Synonyms: Coccidium bigeminum var. hominis, Lucetina hominis.
Location: Small intestine.
Geographic Distribution: Worldwide, but more common in the tropics than in the temperate zone.
Prevalence: This species is quite rare in man. However, Elsdon-Dew and Freedman (1953) found it in 23 persons in Natal, and thought that it was often missed because people did not look for it.
Morphology: The oocysts are sporulated when passed. The oocyst wall is very thin, stretched around the sporocysts and usually constricted between them, and sometimes not visible. It is often ruptured, releasing the sporocysts. The oocysts are about 20 by 15 u. Micropyle, oocyst polar granule and residuum are absent. The sporocysts are ellipsoidal or with one side flattened, about 15 by 10 u, without a Stieda body. A sporocyst residuum is present.
Life Cycle: Unknown.
Pathogenesis: Most infections appear to be subclinical and self-limiting. However, I. hominis may cause a mucous diarrhea. In 31 of 33 cases of Isospora infection studied by Barksdale and Routh (1948), anorexia, nausea, abdominal pain and diarrhea were present.
Remarks: This species resembles I. bigemina very closely, and it may well be the same species (see Becker, 1956, Elsdon-Dew and Freedman, 1953; Routh, McCroan and Hames, 1955). Elsdon-Dew (1954) failed to infect a dog with I. hominis from man, but the animal was an adult and could have been immune.