Humans are the only recognized source of I. be//i infection and, thus, unlike cryptosporidiosis, iso-sporiasis is not a zoonotic infection (Kirkpatrick, 1988). Given the usual need for sporulation of the oocysts of I. be//i outside the human host, I. be//i is believed to be transmitted primarily by contaminated food or water. Person-to-person spread through oral-anal contact in individuals with AIDS has been suggested (DeHovitz et a/., 1986; Forthal and Guest, 1984; Ma and Soave, 1983).

I. be//i is reported more commonly from tropical and subtropical areas of the world, but its true prevalence is unknown (Faust et a/., 1961; Hunter et a/., 1992; Soave and Johnson, 1988). In recent years, more attention has been drawn to this parasitic infection because of the recognition that it was a relatively common and treatable cause of persistent or chronic diarrhea in patients with AIDS living outside the USA (DeHovitz et a/., 1986; Pape et a/., 1989). In the USA, <0.2% of patients with AIDS have been recognized to have I. be//i infection, whereas this infection is identified in approximately 15-20% of patients with chronic diarrhea and AIDS from, for example, Haiti, Zambia, Uganda and the Democratic Republic of Congo (formerly Zaire [1971-1997]) (Colebunders et a/., 1988; Conlon et al., 1990; Henry et al., 1986; Hunter et al., 1992; Sewankambo et al., 1987; Soave and Johnson, 1988). I. belli infection is recognized as a cause of acute, persistent (> 14-30 days) or chronic (>30 days) diarrhea in travelers (God-iwala and Yaeger, 1987; Shaffer and Moore, 1989). Occasional institution-based epidemics of I. belli infection have been reported suggesting that nosocomial spread or spread within daycare centers is feasible (Henderson et al., 1963).

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