Brain and Spinal Cord Infection in Horses

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Equine Protozoal Myeloencephalitis

Equine Protozoal Myeloencephalitis, or EPM for short, is a disease that affects a horse's nervous system, commonly displayed as incoordination of the limbs, muscle atrophy, or lameness. EPM appears to be a condition strictly located in the western hemisphere. EPM is a serious disease but signs can sometimes develop slowly and be difficult to recognize. However, once diagnosed, this disease should be treated as soon as possible to prevent further neurological damage.


Since EPM is a neurologic disease, affected horses will display a variety of neurological symptoms that can be disturbing to a horse owner; among them:

  • Lameness
  • Loss of muscle movement coordination (ataxia)
  • Paralysis of the lips/ears
  • Droopiness in the eyelids
  • Difficulty eating (i.e., inability to chew or swallow food)
  • Muscle atrophy
  • Weakness
  • Seizures (very rare)


EPM is an infection due to the single-celled protozoal organism Sarcocystis neurona. This organism survives in the environment through its natural host, the opossum. In the body of the opossum, this protozoa undergoes several complex stages of reproduction. Its eggs, called sporocysts, are released into the environment through the feces of the opossum and ingested by other animals such as raccoons, armadillos, and even cats.

Each of these animals are called intermediate hosts, since they are necessary for the further development of the protozoa. Sarcocystis neurona does not cause harm to either the opossum or these other intermediate hosts. However, if a horse consumes infected fecal material from an opossum, the horse becomes an aberrant host, meaning it is not the correct host for this protozoa.

As such, the protozoa causes problems in the equine species. Horses are not able to pass the infection along to another horse, because the protozoa is unable to continue its development in the body of the horse. Once in the horse, this protozoa migrates to the nervous tissue in the spinal cord and occasionally the brain stem, where it causes severe inflammation and damage.


Diagnosis of this disease can be challenging. Serum samples taken from your horse can detect the presence of antibodies against this organism, yet if present, these antibodies only indicate exposure and not necessarily active infection. A CSF tap (cerebral spinal fluid) may also help indicate infection. A few other laboratory tests are available and each comes with its own set of false positives and false negatives. Your veterinarian will typically rule out the numerous other neurological conditions before conducting tests to diagnose EPM in the animal.

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