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Cystoisospora belli, formerly known as Isospora belli, is a coccidian unicellular protozoan parasite of the phylum Apicomplexa that primarily infects the human intestinal epithelium and has no known animal reservoir. The parasite is an uncommon cause of diarrhea in immunocompetent hosts and an opportunistic cause of watery diarrhea and weight loss in immunocompromised children and adults.


Transmission occurs primarily by the fecal-oral route through the ingestion of food or water contaminated with human feces. After the host ingests the mature sporulated oocyst, excystation releases sporozoites in the proximal small bowel, possibly in response to exposure to bile. Occasionally, sporozoites leave the intestinal tract to infect extraintestinal sites, primarily in patients with human immunodeficiency virus (HIV)/acquired immunodeficiency syndrome (AIDS). However, the sporozoites typically invade the enterocytes of the duodenum and jejunum and mature into trophozoites. The trophozoites then mature into merozoites by asexual replication and invade new enterocytes or replicate sexually to produce immature unsporulated oocysts that are excreted in the stool. While outside the host, the oocysts ripen in 48 to 72 hours into mature sporulated oocysts (the infectious form). This maturation that occurs outside the host may explain why person-to-person transmission is uncommon. The infectious oocysts can remain viable in a cool, moist environment for months.

First described by Virchow in 1860, C belli is found worldwide but is more common in tropical and subtropical regions, especially Latin America and the Caribbean Islands (eg, Haiti, Mexico, El Salvador, and Brazil), tropical Africa, the Middle East, Southeast Asia, and Australia. Cystoisospora infection is not as common as infections from the closely related genera of Cryptosporidium, Toxoplasma, or Cyclospora. In the United States, sporadic outbreaks (mostly waterborne) of cystoisosporiasis have been reported among institutionalized individuals and attendees of childcare centers. It has been recognized as a cause of diarrhea among travelers to and immigrants from endemic regions and in persons with cell-mediated immunocompromising conditions such as a hematologic malignancy, Hodgkin disease, non-Hodgkin lymphoma, use of steroids or tumor necrosis factor-α antagonists, or HIV/AIDS. In persons with HIV/AIDS, low CD4 cell count (especially < 50 cells/μL), multiple infections, and poor sanitation have been noted as risk factors for cystoisosporiasis. It has been noted that the HIV/AIDS patients receiving trimethoprim-sulfamethoxazole as prophylaxis against pneumocystis pneumonia have a reduced risk of acquiring the disease. It is estimated that between 8% and 40% of HIV/AIDS patients in developing countries are infected with C belli. In Africa, cystoisosporiasis is the initial AIDS-defining illness in 2% to 3% of patients. The estimated prevalence of cystoisosporiasis among AIDS patients with chronic diarrhea varies geographically from about 7% to 10% in South America to up to 20% in Africa. Person-to-person transmission seems to be uncommon. In fact, immunocompetent spouses and children cohabitating with HIV/AIDS patients with cystoisosporiasis infection had no evidence of transmission of the infection. However, there have been ...

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