Equine Protozoal Myeloencephalitis (EPM) is an infectious disease of the horse’s central nervous system.

It is one of the most difficult diseases for veterinarians to diagnose because it often mimics other conditions and has a wide range of symptoms that affect multiple parts of the horse’s body.

Horses affected by EPM may show a lack of coordination in their movements, usually worse on one side of the body. Affected horses may also exhibit lameness, muscle loss, weakness, or drooping facial features.

EPM was first recognized in 1964 by a veterinarian in Kentucky. In the 1970s, researchers identified that a protozoa was the cause of EPM. Since that time, research has been ongoing to better understand this often-devastating neurological illness.

Researchers have found that EPM tends to occur sporadically, seldom involving more than one horse at a farm. [5] There are several FDA-approved treatments available that are effective if administered early.

This disease should never be taken lightly as it can be life-threatening. If you suspect that your horse has EPM, it’s important to have him examined by a veterinarian immediately.

Causes of EPM

Equine Protozoal Myeloencephalitis is caused by an infectious protozoa (a type of single celled organism). It is most commonly caused by Sarcocystis neurona and – more rarely – Neospora hughesi.

Both of these protozoa are carried by opossums. These animals pick up the protozoa by feeding on cat, raccoon, skunk, or armadillo carcasses, all of which act as intermediate hosts.

The protozoa are transmitted to horses when they consume forage, feed, or water that is contaminated by opossum feces containing the infective sporocysts.

Researchers aren’t quite sure how the protozoa gets into the horse’s central nervous system (CNS), but they suspect it enters the bloodstream and then crosses the blood-brain barrier to infect the brain and spinal cord. [9]

Once these sporocysts enter the CNS, they begin to attack the nervous system, causing one or several of many possible symptoms.

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Risk Factors

Horses in certain geographic locations – such as the eastern U.S. – are more likely to develop Equine Protozoal Myeloencephalitis.

Most EPM cases occur in the following states:

  • Kentucky
  • Michigan
  • Missouri
  • New Jersey
  • New York
  • Ohio
  • Pennsylvania
  • Tennessee

These areas are at greater risk because they have higher opossum populations and protozoa are better able to survive in the environment. [2][6]

Younger horses are also at higher risk of this condition and male horses are twice as likely to develop EPM compared to female horses.

Researchers have also found a higher incidence of EPM among certain breeds including standardbreds, Tennessee Walking Horses, Thoroughbreds, Warmbloods, and stallions. Quarter horses, other breeds, ponies, and drafts seem to have a lower incidence rate. [9]