ABSTRACT
Cystoisospora belli is an important opportunistic enteric pathogen, causing chronic emaciating diarrheal illness in immunocompromised patients, especially HIV-infected (AIDS) patients, and various clinical manifestations (from asymptomatic infection to persistent diarrhea) in immunocompetent individuals. Belonging to the phylum Apicomplexa, C. belli is a member of coccidian protozoan with a close relationship to other human intestinal epithelial cell-dwelling parasites in the genera Cyclospora and Cryptosporidium. First observed as globular and ovoidal bodies in the bile of a cow by Leewenhoek in 1674 [1], the coccidians are a diverse group of protozoa parasitizing various species of higher invertebrate and vertebrate hosts. Other human pathogenic members of the phylum Apicomplexa include erythrocyte-dwelling parasites in the genera Plasmodium and Babesia, and enteric and tissue-dwelling protozoa in the genera Toxoplasma and Sarcocystis. All enteric coccidian protozoa produce gastrointestinal symptoms indistinguishable from one another; therefore, laboratory investigation is crucial to make definite diagnosis and to implement proper management. Although a number of species in the genus Cystoisospora are known to infect mammalian hosts, C. belli is the only pathogenic species to humans, causing cystoisosporiasis. Formerly named Isospora belli, cumulative evidence from the structure of the sporocyst and molecular phylogeny have led to its reclassification as C. belli. The description of characteristic oocysts of C. belli in stools was initially observed among military personnel stationed in Egypt, the Middle East, and eastern Mediterranean countries during 1914 and 1921 [2]. It is well recognized that cystoisosporiasis has a cosmopolitan distribution. Although oocysts of Isospora natalensis were reportedly found in human fecal samples, none of the subsequent studies could reaffirm the presence of these oocysts as a causative agent of diarrheal illness in humans since its first identification in 1953, raising the possibility that it may not be a valid pathogenic species of humans [3]. While immunocompetent hosts are susceptible to enteric coccidian infections, most of the enteric coccidian parasites of humans are considered to be opportunistic pathogens except the genus Sarcocystis. The prevalence of these coccidian infections appears to be on the increase, with the number of immunocompromised patients, especially those with AIDS, rising constantly. In general, the illness caused by C. belli seems to be more severe in patients with compromised immunity than in those with competent immune status. However, chronic emaciating infections have been reported in immunocompetent patients, suggesting that host immunity, per se, may not always determine the severity of cystoisosporiasis [4,5]. Further investigation is obviously needed to clarify this issue.
