Equine Protozoal Myeloencephalitis (EPM)

Equine Protozoal Myeloencephalitis (EPM) is a master of disguise. This is a serious disease, which attacks the horse’s central nervous system.  It can be difficult to diagnose because its signs often mimic other health problems in the horse and signs can range from mild to severe.

EPM is caused by one of two protozoal organisms, Sarcocystis neurona or Neospora hughesi, though almost all infections in the United States are caused by S. neurona. More than 50 percent of all U.S. horses have been exposed to S. neurona, which is carried by the opossum and spread through hay, water and pasture contaminated with opossum feces.  Fortunately, not all horses exposed to the parasite develop the disease. Many horses are able to mount an immune response to the protozoa before it is able to affect the central nervous system.  These horses will carry the antibodies generated by this immune response for life.  Future diagnostic testing will read these antibodies as a positive sign of the disease, complicating the interpretation of the test results. Researchers are unable to identify why some horses are able to successfully fight the protozoa and some are not.


If the protozoa are able to cross the blood-brain barrier and enter the central nervous system, they live inside cells in the central nervous system and are not able to be attacked by the immune system.  Changes in the body’s chemistry typically caused by significant or stressful events (trailering, high levels of athletic performance, injury, pregnancy, poor nutrition/weight loss, etc) cause the protozoa to reproduce and the stressed immune system cannot stop their spread into other areas of the brain or spinal cord.  As they spread they cause lesions and inflammation that cause the clinical signs of EPM.  The amount of time from horse consuming the protozoa and clinical disease can be anywhere from weeks to two years.


The clinical signs of EPM can be quite varied, depending on the severity and location of the lesions that develop in the brain, brain stem or spinal cord. Clinical signs are usually asymmetrical (not the same on both sides of the horse). The American Association of Equine Practitioners (AAEP) uses the following checklist of symptoms from when assessing a  horse’s condition for the possibility of EPM:


  • Ataxia (incoordination), spasticity (stiff, stilted movements), abnormal gait or lameness.

  • Incoordination and weakness which worsens when going up or down hills or when head is elevated.

  • Muscle atrophy, most noticeable along the topline or in the large muscles of the hindquarters, but can sometimes involve the muscles of the face or front limbs.

  • Paralysis of muscles of the eyes, face or mouth, evident by drooping eyes, ears or lips.

  • Difficulty swallowing.

  • Abnormal sweating.

  • Loss of sensation along the face, neck or body.

  • Head tilt with poor balance; horse may assume a splay-footed stance or lean against stall walls for support.

  • Seizures or collapse.


If left undiagnosed and untreated, EPM can cause devastating and lasting neurological damage.  Contact your veterinarian immediately if you suspect your horse has developed signs of a neurological disease.  The sooner a diagnosis is made and treatment begins, the better the horse’s chances for recovery.  For more information on methods of prevention and the treatment options (there are many options, opinions, and varied results for all), contact your veterinarian.