Cryptosporidium is a protozoan parasite of the subclass Coccidia, which also includes Toxoplasma, Isospora, Cyclospora, Eimeria, and Sarcocystis. Cryptosporidium parvum is the species that most commonly causes disease in human beings. Previously regarded only as a disease of immunodepressed individuals (especially those with AIDS), Cryptosporidium is now recognized as an important and increasingly common cause of diarrhea, including traveler's diarrhea, worldwide.

Like Giardia, Cryptosporidium is waterborne, as has been well documented in England and the United States, most recently in Milwaukee. Like Giardia, Cryptosporidium causes diarrhea by altering the microvillous tips of the cells lining the small intestine. It can infect other mammals, including cows, and waste water (raw sewage) and runoff from dairies and pastures may be the source of oocysts that contaminate reservoirs and pools.

The diarrhea of cryptosporidiosis, in both immunocompetent and immunocompromised patients, is profuse and watery (cholera-like), usually without blood, fecal leukocytes, and mucus. It is occasionally associated with crampy abdominal pain, nausea and vomiting, low-grade fever, and weight loss. Although the illness is self-limited in most immunocompetent individuals (lasting several days to 2 weeks), it can be relentless and debilitating in the immunocompromised, often resulting in severe dehydration, malabsorption, weight loss, and death. Rarely, Cryptosporidium can also cause respiratory and biliary tract disease.

The diagnosis is made by finding oocytes in the stool. Concentration of the stool specimen using formalin ether gives a higher yield. A modified Ziehl-Neelsen acid-fast stain is used to visualize the oocysts. A serum ELISA for antibodies to Cryptosporidium is available.

Treatment is mainly supportive (oral or intravenous hydration and antidiarrheal agents). Treatment of immunocompromised patients is difficult, and at present there is no accepted regimen. Paromomycin, a somatostatin-like agent, octreotide, and azithromycin have all been used with some success.

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