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Cystoisospora belli, Molecular Detection

Molecular testing for Cystoisospora belli is used for the diagnosis and laboratory documentation of cystoisosporiasis.

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Cystoisospora belli, previously known as Isospora belli, is a parasite that causes an intestinal disease known as cystoisosporiasis. This protozoan parasite primarily exists in the epithelial cells of the small intestine and develops in the cell cytoplasm. 

The distribution of this coccidian parasite is cosmopolitan but is mainly found in tropical and subtropical areas of the world. Many cases of C. belli infections have been reported in patients with HIV, and in patients undergoing immunosuppressive therapy for organ transplants or those treated for tumors worldwide. Cystoisospora belli belongs to Coccidia subclass, a subclass of microscopic, spore-forming, single-celled obligate intracellular parasites. In Coccidia subclass are classified the more common parasites, Cryptosporidium and Toxoplasma.

The life cycle of C. belli is similar to that of Cryptosporidium, except that oocysts passed in stool are not sporulated. Thus, when freshly passed in stools, the oocysts are not infective, and direct fecal-oral transmission cannot occur. The oocysts require days to weeks in the environment to sporulate and, therefore, direct person-to-person transmission is unlikely. The sporulated oocysts are ingested in contaminated food or water and exocyst in the gastrointestinal tract, releasing sporozoites. The sporozoites invade the epithelial cells of the small intestine, replicate, and mature into oocysts, which are shed in stool.

The Infection causes acute, non-bloody diarrhea with crampy abdominal pain, which can last for weeks and result in malabsorption and weight loss. In immunosuppressed patients, and in infants and children, diarrhea can be severe. Eosinophilia may be present.

Microscopic demonstration in the stool of the large, typically shaped oocysts is the basis of diagnosis. Because the oocysts may be passed in small amounts and intermittently, repeated stool examinations and concentration procedures are recommended. Molecular testing of feces for the presence of Cystoisospora belli DNA using the technique of real-time PCR (RT-PCR), is a faster, more sensitive, and more effective method for detecting the parasite.


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