Dientamoeba fragilis

Cosmopolitan intestinal protozoan.  Initially thought to belong to genus Entamoeba, but subsequent electronic microscope examination and genetic analysis showed that Dientamoeba fragilis is phylogenetically related to Histomonas. Today Dientamoeba fragilis is classified in the Trichomonadidae family. This species has no known cyst stage.

Data on prevalence and geographical distribution are certainly underestimated due to the difficult identification of the parasite in fresh wet mounts, in concentrated samples, or in stained wet mounts, because in these preparations the nuclei are not visible and do not stain. Moreover, Dientamoeba fragilis trophozoites display considerable variability in shape and size and can either be overlooked or mistaken for faecal debris, white blood cells or intestinal cells.

The pathogenicity of Dientamoeba fragilis remains controversial.  In fact, the parasite can be found in feces of patients with enteritis of varying grades of severity who can successfully be treated with antiparasitic drugs, but it is also found in feces from many asymptomatic individuals. Recent molecular genetic studies have demonstrated the existence of two distinct genotypes of Dientamoeba fragilis, with different degrees of virulence that can explain the wide variety of symptoms.

The way in which the parasite is transmitted is not known since no cyst stage has been found. Dientamoeba fragilis is probably transmitted by fecal-oral route, although transmission via eggs of helminths (e.g. Ascaris lumbricoides, Enterobius vermicularis) has also been postulated.

Trophozoite

Colonizes the colon. At room temperature, the trophozoite is immobile and rounded in fresh formed stools, while it is very active in loose or watery stools: short transparent pseudopods are formed at different positions one at a time and immediately withdrawn, without achieving any progressive movement.  Dientamoeba fragilis trophozoites show intense phagocytic activity, like those of Entamoeba coli.  The term “fragilis” is improper, because in watery or loose stools (or in stools diluted in physiological saline), trophozoites remain mobile for at least 24 hours even at room temperature. If tap water is used for dilution, trophozoites lyse in a few minutes.

Size: very variable, from 3 to 20 μm.

Morphology: usually rounded but also elongated, sometimes pointed at one end or racket-shaped. Microscopists who only perform direct stool examination must be aware that Dientamoeba fragilis trophozoites appears like polymorphic refractive bodies and may therefore be misdiagnosed as artifacts.

In about 70% of cases, the characteristic binucleate form of the Dientamoeba fragilis trophozoite is seen, while the remaining population is mononucleate. Cases have been reported, however, with up to 80% mononucleate forms. These forms derive from a recent binary fission and appear a little smaller than the binucleate forms. The early phase of nuclear division can be observed in mononucleate trophozoites only. When the trophozoite is in the process of degeneration, a large vacuole forms in the cytoplasm, pushes the two nuclei towards the rim and gives the cell the appearance of a Blastocystis. Nuclei are never visible in unstained or stained wet mounts; at the most, they may be seen as uncolored areas in the cytoplasm and are thus difficult to identify. In permanent stained smears (trichrome or hematoxylin), the nucleus (or nuclei) is small (about 2-3 μm) and contains a cluster of 3-8 chromatin granules. Diagnosis may be difficult when the nuclear chromatin is covered with stain deposits or when fragmentation is not evident (poor differentiation during the destaining process), especially in mononucleate trophozoites (also depending on the stage of nuclear division). In such cases, Dientamoeba fragilis trophozoites may be mistaken for those of Endolimax nana. Peripheral chromatin is absent; the nuclear membrane is very thin and often hard to distinguish. In the binucleate forms, the nuclei are sometimes linked by a thin, connecting thread (centrodesmus or desmose);  this appearance is representative of the arrest of nuclear division in telophase. The cytoplasm contains little granules but may also contain bacteria, yeast or starch granules. The trophozoite is often parasitized by Sphaerita.

Identification of Dientamoeba fragilis is usually made on smears of fresh stool stained with Giemsa’s, May-Grünwald-Giemsa’s or Field’s stain. However, a great deal of experience is required to avoid confusion with cysts or trophozoites of other amoebae species that can be present in the same specimen, and to correctly interpret degenerated and vacuolated forms of Dientamoeba fragilis that can resemble Blastocystis sp.

Images