Extension Logo
Extension Logo
University of Minnesota Extension

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.
  • Keep your horses healthy by storing grain in sealed bins and controlling opossum populations around your barn.

What is EPM?

EPM is a disease that affects the brain and spinal cord. It’s caused by the microbe, Sarcocystis neurona, commonly found in the opossum. Horses that come in contact with infected opossum feces can develop neurologic disease.

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

    • 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


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



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.


  • Keep grain in covered bins

  • Control opossum populations around your barn

Author: Annette McCoy, DVM, former student, College of Veterinary Medicine

Reviewed in 2021

Share this page:
Page survey

© 2023 Regents of the University of Minnesota. All rights reserved. The University of Minnesota is an equal opportunity educator and employer.