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EPM and Horses: 9 Fast FactsBy Kentucky Equine Research Staff · March 30, 2017

Equine protozoal myeloencephalitis (EPM) continues to plague horses, causing neurological deficits that threaten athletic careers and lives. EPM still poses diagnostic challenges and can be difficult to differentiate from other neurological diseases. Improve your understanding of the disease*,**, and learn how to feed horses to minimize the chance of infection.

  • Infection with one of two parasites, either Sarcocystis neurona or Neospora hughesi, causes EPM. Most discussions pertain to S. neurona because less information about N. hughesi currently exists.
  • Many horses (up to 85%) have antibodies against S. neurona. This indicates that horses have been exposed to the parasite, though infection did not necessarily take hold. The immune system is capable of fighting off the parasite without evidence of an active infection.
  • Horses become infected by consuming opossum feces containing S. neurona. The parasite then migrates through the horse’s body before taking residence in the central nervous system (CNS)—the brain and/or spinal cord. Because each parasite essentially “lands” in a different part of the CNS, the clinical signs an infected horse shows end up being completely individual.

“Creating a opossum-free pasture may not be feasible for most equine operations; however, feeding off the ground and preventing opossums from accessing the barn and the feed room by ensuring hay, concentrates, and other feeds are out of the opossum’s reach and sealed in bins with tight-fighting lids will certainly help exposure to S. neurona,” advised Kathleen Crandell, Ph.D., a nutritionist for Kentucky Equine Research (KER).

  • The vast and varied clinical signs of EPM include the following: incoordination, weakness, spasticity, decreased reflexes, depression, head tilt, facial nerve paralysis, difficulty swallowing, upper airway dysfunction, and seizures, to name several but not all.
  • EPM remains a “diagnosis of exclusion,” which means that all other causes of neurologic disease must be excluded before making a presumptive diagnosis of EPM. Even with the available tests, diagnosing EPM with complete certainty can only be achieved by visualizing the parasite within the tissues of the CNS, which can only be achieved through necropsy.
  • Horses presumptively diagnosed with EPM may be treated with one of three FDA-approved medications.
  • Other treatments include compounded medications (e.g., decoquinate, levamisole), as well as anti-inflammatory medications such as flunixin meglumine and DMSO.
  • Secondary oxidative damage also occurs, which explains why vitamin E, a natural antioxidant, has been advocated for horses with EPM.

“Experts recommend administering between 5,000 and 10,000 IU of natural vitamin E per day for horses with EPM. Nano•E, a natural form of vitamin E that is water-soluble and rapidly absorbed, is often recommended by veterinarians for use in horses with EPM,” shared Crandell.

  • Treated horses may show some residual signs of neurological deficits due to permanent damage to the CNS caused by the parasite.

*Reed, S.M., M. Furr, D.K. Howe, et al. 2016. Equine protozoal myeloencephalitis: An updated consensus statement with a focus on parasite biology, diagnosis, treatment, and prevention. Journal of Veterinary Internal Medicine 30(2):491–502.

**Pusterla, N., and T. Tobin. Therapeutics for equine protozoal myeloencephalitis. Veterinary Clinics of North America. Equine Practice. In press.