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Equine protozoal myeloencephalitis (EPM)

Quick facts

  • EPM is a neurologic disease that horses get from eating infected opossum feces.
  • Incoordination, muscle atrophy and loss of feeling around the body are a few signs of illness.
  • Storing feed in animal proof containers and keeping your barn tidy and free of openings can help prevent opossum from making your place their home.

What is EPM?

Equine protozoal myeloencephalitis, or EPM, is a disease that affects the brain and spinal cord. It’s caused by the microbe, Sarcocystis neurona, commonly found in the opossum. Research in other states suggests that about 10 to 33 percent of opossums are infected with the microbe in Michigan and Missouri. Horses that come in contact with infected opossum feces can develop neurologic disease. While EPM is considered rare, it is a serious disease that some horses may not survive even with treatment.

Other hosts of this microbe include

  • Armadillos

  • Skunks

  • Domestic cats

Unlike the opossum, these hosts can’t directly pass the disease to horses.

Signs of illness

Many of the EPM signs mimic other neurologic disease or may come and go.

  • Incoordination; stiff, stilted movements; abnormal gait or lameness

  • Incoordination and weakness

    • This worsens when going up or down slopes or when the head is up

  • Muscle atrophy

    • Most noticeable along the topline or in the hindquarters


Two EPM tests are available for diagnosis. Each of them has pros and cons, which you must take into consideration when deciding which to perform.

Serum antibody test

This blood test allows your veterinarian to look for any S. neurona antibodies. If none are present, your horse doesn’t have the disease. If these antibodies are present, it means your horse was exposed to the microbe sometime in its life. It doesn’t mean your horse has the disease.

About 50 to 60 percent of horses have been exposed to S. neurona but only 0.14 percent develop the disease.

Spinal tap

Spinal tapping allows your veterinarian to look at your horse’s cerebrospinal fluid (CSF).  A spinal tap better detects infection but it’s more invasive than a blood test. Blood contamination of the CSF sample can result in false positive tests.

  • Sometimes involves the face or front limb muscles

  • Drooping eyes, ears or lips

  • Difficulty swallowing

  • Seizures or collapse

  • Abnormal sweating

  • Loss of feeling along the face, neck or body

  • Head tilt with poor balance

    • May stand splay-footed or lean against stall walls for support


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.


About 60 to 70 percent of horses treated for EPM will improve, and 15 to 25 percent will recover completely. Starting treatment early will lead to the best results. The greatest amount of improvement is seen within the first four weeks.

About 80 percent of horses will remain positive on the spinal tap despite treatment. They may not even show signs of illness. Relapses occur within two years in about 10 to 20 percent of these horses.


Opossums are important members of our wildlife ecosystem and can eat large numbers of ticks, which can also cause disease in horses. 

Precautions to prevent EPM include

  • Keep feed in enclosed containers to prevent access by opossums and other wildlife.
  • Sanitation is key. Clutter tends to invite rodents.
  • Regularly check your facility, especially feed rooms, for signs of wildlife presence including denning, feces, and evidence of chewed material.
  • Eliminate external entrances for rodents. Steel wool can be used as a temporary plug to deter rodents.
  • Stack firewood off the ground (in racks or on pallets) and frequently inspect piles for signs of denning. It’s best to keep wood piles away from house or barn areas.
  • Farm garbage can also attract wildlife and should be secured.
  • Food for barn cats and dogs will attract unwanted visitors. It is best to feed pets in areas away from where feed is stored and horses are fed.

Authors: Annette McCoy, DVM, and Anna Firshman, College of Veterinary Medicine

Reviewed in 2024

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