Protozoa is a loose term for certain nucleated, unicellular organisms (eukaryotes) that lack a cell wall and are neither animals, plants, nor fungi. The most important intestinal protozoan pathogens that cause enteric infections in humans include
Cystoisospora belli (previously known as Isospora belli—see Cystoisosporiasis Cyclosporiasis Cyclosporiasis is infection with the protozoan Cyclospora cayetanensis. Symptoms include watery diarrhea with gastrointestinal and systemic symptoms. Diagnosis is by detection of characteristic... read more
)Giardia duodenalis (previously known as G lamblia, G intestinalis—see Giardiasis Giardiasis Giardiasis is infection with the flagellated protozoan Giardia duodenalis (G. lamblia, G. intestinalis). Infection can be asymptomatic or cause symptoms ranging from intermittent... read more )
Members of the phylum Microsporidia used to be classified as protozoa but are now, based on DNA studies, thought to be fungi or closely related to them.
Multiple pathogenic parasites and nonpathogenic commensal organisms may be present in the intestine at the same time.
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 may occur in mental institutions and day care centers. Occasionally, large foodborne and waterborne outbreaks of intestinal protozoan infection have occurred in the US (eg, the multi-state outbreak of Cyclospora cayetanensis infection attributed to contaminated fresh vegetable trays in 2018 and the massive waterborne Cryptosporidium outbreak in Milwaukee in 1993). Some gastrointestinal protozoa are spread sexually, especially with practices involving oral-anal contact. Several protozoan species cause severe opportunistic infections in immunocompromised patients, including those with AIDS Immune system Human immunodeficiency virus (HIV) infection results from 1 of 2 similar retroviruses (HIV-1 and HIV-2) that destroy CD4+ lymphocytes and impair cell-mediated immunity, increasing risk of certain... read more 
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Systemic protozoal diseases are discussed elsewhere and include malaria Malaria Malaria is infection with Plasmodium species. Symptoms and signs include fever (which may be periodic), chills, rigors, sweating, diarrhea, abdominal pain, respiratory distress, confusion... read more 
, babesiosis Babesiosis Babesiosis is infection with Babesia species of protozoa. Infections can be asymptomatic or cause a malaria-like illness with fever and hemolytic anemia. Disease is most severe in asplenic... read more 
, toxoplasmosis Toxoplasmosis Toxoplasmosis is infection with Toxoplasma gondii. Symptoms range from none to benign lymphadenopathy, a mononucleosis-like illness, to life-threatening central nervous system (CNS) disease... read more 
, leishmaniasis Leishmaniasis Leishmaniasis is caused by species of Leishmania. Manifestations include cutaneous, mucosal, and visceral syndromes. Cutaneous leishmaniasis causes painless chronic skin lesions ranging... read more 
, Chagas disease Chagas Disease Chagas disease is infection with Trypanosoma cruzi, transmitted by Triatominae bug bites or, less commonly, via ingestion of sugar cane juice or foods contaminated with infected Triatominae... read more 
, and African trypanosomiasis African Trypanosomiasis Human African trypanosomiasis is infection with protozoa of the species Trypanosoma brucei, transmitted by the bite of a tsetse fly. Symptoms include characteristic skin lesions, intermittent... read more 
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Diagnosis
Making a diagnosis of intestinal protozoan and microsporidia infections based on symptoms and physical findings is difficult; stool testing for parasite antigens or DNA and microscopic examination of stool for cysts or organisms are necessary.
Fecal antigen tests that are sensitive and specific are available for
Cryptosporidium species
E. histolytica
G. duodenalis
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 ).
Molecular diagnosis using polymerase chain reaction-based assays is available for many enteric protozoa.
