Equine Protozoal Myelitis (EPM)

By: Megan Slamka, DVM

What is Equine Protozoal Myelitis (EPM) and how can my horse get this?

EPM pic 1EPM is a neurologic disease characterized by ataxia or incoordination in horses caused by ingestion of the parasite Sarcocystis neurona.  Opossums shed S. neurona in their feces which may contaminate your horse’s environment.  If your horse subsequently ingests the organism, they may develop EPM. Horses under high amounts of stress or with other disease syndromes like Cushing’s, are at the most risk for developing clinical signs. Performance horses between the ages of 15 months and 4 years are more often affected. There is no known genetic predisposition and EPM appears less common during winter months. Opposums carry S. Neurona in their feces, horses ingest the parasite and it travels to their brain and spinal cord where it causes damage. 

What are the clinical signs?

Upon ingestion, S. neurona can migrate to the spinal cord and brain causing extensive EPM pic 2damage. The signs vary depending on which portion of the brain or spinal cord is affected. Horses are often bright and alert, but may appear uncoordinated or ataxic. Ataxia is the most common clinical sign. Other signs include muscle weakness, decrease in muscle mass and back soreness. The horse may have difficulty eating, a drooping lip, a head tilt or dribbling of urine all related to further nerve damage.

How is EPM diagnosed and how can we treat it?

Diagnosis of EPM can be difficult, due to ataxia being mistaken as lameness or a spinal injury. Your veterinarian will take a thorough history and rule out such things as trauma, nutritional deficits, toxicities or other infectious causes of neurologic disease. There are many blood tests to detect EPM that detect antibodies to the parasite. One of the most recommended diagnostic test is the Immunofluorescent Antibody test (IFA) on cerebrospinal fluid. The IFA is the most sensitive and specific test for S. neurona. This means you can better trust the test results are accurately telling you your horse is positive or negative. Cerbrospinal fluid is collected by performing a spinal tap.

Treatment is aimed at providing nursing care as well as using antiprotozoals to treat the infection. Severely effected horses may need to be put in a sling. They may need protective gear and nutritional modifications as well as deep bedding. There are several different treatments available, and the option that your veterinarian chooses will be based on the symptoms that your horse is showing.

What is the prognosis? and How can we prevent it?

Most horse’s neurological deficits improve. However, full recovery occurs in less than 25% of affected horses. Improvement is often observed within the first week of therapy and increases steadily. Performance animals should stay out of training while undergoing therapy. Unfortunately, chronic signs such as muscle atrophy rarely ever improve.

As with most life threatening disease, prevention is key.

Preventative measures include:

  • Cover feed in storage bins
  • Don’t leave grain uncovered for opossums to get into
  • Elevate hay off the ground to prevent contamination from feces on the ground
  • Minimize stressful scenarios such as long trailer rides that may make your horse more susceptible
  • Use good standards of hygiene such as regular cleaning of hay and water troughs


  • EPM is a parasitic disease that causes neurologic signs such as ataxia in horses.
  • Horses become infected by ingesting the parasite which is transmitted by opossum feces.
  • Treatment includes Ponazuril for 28 days along with supportive care to prevent self injury.
  • Prognosis is good for survival, but poor for full return to previous level of performance.
  • Prevention involves minimizing overtly stressful scenarios that may make your horse more susceptible and minimizing contact with opossum feces.