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Equine Protozoal Myeloencephalitis

Equine Protozoal Myeloencephalitis

Equine Protozoal Myeloencephalitis, more commonly known as EPM, is a rare disease that attacks the nervous system of horses. Researchers at Oregon State University's Equine Science Program estimate that 45 percent of Oregon’s horses have been exposed, with 60 percent or more exposed west of the Cascades. Fortunately, the actual rate of horses that become ill or die from EPM is still less than one percent.

Early detection will provide your horse with the best chance for recovery. Please contact your veterinarian for the latest recommended treatment if you suspect EPM in your horse.


Fast Facts

  • EPM can affect horses of any age, breed or gender.
  • EPM cannot be passed from horse to horse.
  • Caused by a protozoa, horses acquire EPM through pasture hay, grain, and water contaminated with opossum feces.
  • Early detection and treatment increases chance for recovery.
  • Some horses do not respond or relapse after treatment is discontinued.

Preventing EPM

  • Follow a regular worming and vaccination schedule.
  • Keep opossums and birds out of the barn.
  • Keep hay covered and away from opossum feces and bird droppings.
  • Clean up garbage and keep it securely covered.
  • Purchase hay and grain from reliable sources. Follow a regular feeding schedule. Use high-quality feed, especially hay.
  • Discard any old, leftover hay.
  • Give vitamin supplements, especially 2,000 IU (international units) of vitamin E daily. Supplement separately with selenium if you are feeding hay from a selenium-deficient area.
  • Minimize grain spillage.
  • Observe your horse. Watch how it moves. Look for atrophy of muscles, abnormal behavior or lameness.
  • Avoid high-stress situations. Provide shade when it's hot, cover when it's wet. Avoid excessive workouts, especially if the horse is in poor condition.

Signs of EPM

  • Uncharacteristic stumbling and falling
  • Weakness in a limb
  • Muscle atrophy
  • Peculiar hoof placement
  • Marked incoordination
  • Toe dragging and lameness

Diagnosing EPM

  • Response to treatment
  • Analysis of cerebrospinal fluid obtained from a spinal tap

Treating EPM

  • There are two treatment options for EPM.
    • A six-month course of antibiotic (trimethoprim-sulfonamide) and antiprotozoal agent (pyrimethamine).
    • A 28-day course of antiprotozoal (ponazuril). Horses may need a second round of ponazuril in some cases. This is the only FDA-approved treatment for EPM.
  • Depending on your horse’s condition, your horse may need general supportive care.
  • Consult your veterinarian about treatment options.

Updated: June 1, 2015