Equine Protozoal Myeloencephalitis (EPM) is a neurologic disease of horses that affects the brain and spinal cord. It is one of the most commonly discussed neurologic conditions in equine medicine and also one of the most misunderstood.
This overview is based on the American College of Veterinary Internal Medicine (ACVIM) consensus statement, which represents the most current, evidence-based guidance on EPM.
What Is EPM?
EPM is caused by microscopic protozoal parasites that can invade the central nervous system. The most common cause is Sarcocystis neurona. A second parasite, Neospora hughesi, can also cause EPM but is less common.
Importantly, many horses are exposed to these parasites during their lifetime, but only a small percentage develop neurologic disease.
Where Does EPM Come From?
The primary source of Sarcocystis neurona is the opossum. Horses become infected by ingesting feed or water contaminated with opossum manure. EPM is not contagious between horses.
Why Do Only Some Horses Get Sick?
A horse’s immune system plays a major role in preventing disease. Stressful events such as heavy training, transport, illness, injury, or surgery may increase risk, but stress alone does not cause EPM.
What Are the Signs of EPM?
Clinical signs vary widely and depend on which part of the nervous system is affected. Common signs include:
– Asymmetric weakness or incoordination
– Gait abnormalities that resemble lameness
– Muscle atrophy, often affecting one side
– Difficulty swallowing
– Head tilt or facial nerve deficits
Why Is EPM Hard to Diagnose?
There is no single definitive test for EPM in the live horse. Diagnosis relies on a thorough neurologic exam, ruling out other causes, and careful interpretation of blood and cerebrospinal fluid testing.
How Is EPM Treated?
Treatment involves FDA-approved antiprotozoal medications such as ponazuril, diclazuril, or sulfadiazine/pyrimethamine combinations. Duration of treatment depends on clinical improvement rather than test results alone.
What Is the Prognosis?
Many horses improve with early and appropriate treatment, although some may retain residual neurologic deficits. Relapses can occur and require reevaluation.
Can EPM Be Prevented?
Risk reduction includes limiting opossum access to feed and water, avoiding feeding on the ground, reducing stress when possible, and maintaining overall health.