Dientamoeba fragilis Infection

[Dientamoeba fragilis]

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Causal Agent

Despite its name, Dientamoeba fragilis is not an ameba but an intestinal flagellate, most closely related to trichomonads. In human stool specimens, D. fragilis is almost always found solely as a trophozoite. However, the rare presence of putative cyst and precyst forms in clinical specimens has been reported; their transmission potential is being investigated. Other aspects of the transmission and pathogenicity of D. fragilis also are poorly understood.

Life Cycle

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The complete life cycle of Dientamoeba fragilis has not yet been elucidated; assumptions have been made on the basis of clinical observations and the biology of related species (in particular, Histomonas meleagridis, a parasite of galliform birds). Trophozoites are found in the lumen of the large intestine, where they multiply via binary fission, and are shed in the stool . Historically, only the trophozoite stage of D. fragilis had been detected. However, rare putative cyst and precyst forms have been described in human clinical specimens; whether and in what settings transmission to humans occurs via ingestion of such forms in contrast or in addition to other fecal-oral transmission routes is not yet known . Transmission via helminth eggs (e.g., via Enterobius vermicularis eggs) has been postulated .

Hosts

Dientamoeba fragilis is primarily a parasite of humans. Trophozoites have been identified in some other mammals (e.g., non-human primates, swine), but the epidemiologic significance of these hosts is unknown.

Geographic Distribution

Dientamoeba fragilis is found worldwide. Infection appears to be more common in children.

Clinical Presentation

The pathogenicity and clinical importance of D. fragilis continue to be investigated, including whether particular genotypes, subtypes, or strains of D. fragilis are associated with symptomatic infection in humans. Both asymptomatic and symptomatic infection (e.g., with various nonspecific gastrointestinal symptoms) have been reported. The reported clinical manifestations have sometimes been described as similar to those of colitis, appendicitis, or irritable bowel syndrome.

Dientamoeba fragilis binucleate trophozoites stained with trichrome

Dientamoeba fragilis is a flagellate that lacks external flagella and therefore must be morphologically differentiated from the small nonpathogenic amebae (e.g., Endolimax nana, Entamoeba hartmanni). Dientamoeba fragilis trophozoites measure 5 to 15 µm; cyst-like stages are rare. Pseudopodia are angular to broad-lobed and transparent. Although most trophozoites are binucleate, some have only one nucleus. The nucleus typically has a fragmented karyosome with discrete chromatin granules, and a thin nuclear membrane may be visible.

Figure A: Binucleate form of a trophozoite of D. fragilis, stained with trichrome. The two small nuclei (arrows) have fragmented karyosomes containing discrete chromatin granules (darts).

Figure D: Binucleate form of a trophozoite of D. fragilis, stained with trichrome.

Figure B: Binucleate form of a trophozoite of D. fragilis, stained with trichrome.

Figure E: Binucleate form of a trophozoite of D. fragilis, stained with trichrome.

Figure C: Binucleate form of a trophozoite of D. fragilis, stained with trichrome.

Figure F: Binucleate form of a trophozoite of D. fragilis, stained with trichrome.

Dientamoeba fragilis uninucleate trophozoites stained with trichrome.

Although most D. fragilis trophozoites are binucleate, some have only one nucleus.

Figure A:Uninucleate form of a trophozoite of D. fragilis, stained with trichrome.

Figure D: Binucleate and uninucleate forms of trophozoites of D. fragilis, stained with trichrome.

Figure B: Uninucleate form of a trophozoite of D. fragilis, stained with trichrome.

Figure C: Uninucleate form of a trophozoite of D. fragilis, stained with trichrome.

Laboratory Diagnosis

Infection typically is diagnosed by detection of D. fragilis trophozoites in fecal smears stained with trichrome or another permanent stain. The trophozoite stage of the parasite is not usually detectable if stool concentration methods are used. Dientamoeba fragilis trophozoites can easily be overlooked or misidentified because they are pale-staining and their nuclei sometimes resemble those of Endolimax nana or Entamoeba hartmanni.

More on: Morphologic comparison with other intestinal parasites.

Laboratory results, including prevalence rates, based on real-time PCR analysis should be approached with caution because of the performance variability across platforms and the potential for low specificity.

Laboratory Safety

Standard precautions for processing stool specimens apply.