Sarcocystis - Life Cycle

Life Cycle

The heteroxenous (more than one obligatory host in its life cycle) life cycle of these apicomplexan parasites remained obscure until 1972 when the prey-predator relationship of its definitive and intermediate hosts was recognised. The life cycles of about 60 of these species is now known.

In outline gametogony and sporogony occur in the intestine of the definitive host while both schizogony which occurs in various tissues and the formation of sarcocysts (containing bradyzoites and metrocytes) occurs principally in the muscles of the intermediate host. In some cases a single species may act as both the definitive and intermediate host.

Oocysts are passed in the feces of an infected definitive host. The oocyst undergoes sporogony creating two sporocysts. Once this is complete the oocyst itself undergoes lysis releasing the sporocysts into the environment. Sporocysts typically contain 4 sporozoites and measure 15-19 by 8-10 micrometres.

An intermediate host such as a cow or pig then ingests a sporocyst. Sporozoites are then released in the body and migrate to vessels where they undergo the first two generation of asexual reproduction. These rounds result in the development of meronts. This stage lasts about 15 to 16 days after ingestion of sporocysts. Merozoites emerge from the second generation meronts and enter the mononucleate cells where they develop by endodyogeny. Subsequent generations of merozoites develop downstream in the direction of blood flow to arterioles, capillaries, venules, and veins throughout the body subsequently developing into the final asexual generation in muscles.

Merozoites entering muscle cells round up to form metrocytes and initiate sarcocyst formation. Sarcocysts begin as unicellular bodies containing a single metrocyte and through asexual multiplication numerous metrocytes accumulate and the sarcocyst increases in size. As the sarcocyst matures, the small, rounded, noninfectious metrocytes give rise to crescent-shaped bodies called bradyzoites that are infections for the definitive host. Time required for maturation varies with the species and may take 2 months or more.

In species in which symptoms develop these typically occur 20–40 days after ingestion of sporocysts and during the subsequent migration of sporozoites through the body vessels. Acute lesions (oedema, hemorrhages and necrosis) develop in the affected tissues. The parasite has a predilection for skeletal muscle (myositis), cardiac muscle (petechial hemorrhages of cardiac muscle and serosae), and lymph nodes (oedema, necrosis and hemorrhage). These lesions are associated with maturation of second generation of meronts within the endothelial and subendothelials cells. Occasionally mononuclear infiltration or hyperemia has been observed in the lamina propria of the small intestine. After the acute phase cysts may be found in various muscular tissues, generally without pathology.

Once the intermediate host is eaten by the definitive host such as a dog or human, the parasite undergoes sexual reproduction within the gut to create macrogamonts and microgamonts. Most definitive hosts do not show any clinical sign or symptoms. Fusion of a macrogamont and a microgamont creates a zygote which develops into an oocyst. The oocyst is passed through the faeces completing the life cycle.

A second life cycle has more recently been described whereby carnivores and omnivores pass the infectious stages in their faeces. Ingestion of this material may lead to successful infection of the ingesting animal.