- Equine Protozoal Myeloencephalitis (EPM) is a neurologic disease of horses caused by the protozoan Sarcocystis neurona. This disease agent is common throughout most of the continental United States.
- Some serological surveys suggest that as many as 50 percent of the horses in the U.S. may have been exposed to this disease-causing organism. However, the number of horses that come down with disease is low. Outbreaks of the disease are typically sporadic and isolated. It can be attributed to levels of stress in the horses or infection patterns as dictated by the source of the infection, the opossum.
- The opossum acts as the definitive host for the disease. It transmits the infectious agent by shedding it through its feces. Horses usually become exposed when they ingest contaminated feed, water, hay or pasture forage. This disease can affect horses of any age or breed, but the horse becomes a dead end host and cannot continue to transmit the disease.
- Signs can range from subtle incoordination, weakness, poor balance or stability, droopy lip or eyelid, muscle atrophy, dragging a toe, complete paralysis and death.Once inside the horse’s body, the protozoan eventually moves to the brain or spinal cord and attacks the central nervous system causing neurologic signs to develop. Clinical signs most commonly seen with the disease typically begin with a slow onset but can appear acutely. These signs may be asymmetric and can be varied depending upon what part of the brain or spinal cord the organism has infected.
- Diagnosis of EPM can be difficult. Since many horses are exposed to the protozoan, many will already have a positive blood test. Other testing can be performed on the fluid from the spinal column.
- However, since spinal taps can be of some risk and difficult to perform, many times treatment is begun based on a positive blood test plus the demonstration of clinical signs.
- Treatment for EPM can be expensive, but it has been shown to be effective against the disease.