Cystoisosporiasis, which was previously known as isosporiasis, is an uncommon diarrheal illness caused by the protozoan Cystoisospora belli (formerly known as Isospora belli). C belli was first described by Virchow in 1860. The genus Cystoisospora is related closely to the generaCryptosporidium, Cyclospora, and Toxoplasma. However, Cystoisospora infection is not as common as infection with Cryptosporidium or Toxoplasma. The first case of human infection with C belli was described In 1915.  See the image below.
Humans are the only known hosts for C belli, which has no known animal reservoir. Cystoisosporiasis has a worldwide distribution, although it is more common in tropical and subtropical climates. (See Epidemiology.)
C belli infection usually causes a mild and protracted illness unless the patient is immunocompromised. (See Clinical.) Clinical presentation may mimic those of inflammatory bowel disease and irritable bowel syndrome.
The diagnosis of cystoisosporiasis is based on a combination of clinical, epidemiological, and diagnostic tests. (See Workup.) Cystoisosporiasis is an AIDS-defining illness, so an appropriate workup for HIV infection should be performed, if necessary.
Also see Common Intestinal Parasites, a Critical Images slideshow, to help make an accurate diagnosis.
Although cystoisosporiasis is generally a self-limited infection, patients who are treated tend to improve in 2-3 days, whereas those who are not remain sick considerably longer. (See Treatment.) Immunocompetent hosts generally respond very rapidly to antiparasitic therapy. Immunocompromised hosts also respond well, though less rapidly; however, they have a high relapse rate once therapy is stopped and thus typically require indefinite prophylaxis after therapy.
C belli is ingested in contaminated food or water, and its life cycle requires a stage outside the host. After mature C belli oocysts are ingested, they liberate sporozoites (possibly in response to bile in the small intestine), which invade the enterocytes of the proximal small intestine. Here, they become trophozoites, and asexual multiplication (schizogony) produces merozoites that invade previously uninfected cells.
Shortly thereafter, a sexual multiplication cycle (sporogony) begins, generating oocysts that may pass into the environment. Outside the host, oocysts mature and become infectious 2-3 days later. The oocysts of C belli are resistant and remain viable in the environment for months.
Symptoms of isosporiasis suggest a toxin-mediated mechanism, but no toxin has been identified. In humans, extraintestinal forms of cystoisosporiasis are rare; however, they have been reported in patients with AIDS.
The causative pathogen of cystoisosporiasis is C belli, a protozoan that belongs to the subclass Coccidia in the phylum Apicomplexa. The mode of transmission of isosporiasis is fecal-oral, ie, through food or water contaminated with human feces. In immunocompetent hosts, C belli infection causes a self-limited diarrheal illness. In individuals with immunocompromise, it may cause chronic life-threatening diarrhea and dehydration.
Exposure to contaminated food or water predisposes to this infection. Because an external stage in the environment is required for the oocysts to mature, direct person-to-person transmission is unlikely. Accordingly, isosporiasis is more common in areas with poor sanitation. The disease is also more common in patients with AIDS.
United States statistics
The exact incidence of cystoisosporiasis in humans is unknown. C belli has been reported as the cause of outbreaks of diarrheal illness in daycare centers and mental institutions and has been implicated in traveler’s diarrhea in endemic areas. 
C belli infection is distinctly rare among immunocompetent individuals. Cystoisosporiasis is more common in persons with AIDS, 0.2-3% of whom have stools positive for C belli. However this increased prevalence has been reduced by the widespread use of Pneumocystis jiroveci pneumonia (PCP) prophylaxis with trimethoprim-sulfamethoxazole (TMP-SMZ) among patients with HIV infection.
In Los Angeles from 1985 to 1992, 1% of patients with AIDS had stools positive for C belli.  Infection was more common in foreign-born and Hispanic residents with AIDS (eg, those from El Salvador [7.4%] or Mexico [5.4%]) and less common in those receiving TMP-SMZ prophylaxis for Pneumocystis infection. 
Cystoisosporiasis has also been reported in patients with lymphoma and leukemia and recipients of renal and liver transplants.
Endemic areas of cystoisosporiasis include Africa,  Australia, the Caribbean Islands, Latin America, and Southeast Asia. One study found positive examination findings in up to 15% of Haitians infected with AIDS.  In developing countries, 8-40% of patients with AIDS are infected.
Cystoisosporiasis is the initial AIDS-defining illness in approximately 2-3% of patients with AIDS who are from Africa. Among patients with AIDS who are from South America, 10% with chronic diarrhea have isosporiasis. In patients with AIDS who are from Haiti and Africa, 7-20% with chronic diarrhea have cystoisosporiasis.
Age-, sex- and race-related demographics
People of all ages are susceptible to C belli infection, although it tends to be more serious in infants and young children, possibly as a result of the risk of dehydration in this population. C belli can cause severe diarrhea in infants.
No gender predilection for infection has been noted, aside from the gender distribution of people with AIDS, the risk factor most commonly associated with this disease.
No racial predilection for isosporiasis has been reported, other than the racial distribution of people with AIDS. Among people with AIDS in the United States, Hispanics appear to be more at risk than blacks or whites, likely secondary to importation.
Prognosis is excellent with therapy (and prophylaxis, if appropriate).
In immunocompetent patients, cystoisosporiasis is usually a transient, self-limited illness but can sometimes result in a protracted diarrheal illness. Cystoisosporiasis has also been reported as a contributor to malabsorption syndrome in immunocompetent patients. 
In patients with AIDS, cystoisosporiasis can range in severity from a chronic and intermittent illness to a severe and life-threatening diarrheal illness.
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