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[Cystoisospora belli (syn. Isospora belli)]

Immature oocyst of C. belli stained with safranin, containing a single sporoblast.

Cystoisospora belli oocyst, stained with safranin.

Immature oocyst of C. belli in an unstained wet mount showing two sporoblasts.

C. belli oocyst, unstained wet mount.

Immature oocyst of C. belli in an unstained wet mount showing a single sporoblast.

C. belli oocyst, unstained wet mount.

Causal Agents

The coccidian parasite, Cystoisospora belli, infects the epithelial cells of the small intestine, and is the least common of the three intestinal coccidia that infect humans.

Life Cycle

Life cycle of cystoisosporiasis

At time of excretion, the immature oocyst contains usually one sporoblast (more rarely two)The number 1. In further maturation after excretion, the sporoblast divides in two (the oocyst now contains two sporoblasts); the sporoblasts secrete a cyst wall, thus becoming sporocysts; and the sporocysts divide twice to produce four sporozoites eachThe number 2. Infection occurs by ingestion of sporocysts-containing oocysts: the sporocysts excyst in the small intestine and release their sporozoites, which invade the epithelial cells and initiate schizogonyThe number 3. Upon rupture of the schizonts, the merozoites are released, invade new epithelial cells, and continue the cycle of asexual multiplicationThe number 4. Trophozoites develop into schizonts which contain multiple merozoites. After a minimum of one week, the sexual stage begins with the development of male and female gametocytesThe number 5. Fertilization results in the development of oocysts that are excreted in the stoolThe number 1.

Geographic Distribution

Worldwide, especially in tropical and subtropical areas. Infection occurs in immunodepressed individuals, and outbreaks have been reported in institutionalized groups in the United States.

Clinical Presentation

Infection causes acute, nonbloody diarrhea with crampy abdominal pain, which can last for weeks and result in malabsorption and weight loss. In immunodepressed patients, and in infants and children, the diarrhea can be severe. Eosinophilia may be present (differently from other protozoan infections).

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  • Page last reviewed November 29, 2013
  • Page last updated November 29, 2013
  • Content source: Global Health - Division of Parasitic Diseases and Malaria
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