the clinical signs associated with EPM.

From our veterinarian, Sarah Reuss, VMD, DACVIM, Merial Large Animal Veterinary Services, Internal Medicine.

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