Equine protozoal myeloencephalitis (EPM) is a progressive, degenerative disease of the central nervous system of the horse. The disease is characterized by asymmetrical ataxia with or without muscle atrophy.


EPM is a neurological disease that occurs when protozoal parasites infect and invade the central nervous system. 

At least two protozoal parasites cause EPM: Sarcocystis neurona and less commonly, Neospora hughesi. 

EPM infection results in characteristic lesions in the brain and spinal cord that are evident during necropsy. The presence of these lesions correlates well with the clinical signs generally attributed to EPM (incoordination, muscle atrophy, etc). 

The horse is considered a dead end host for S. neurona, meaning that it cannot transmit the disease to other horses. 

Opossums have been identified as the definitive host for S. neurona and they shed the infective egg-like stages (sporocysts) in their feces. Horses become infected by ingesting food or water that has been contaminated with opossum feces containing the infective sporocysts. 

Once ingested by a horse, the sporocysts migrate from the intestinal tract into the bloodstream and cross the blood/brain barrier. There, they begin to attack the horse's central nervous system. 

The onset of the disease may be slow or sudden and the signs vary depending on the type of damage to the central nervous system. If left undiagnosed and untreated, EPM can cause devastating and lasting neurological deficits. 

Most current diagnostic tests are based on the presence of antibodies to the parasites that cause EPM. The presence of antibodies only means that the horse has been exposed to these organisms. Not all horses that are exposed actually come down with the disease as some horses mount an immune response and are not affected by the organisms

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