If a horse suddenly develops weakness, lameness, has seizures and vision problems, he may be infected with Sarcocystis neurona protozoa which causes the fatal equine protozoal myeloencephalitis (EPM). Horses ingest the protozoal sporocysts in contaminated feed and pastures. A major source of infection is opossum feces, contaminating feed and pasture. The protozoa invade the tissue of the central nervous system, causing fatal neurologic damage. The initial signs are loss of appetite, fever, diarrhea, which are quickly followed by incoordination and head-pressing.
The key to diagnosis is recognition of subtle changes in the horse's stance and way of traveling: toe dragging when tired, mild lameness in the hind legs, back soreness, choppiness of gait, or awkward stance. As the disease gets worse, the ears or lips droop on one side, the head tilts, and the animal leans to one side. Horses in early stages of the disease are more likely to respond to treatment.
Sarcocystis neurona is a two-host parasite, the secondary hosts being the armadillo, the skunk, the raccoon, and the cat with the horse as the end host. Mainly, EPM occurs in horses under the age of 5. There are no licensed vaccines available.