Equine Protozoal Myeloencephalitis, Myelitis is the most common infectious cause of neurologic disease in horses. It is caused by one of two protozoal parasites (Sarcocystis neurona or Neospora hughesi), that attack the brain and/or spinal cord and damage the nerve tissue.

Sarcocystis neurona has been implicated in the disease for years. Neospora as a causative agent was discovered more recently. Sarcocystis has a complex life cycle involving the opossum as definitive host, so it is more prevalent in areas where opossums live.

Usually, a horse contracts Sarcocystis by ingesting contaminated opossum feces that have been deposited in water, feed, hay and pasture areas.

Signs of this disease are similar to those associated with other spinal cord disease disease and severity can range from mild to severe. Classic signs are wobbliness, weakness and areas of muscle loss. Often, signs appear asymmetrically, and are worse on one side.

A great deal is written about EPM and much can be found on the Internet. However, there is also a great deal not known about the disease. The hardest part of dealing with this disease is establishing a definitive diagnosis. There is a great deal of misdiagnosis surrounding EPM.

There are many limitations and uncertainties associated with this diagnosis. One of the greatest difficulties in diagnosing this disease is separating horses that have been exposed to the disease from those that truly have the disease. In addition, many cases are complicated by unclear or confusing laboratory results.

There currently are several different tests being used. We use the UC Davis IFAT test, which returns a likelihood of infection.

Your vet may lean toward this diagnosis after taking a careful history and performing a careful physical and neurologic exam. Usually the signs are worse in a localized area or on the left or right side. Antibody testing is performed on a blood sample, and cerebrospinal fluid may be taken and analyzed. Radiographs may be taken to rule out other possible diseases.