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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.

Oocyst of Cystoisospora belli with 2 sporoblasts.

Oocyst of Cystoisospora belli with 2 sporoblasts. From the Image Library, Division of Parasitic Diseases at the National Center for Infectious Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, Georgia.

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.

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