The Life Cycle of Sarcocystis Neurona

Definitive Host

• The life cycle of S. neurona begins in the definitive host of an opossum, of either the d. virginiana or d. albiventris species. The animal passes both oocysts and sporocysts of the protozoan in its feces.

Intermediate Hosts--Initial Infection

• Intermediate hosts, including raccoons, otters, skunks, armadillos and cats, then ingest the sporocysts. Horses can also acquire the disease. The ingested sporocysts release sporozoites within the intestinal tract of the intermediate host.

Intermediate Hosts--Reproduction

• The sporozoites then penetrate the intestinal wall and enter cells within the circulatory system. These then develop into schizonts within the cells. The cells then burst to release merozoites in the bloodstream. This process will be repeated, creating large numbers of merozoites.

Intermediate Hosts--Systemic Infection

• Merozoites will then infect tissues within the central nervous system, including the brain, cranial nerves and spinal cord. These then will form schizonts and continue the reproductive process. Sarcocysts also will form in the skeletal muscle tissue of the intermediate host.

Definitive Host--Initial Infection

• Schizonts and merozoites in the circulatory and central nervous systems and sarcocysts within muscle tissue remain there, unable to transmit the infection directly.

  The disease is eventually transmitted to the opossum when the animal ingests muscle tissue from an infected intermediate host, presumably during scavenging.

Definitive Host--Reproduction

• The sarcocysts then release bradyzoites in the body of the opossum. These reproduce sexually within the animal. Eventually they form sporulated oocysts that are passed through the intestinal tract and excreted in the feces.

  Because only schizonts and merozooites have been identified in horses, they are considered a dead-end host, unable to transmit the parasite to continue the life cycle.

Clinical Features, Treatment and Control of EPM

• Within horses, early signs of the disease present as stumbling, which is often confused with lameness. Other symptoms include ataxia, spastic activity in the limbs, depression, facial paralysis, difficulty swallowing and abnormal gait.

  Treatment of EPM in horses should start as soon as clinical signs are identified. The treatment of choice is sulfonamides such as sulfadiazine or pyrimethamine. Diclazuril and toltrazuril, are currently being evaluated as treatments. To prevent exposure to the parasite, use proper hygiene measures and eliminate potential access points by opossums to horse feed and pasturage.