HISTORY DIAGNOSIS AND TREATMENT OF EQUINE PROTOZOAL MYELOENCEPHALITIS. Equine Protozoal Myeloencepalitis is a serious neurological disease in horses caused by a parasite protozoa thought to be sarcosystis neurona. The disease was first identified in the 1960’s when lesions and inflammation were seen in the brain and spinal cords of horses that had died of severe neurologic disease. Protozoa were discovered on the lesions in 1974, however the vector was unknown and the disease considered rare. Recently the opossum has been isolated as the probable vector and the likely parasite organism identified as Sarcosystis falcatula. (Fenger, 1996)The Sarcosystis protozoan parasites have a complicated life cycle. They exist in two distinct forms. One in the definitive host (a carnivore like the opossum) and one in a secondary or intermediate host (wild birds that the opossum feeds on). It reproduces in the digestive tract of the definitive host and passes infective sporocysts in the feces, which are then taken in by the secondary host. There the sporocysts migrate to the muscles and can persist for years until the secondary host is eaten by the definitive host. The protozoa are usually very host specific (requiring a specific carnivore to reproduce), and most likely neither host show any clinical signs of disease. (U. Missouri… 1999)The horse is an unfortunate casualty in this cycle; it is infected when feces of the opossum are ingested with their food material. The horse is a dead-end host, meaning it is not required in the parasite’s life cycle and therefore the disease cannot be transmitted from horse to horse. Several interesting facts put forth by the University of Missouri: EPM is seen only in parts of the world that are inhabited by opossum specifically the Americas. It has been estimated that seroprevalence (indicating exposure) in Pennsylvania, Ohio, and Kentucky is as high as 50%, however only a small percentage (2-3%...
