Sarcocystis is a genus of protozoan parasites, with many species infecting mammals, reptiles and birds. Its name is dervived from Greek sarx = flesh and kystis = bladder.
The lifecycle of a typical member of this genus involves two host species, a definitive host and an intermediate host. Often, the definitive host is a predator and the intermediate host is its prey. The parasite reproduces sexually in the gut of the definitive host, is passed with the feces, and ingested by the intermediate host. There, it eventually enters muscle tissue. When the intermediate host is eaten by the definitive host, the cycle is completed. The definitive host usually does not show any symptoms of infection, but the intermediate host does.
About 130 recognized species are in this genus. Revision of the taxonomy of the genus is ongoing, and all the currently recognised species may be a much smaller number of species that can infect multiple hosts.
History
The organism was first recognised in a mouse by Miescher in 1843. His findings were not initially interpreted as involving a protist, and the literature referred to the structures he described as "Miescher's tubules". Incidentally, Miescher's son, Johann Friedrich Miescher, discovered DNA. Similar structures were found in pig muscle in 1865, but these remained unnamed until 1899, when the name Sarcocystis miescheriana was proposed for them.
Initially, whether these organisms were fungi or protozoa was unclear . This uncertainty was resolved in 1967 when electron microscopic studies showed that they were protozoa, related to Toxoplasma and Eimeria. The lifecycle remained unknown until 1970, when bradyzoites from sarcocysts in bird muscles were inoculated into cultured mammalian cells and seen to undergo development into sexual stages and oocysts. Transmission studies with Sarcocystis of cattle (then considered a single species, Sarcocystis fusiformis) in dogs, cats, and humans revealed three morphologically distinct species, which were named S. bovicanis, S. bovifelis, and S. bovihominis. This and post-1972 research on Sarcocystis was reviewed during the same decade; and that account is still a very useful source of information today.
Lifecycle
The heteroxenous (more than one obligatory host) lifecycle of these apicomplexan parasites remained obscure until 1972, when the prey-predator relationship of its definitive and intermediate hosts was recognised. The lifecycles of about 60 of these species are 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. These sporocysts of Sarcocystis characteristically contain four sporozoites and measure approximately 15–19 by 8–10 μm. Oocysts of Sarcocystis are thin-walled and easily break open. The result is that sporocysts recovered from feces outnumber intact oocysts.
Intermediate hosts such as cattle or pigs then ingest sporocysts. Sporozoites are released in the body and migrate to vessels, where they undergo the first two generations 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 mononucleated 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 (also known as "bradyzoic merozoites") that are infectious for the definitive host. The time required for maturation varies with the species and may take 2 months or more.