Sarcocystis neurona Life Cycle

Horses are infected by ingesting infective S. neurona sporocysts. Sarcocystis neurona has an obligate 2-host species life cycle, including the natural intermediate host which is a prey species of some kind, and a definitive host, the opossum. Until recently, S. neurona was considered likely to be S. falcatula, a species of Sarcocystis that utilizes birds of passeroroid (grackles, cowbirds, starlings), psittacorid (budgerigars) or columborid (pigeons) birds as intermediate hosts, but recent research conducted independantly at the University of Florida and Cornell University have revealed that S. neurona and S. falcatula are not the same species. However, the opossum (Didelphis virginiana) is the definitive host for both species, the opossum is the source of the infective sporocysts to horses. The parasite encysts in the muscle tissue of the intermediate host. When this tissue is eaten by opossums, the organism undergoes sexual reproduction in intestinal epithelium, and forms infective sporocysts contained within an oocyst. Oocysts and sporocysts are found in the intestinal contents but the fragile oocyst is commonly disrupted by the time feces are passed. The intermediate host become infected by ingesting sporocysts which presumably contaminate the feed or water. Sporozoites emerge from the sporocysts, penetrate the intestines, and become tachyzoites (merozoites), which undergo a series of replicative cycles in the vascular endothelial cells, and possibly white blood cells. The protozoa enter host cells, and become the intracellular stage, the schizont, or meront. This schizont is a "mother cell" that divides asexually into many tachyzoite offspring.

In the natural intermediate hosts, later generations of tachyzoites migrate to muscle, and encyst in myocytes, forming sarcocysts. These later generations of tachyzoites develop into a slowly dividing stage, called bradyzoites. Bradyzoites divide slowly over the course of the host's lifetime. Some species of Sarcocystis cause myositis in the intermediate host (e.g. eosinophilic myositis caused by S. cruzi in cattle) while others evade the host immune system, and remain asymptomatic (e.g. S. fayeri in horses is rarely associated with myositis). When the natural intermediate host is killed, or dies and is eaten, these cysts are activated in the gastrointestinal tract of the definitive host, and the cycle starts again. Most Sarcocystis spp. are found encysted in the muscle tissue of the intermediate host, but some Sarcocystis spp., as well as the closely related protozoa, Toxoplasma gondii, have been found in cysts in the central nervous system.

Horses are an aberrant intermediate host of S. neurona. Sporocysts are eaten, pass into the small intestines and excyst in the horse. From there, the infective stage of the organism, the sporozoites, enter the horse's blood stream. In some horses, they undergo several replicative cycles in endothelial cells (in blood vessels), becoming tachyzoites, and migrate to the central nervous system. They replicate asexually within neurons and microglial cells, without forming tissue cysts. In the central nervous system of the horse, they slowly divide and grow, gradually destroying the nervous tissue, causing incoordination and the other clinical signs that result from EPM. The stage of the organism found horses cannot be transmitted to other horses. Because the organism does not encyst in the tissues, it cannot be transmitted to opossums, even if the opossum were to eat the tissue. Therefore, the horse is a dead end host for the protozoan.

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