Equine Protozal Myelitis (EPM) is one neurologic disease that can be difficult to diagnose. Without rapid veterinary intervention, the horse’s chances for recovery or even survival are at risk.

EPM is caused by two protozoan organisms that invade the nervous system. They are Sarcocystis neurona, which is the most common, and Neospora hughesi, a more sporadic cause. However, researchers found that nearly one-third of 5,200 healthy horses tested positive for Neospora hughesi. 

Sarcosystis neurona needs an intermediary host before infection can be transmitted to a horse. Certain geographic areas are more likely to increase the risk of exposure based on the presence of specific host reservoir animals. The most significant reservoir for EPM is the opossum, although other animals eaten by opossums have been implicated in the transmission cycle. Those include raccoons, skunks, armadillos, sea otters, and cats. An opossum passes sporocysts in their feces, which then might contaminate a horse’s feed, pasture, or water to be ingested by the horse. From the intestines, the protozoa enter the bloodstream, cross the blood-brain barrier, and attack the central nervous system. 

There is no transmission possible from horse to horse, and not all horses exposed to the protozoa will develop the disease, possibly due to host immune factors. Exposure might be identified with blood testing, but a positive blood test does not confirm that a horse has developed the neurologic disease. The blood test is useful only as a screening tool. If the blood test is negative, there is little likelihood that the horse has been exposed and not at all likely to be infected. If the blood test comes back positive, then further diagnostic testing of cerebral spinal fluid might be indicated when the horse is showing clinical signs. The titer of the cerebral spinal fluid is compared to the blood titer to determine a ratio that might then indicate active disease.

It is estimated that clinical disease from EPM occurs in only about 1% of horses exposed to the protozoal organisms. 

By Nancy S. Loving, DVM