Dientamoeba Fragilis

Dientamoeba fragilis is a protozoan parasite that lives in the human large intestine. Despite the name, it does not have flagella; it exists mainly as a trophozoite, a fragile, motile form with two nuclei that quickly disintegrates outside the body. Because of this fragility, traditional stool microscopy often failed to detect it, and its significance remained unclear for decades.

D. fragilis reproduces in the colon and is transmitted between people, either via contaminated hands, food, or possibly through co-carriage with pinworm (Enterobius vermicularis) eggs. A cyst form, which could explain transmission without a host, has been proposed but is rarely seen.

Molecular testing has revealed that D. fragilis is extremely common worldwide, particularly in children. PCR-based studies find prevalence rates ranging from 4% to more than 30%, showing that many healthy individuals harbor the organism without any symptoms. In some, however, D. fragilis has been associated with abdominal pain, loose stools, bloating, or fatigue, and symptoms often resolve after treatment. These mixed findings have led researchers to classify D. fragilis as a potentially opportunistic or conditionally pathogenic organism, i.e. harmless in most hosts but capable of causing disease under certain microbiome or immune conditions.

The parasite load may be key to distinguishing colonization from infection. Higher DNA quantities detected by quantitative PCR (qPCR) are more often found in symptomatic individuals, suggesting that D. fragilis may trigger inflammation when it proliferates excessively. Treatment is typically considered only when symptoms are present, no other cause is identified, and the organism is confirmed by PCR.