Dientamoeba fragilis

Dientamoeba fragilis is a single celled parasite that infects the gastrointestinal tract of humans. Trophozoites of D. fragilis characteristically have two nuclei, hence the 'DI' prefix to the genus name. However, the rest of the genus name indicates that it is an ENTeric AMOEBA and not that it is related to intestinal parasites of the genus Entamoeba. The species name refers to the fact that the trophozoite stages are fragile - they do not survive long in the stool after leaving the body of the human host.

Dientamoebiasis
Infection with Dientamoeba fragilis is called Dientamoebiasis and is associated variously with symptoms of abdominal pain, diarrhea, weight loss, and fever.

Phylogenetics
Dientamoeba fragilis is a type of trichomonad. Trichomonads are flagellated organisms but D. fragilis lacks flagella, having secondarily 'lost' them over evolutionary time. Thus, it is an amoeba of flagellate ancestry.

Life Cycle
The life cycle of this parasite has not yet been completely determined, but some assumptions have been made based on clinical data. To date, a cyst stage has not been identified in D. fragilis, and the trophozoite is the only stage found in stools of infected individuals. Like other intestinal parasites, D. fragilis is probably transmitted by the fecal-oral route. In the absence of a cyst form, transmission via helminth eggs (e.g., Ascaris, Enterobius spp.) has been postulated. The rationale for this suggestion is that D. fragilis is closely related to the turkey parasite Histomonas, which is known to be transmitted via the eggs of the helminth Heterakis.

Microbiology
Dientamoeba fragilis replicates by binary fission and moves by pseudopodia. D. fragilis feeds by phagocytosis. The cytoplasm typically contains numerous food vacuoles that contain ingested debris, including bacteria. Waste materials are eliminated from the cell through digestive vacuoles by exocytosis. D. fragilis possesses some flagellate characteristics. In the binucleate form there is a spindle structure located between the nuclei, which stems from certain polar configurations adjacent to a nucleus—these configurations appear to be homologous to hypermastigotes’ atractophores. There is a complex Golgi apparatus; the nuclear structure of D. fragilis is more similar to that of flagellated trichomonads than to that of Entamoeba. Also notable is the presence of hydrogenosomes, which are also a characteristic of other trichomonads.