Definition: A genus of protozoan parasites, related to the sporozoan genera Eimeria, Isospora, and Toxoplasma, and placed in a distinct family, Sarcocystidae, but with the above genera in the same suborder, Eimeriina, within the subclass Coccidia, class Sporozoea, and phylum Apicomplexa. Tissue stages of Sarcocystis are usually seen as thick-walled cylindric or (often extremely large [1 cm or more]) fusiform cysts (Miescher tubes) in reptile, bird, or mammal striated muscles. Cysts are smooth in the house mouse form or with radial spines (cytophaneres) in sheep or rabbit; contents may be compartmentalized by septa. Variably shaped spores (Rainey corpuscles) probably are peripheral rounded cells (sporoblasts, cytomeres) that divide to form mature “spores” (bradyzoites), motile bodies when released from the cyst; sexual stages have been described in tissue cultures. These parasites are abundant but rarely of pathogenic significance. Humans who have ingested meat containing the mature sarcocysts serve as the definitive hosts; fever, severe diarrhea, abdominal pain, and weight loss have been reported in a small number of immunocompromised hosts. When humans accidentally ingest oocysts from other animal stool sources, the sarcocysts that develop in human muscle appear to cause no inflammatory response.
[sarco- + G. kystis, bladder]
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