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Dientamoeba fragilis is a nonflagellate trichomonad parasite that inhabits the human colon and has been associated with both acute and chronic gastrointestinal symptoms. Discovered in 1918, the organism was initially classified as an amoeba, but findings from molecular studies and other methods showed the organism is closely related to trichomonads.


D fragilis infects the mucosal crypts of the large intestine from the cecum to the rectum, with the cecum and proximal colon usually the most affected areas. D fragilis is not invasive and does not cause cellular damage. Gastrointestinal symptoms are caused mainly by superficial colonic mucosa irritation. Like Cryptosporidium parvum, D fragilis can cause disease regardless of the patient’s immune status. The exact mechanisms of pathogenicity for this organism are yet to be fully determined.

While animals such as mice can be experimentally infected, humans are likely the natural host for D fragilis. The life cycle and mode of transmission of this organism remain poorly defined. Because studies have shown that household contacts may have higher rates of infection, a fecal-oral mode of transmission is most likely.

Worldwide prevalence varies from 0.5% to 16%, and in contrast to many pathogenic protozoa, greater rates of D fragilis infection have been reported from countries with higher levels of health standards with increased prevalence of disease in persons residing in crowded living conditions. Serologic surveys suggest that infection occurs mostly during childhood. When adequate laboratory methods are used, D fragilis has been described to be more commonly identified than Giardia and Cryptosporidium.


Both acute and chronic illnesses have been associated with infection due to D fragilis. Infected adults and children may present with acute watery diarrhea accompanied by abdominal pain; nausea and vomiting can occur, as can bloating and flatulence. Hematochezia is unusual with these infections. Nongastrointestinal complaints such as fatigue, fever, headache, irritability, malaise, urticaria, and weakness are less common but may occur. D fragilis may be associated with chronic abdominal pain lasting months to years as well as bouts of alternating diarrhea and constipation, fatigue, and flatulence. In a study of infected patients, a small percentage (~11%) had no symptoms of infection. On physical examination, usually no specific findings are evident; however, some children have exhibited generalized abdominal tenderness without rebound. No specific mortality is associated with D fragilis infection.


D fragilis should be suspected in patients who have abdominal pain and/or diarrhea for an extended period of time as well as symptomatic household contacts of those patients diagnosed as having an infection with D fragilis. Suspicion of infection should be particularly high for persons residing in institutions as well as those who recently traveled outside the United States.

To increase diagnostic yield, stools from symptomatic patients ...

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