Location: Presumably small intestine. Elsdon-Dew, Roach and Freedman (1953) found oocysts in material from a duodenal intubation.
Geographic Distribution: Presumably 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 32 persons in Natal, and considered that it was often missed because it was not looked for.
Morphology: This species has often been confused with I. hominis (see Elsdon-Dew and Freedman, 1953), but is clearly different. The oocysts are elongate ellipsoidal, 20 to 33 by 10 to 19 u (mean, 30 by 12 u according to Elsdon-Dew and Freedman, 1953). One or both ends of the oocyst may be somewhat narrow. The oocyst wall is smooth, thin, and colorless. A very small micropyle is sometimes visible. An oocyst polar granule may be present in young, incompletely sporulated oocysts, but quickly disappears. An oocyst residuum is absent. The sporocysts are subspherical to ellipsoidal, without a Stieda body, 12 to 14 by 7 to 9 u (mean 11 by 9 u according to Elsdon-Dew and Freedman, 1953). A sporocyst residuum is present. The sporozoites are slender, somewhat crescent-shaped, with the nucleus at one end. Both immature and mature oocysts may be passed in the feces. The sporulation time is up to 5 days.
Life Cycle: Unknown.
Pathogenesis: Most infections with I. belli appear to be subclinical and self-limiting. However, it may cause a mucous diarrhea. In 31 of the 33 cases of Isospora infection studied by Barksdale and Routh (1948), anorexia, nausea, abdominal pain and diarrhea were present. Matsubayashi and Nozawa (1948) reported that symptoms appeared 1 week after experimental infection of 2 human volunteers, presumably with I. belli, and that oocysts appeared in the feces 10 days after infection and persisted for a month.
Cross-Transmission Studies: Jeffery (1956) failed to transmit I. belli from man to 2 monkeys, 2 dogs, 2 pigs, 12 mice, 4 rats, a guinea pig and a rabbit. Robin and Fondimare (1960) were unable to transmit it from man to the guinea pig, rabbit, mouse or rat.