Equine protozoal myeloencephalitis (EPM) was recognized in horses in the 1950’s and Sarcocystis neurona was isolated in 1991. EPM quickly became a very difficult disease for horse owners and veterinarians. The result of EPM can be devastating and catastrophic. Or, the disease can be mild and treatable. Equine protozoal myeloencephalitis is hard to recognize and difficult to diagnose. Proper treatment is equally problematic.
Pathogenes’ approach to making EPM a solvable issue is to dispel myths that pervade the topic.
Sarcocystis neurona is the most common cause of EPM in horses worldwide. It is a single celled parasite belonging to the group called coccidia (Apicomplexa: Sarcocystidae) with opossums as the definitive hosts and a variety of mammals as aberrant or natural intermediate hosts. Opossums shed infectious sporocysts, shown at the right.
What is very difficult about diagnosing EPM in the horse is the antibodies that can detect the response to infections appear when the infection is chronic. Active parasites are present early and disseminate quickly into organs and sometimes, the nervous system of the horse. Spreading of the parasite initiates immune responses that result in inflammation. The disease is biphasic, infection followed by inflammation. Inflammation is responsible for clinical signs in the horse. The horses that have active S. neurona merozoites or chronic inflammation can respond to treatment.
Sarcocystis fayeri is a parasite that naturally infects horses, one third of horses in the US are infected with this parasite! Sporocysts are shed by dogs, active parasites eventually get to the muscles in horses and form cysts. The cysts secrete a toxin, most likely when they breakdown in tissues, that make horses sick.
Horses with S. fayeri toxicosis can look just like horses with S. neurona-EPM! It isn't the muscle cysts, but the toxin released from the cysts, and dying muscle cells, that are associated with inflammation. Our tests use serum and are uniquely designed to distinguish between S. neurona, S. fayeri-toxin, and inflammation. The diseases caused by these three conditions are treated differently. It's important, and easy, to test, send serum with the Equine Consultation Form and have results the same day we get the sample.