Equine protozoal myeloencephalitis

Equine protozoal myeloencephalitis (EPM), is a disease caused by the apicomplexan parasite Sarcocystis neurona that affects the central nervous system of horses.

EPM was first discovered in the 1960s by the American biologist Dr. Jim Rooney. The disease is considered rare, though recently, an increasing number of cases have been reported. Previous research identified the "barn cat" as the definitive host of the disease. However, since that time it has been learned that the definitive host is a number of wildlife not only opossum, while any of a number of mammals can serve as intermediate hosts in the disease's two-host life-cycle. Those with horses should not panic and kill opossums or wildlife rather keep feed covered and stalls clean.

The term EPM refers to the clinical neurologic symptoms caused by the parasite, not infection itself. The majority of horses infected with S. neurona do not exhibit neurologic symptoms consistent with EPM. There are six subspecies of S. neurona which can be identified by surface antigens (SAG). Equine EPM is caused by the parasites that exhibit SAG1, SAG5, and SAG6. SAG1 and SAG5 are responsible for the majority of EPM cases in horses. Horses produce antibodies to these surface antigens. Serum antibody testing is available that measures levels of these antibodies in the blood of horses, which is helpful in diagnosing EPM in an ataxic horse. Serial blood levels are helpful in guiding treatment. In experimentally infected horses it takes 14 days from infection to positive antibody tests. 80% of horses with EPM have positive antibody tests. A negative antibody test in the presence of EPM results if testing is done before 17 days or if the horse has been treated with antiprotozoal drugs which delays antibody production.

EPM is caused by the parasite Sarcocystis neurona. The life cycle of S. neurona is well described. In order to complete its life cycle this parasite needs two hosts, a definitive and an intermediate. In the laboratory, raccoons, cats, armadillos, skunks, and sea otters have been shown to be intermediate hosts. The opossum is the definitive host of the disease, passing the parasite through feces. Horses contract EPM from contaminated feed or water. So killing other wildlife is counterproductive, cleaning your barn and covering feed is how to prevent this disease. However, horses cannot pass the disease among themselves, that is, one horse cannot contract the disease from another infected horse. The horse is a dead-end, or aberrant, host of the parasite.

...
Wikipedia