Neurologic diseases can often be difficult to identify in their early stages unless you know some things to look for. One disease that presents with a variety of abnormal neurologic signs is caused by one or two protozoan organisms, and is referred to as equine protozoal myelitis (EPM). The two possible culprits that invade the nervous system are Sarcocystis neurona, which is the most common cause, 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 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 infective 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 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 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.