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Overview of Intestinal Protozoan Infections

By Richard D. Pearson, MD, University of Virginia School of Medicine

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The most important intestinal protozoan pathogens are Entamoeba histolytica, Cryptosporidium sp, Giardia intestinalis (lamblia), Cystoisospora (Isospora) belli, Cyclospora cayetanensis, and members of the phylum Microsporidia. Multiple pathogenic parasites and nonpathogenic commensal organisms may be present in the intestine at the same time. Nonintestinal protozoan infections are covered in other chapters: For systemic protozoal diseases (malaria, babesiosis, leishmaniasis, toxoplasmosis, trypanosomiasis), Extraintestinal Protozoa; for nematode infections, Nematodes (Roundworms); for fluke infections, Trematodes (Flukes); and for tapeworm infections, Cestodes (Tapeworms).

Intestinal protozoa are spread by the fecal-oral route, so infections are widespread in areas with inadequate sanitation and water treatment. They are also common in the US in settings where fecal incontinence and poor hygiene prevail, as occur in mental institutions and day care centers. Occasionally, large waterborne outbreaks of intestinal protozoan infection have occurred in the US (eg, the massive waterborne Cryptosporidium outbreak in Milwaukee in 1993). Some GI protozoa are spread sexually, especially with practices involving oral-anal contact, and several protozoan species cause severe opportunistic infections in patients with AIDS.


Making a diagnosis based on symptoms and physical findings is difficult; stool testing for parasite antigens or microscopic examination of stool for cysts or organisms is necessary.

Fecal antigen tests that are sensitive and specific are available for

  • G. intestinalis

  • Cryptosporidium sp

  • E. histolytica

Microscopic diagnosis may require several samples, concentration methods, and special stains; thus, the laboratory should be notified which pathogen or pathogens are suspected. Some patients require semi-invasive diagnostic techniques such as endoscopic biopsy (see Table: Collecting and Handling Specimens for Microscopic Diagnosis of Parasitic Infections).

Molecular diagnosis using PCR-based assays is available for some enteric protozoa and holds promise for the future.

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