Equine protozoal myeloencephalitis (EPM), caused by the protozoan Sarcocystis neurona and occasionally other protozoans, is one of the most important neurologic diseases in the horse. Equine protozoal myeloencephalitis (EPM) has been a diagnostic problem in horses. Usually, careful assessment of clinical signs and history will lead you to a short list of probable causes. Recent advances in serological techniques have increased the accuracy of correctly diagnosing this disease or ruling it out. There is much new information on EPM and a conditional vaccine is on the market. In this article we will discuss the clinical signs characteristic of EPM, life cycles, current diagnostic tests, treatment, the vaccine, and other prevention aids. Also, links to past forum discussions and other information on the internet is provided.

Note there are other protozoal infections that cause EPM but are far less common. Unless specifically noted below EPM is synonymous.with neurological disease cause by S. neurona.

EPM occurs throughout the Americas but not elsewhere because of the limited range of the definitive host the opossum. There are no sex predilections but Thoroughbreds and Standardbreds are affected more frequently but it can happen in any breed. Several risk factors have been identified that seem to stem from immune competence and the risk of exposure to possum feces:

• Less than 5 years of age or more than 13 years of age. Several studies have shown that young horses under 5 years of age have much greater rate of developing disease from EPM than older horses. In one study it was a four time greater likelihood of developing EPM than horses between 5 and 20 yr of age. There may be several reasons for this but one is that of the stress of beginning training and the stress of transporting may be tougher on young unexperienced horses. Growing foals seem less likely to have the disease possibly do to maternal antibody protection.
• Greater incidence of disease during the summer and spring. Presumably the opossum are more active during these times of years so they are more likely to wonder into the pasture.
• Previous cases recognized on the farm. EPM often occurs as a isolated events, but several cases can occur in a single location.
• Presence of opossums, skunks or other small wild animals on a farm.
• Availability of the livestocks water and feed to wild animals. Such management practices are sure to attract wildlife.
• Presence of woods in or close to the pastures. Conversely the presence of a stream or river nearby lowered the incidence of EPM possibly by providing the opossum a source of water off the farm.