Equine protozoal myeloencephalitis (EPM) is a serious neurological disease and a common cause of ataxia (lack of coordinated movements) and weakness in horses. The causative agent of EPM is Sarcocystis neurona, a protozoan parasite that infects the central nervous system of horses. Recent evidence indicates that the seroprevalence (presence of antibodies in the blood against S. neurona) of infection in some areas of the United States is greater than 50% of horses. It is important to stress to horse owners that while exposure to S. neurona is very high in certain regions of the United States, only a small percentage (less than 1%) of the horses exposed will develop clinical signs.

EPM tends to occur more commonly in warm and temperate areas of the United States that also have a reasonable rainfall. Areas that are very cold or dry have a very low prevalence of EPM. In one study at The Ohio State University, researchers noted that horses admitted for neurological diseases were three times more likely to have EPM when examined during the summer months, and six times more likely to have EPM when admitted in the fall compared to winter. The authors also noted that the risk factors of horses which resided on premises located near a creek or river were about half that of horses which did not reside around running water.

The risk of developing EPM was about three times greater among horses whose hay and feed were not kept secure from wildlife compared to the horses whose feed was stored completely secure. It is speculated that a creek or river might provide a preferred habitat for opossums, thereby luring them away from the barn and feed supply. They also noted a higher risk when feed was not secured because S. neurona is shed in the feces of opossums, and horses are subsequently exposed through contaminated food and water.

As an owner of a suspect EPM horse, you must understand that having your veterinarian submit a serum sample for Western blot analysis (looking for antibodies against S. neurona) has very little diagnostic value unless a negative result is obtained—this test has a high negative predictive value for EPM (the chances of your horse having EPM with a negative test result is less than 10%).

It is critical to have your veterinarian determine if the horse is truly neurologic, or perhaps is suffering from a musculoskeletal disorder. At the time of diagnosis, your veterinarian will be able to determine if the horse is suffering from spinal cord, brain stem, or forebrain disease. Your veterinarian then will determine if you should pursue the diagnostics for EPM.

There are a lot of neurological diseases to which horses can fall victim that could easily be mistaken for EPM. To help aid in the diagnosis of EPM, the cerebrospinal fluid (CSF) is obtained and a Western blot test for antibodies against S. neurona is performed. The CSF results can only be interpreted in light of the examination of the whole horse. Horses can have a transient neural phase (show antibodies only for a short time) that they clear, therefore having a positive Western blot with no active disease and no long-term consequences.

EPM is a very difficult disease to understand, with many facets of its pathogenesis being currently investigated. Further diagnostics for this disease are being investigated, which will hopefully decrease the confusion when interpreting results. It cannot be stressed enough that the value of determining that a horse has EPM when considering management is critical to the long-term outcome of the patient. If the patient is treated without diagnostics, other diseases might be overlooked. In addition, a large expense can be incurred while treating what might be the wrong disease. Although treatment is gener-ally considered to be safe, there can be complications from therapy.

EPM is found throughout the United States, with a higher prevalence in certain regions. We must remember that it is impossible to obtain a definitive diagnosis of EPM with our current diagnostics. Research is currently being conducted with the development of new diagnostics for EPM. What owners can do to aid in the prevention of EPM is keep horse feed secure from wildlife. Currently there is a vaccine on the market for EPM, and its clinical efficacy is being investigated.

EPM is here, and will continue to be a disease that frustrates owners as well as veterinarians in our community.

About the Author

Nathan M. Slovis, DVM, Dipl. ACVIM, CHT

Nathan M. Slovis DVM, Dipl. ACVIM, CHT, received his DVM from Purdue University. He is board certified in large animal internal medicine and he is currently the Director of the McGee Medical Center at Hagyard Equine Medical Institute in Lexington, Ky. His special interests are in neonatology, infectious diseases, and hyperbaric medicine (in which he is certfied as a hyperbaric technologist).

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