Equine Protozoal Myeloencephalitis (EPM) in Horses

Equine protozoal myeloencephalitis (EPM) is a devastating neurological disease that can leave horses severely disabled and may result in death. It is caused by infection of the horse's central nervous system with a protozoan, which is a single-celled organism. The most common protozoan to cause EPM is Sarcocystis neurona. Less commonly, it can be caused by infection with Neospora hughesi.1

Because EPM can affect any part of the horse's central nervous system, which includes the brain and the spinal cord, symptoms can vary, but common signs of this disorder include gait abnormalities, involuntary muscle movements, lethargy, muscle atrophy, and difficulty swallowing.

Any horse of any age can develop EPM, but it is more common in young horses that race or participate in other athletic activities.1

What Is Equine Protozoal Myeloencephalitis?

Equine protozoal myeloencephalitis (EPM) is a neurological disease that affects the central nervous system of horses. The term "myeloencephalitis" means inflammation of the spinal cord and brain. The disease is caused by a protozoan, usually Sarcocystis neurona. This protozoan's primary host is the opossum. The feces of an infected opossum can contain sporocysts, which are the immature, egg-like form of the protozoan. If a horse ingests grass, water, or other substances containing the sporocysts, the horse may develop EPM.

For reasons that are not well understood, however, only a very tiny percentage of horses exposed to sporocysts from Sarcocystis neurona actually develop EPM. In fact, although as many as 90 percent of the horses in some areas of North America test positive for exposure to the sporocysts, less than 1 percent actually develop the disease.2

Symptoms of EPM in Horses

One of the difficulties with diagnosing EPM is that it can look like many other neurological diseases, particularly in the beginning. It is a progressive disease, meaning it tends to worsen over time. However, some horses progress through the illness faster than others, and it is not uncommon for a horse to develop worsening symptoms, then level off for awhile, sometimes even for months, before once again beginning to deteriorate.2