These are small amoebae, usually with 2 nuclei. The nuclei are vesicular, with a delicate membrane and an endosome consisting of several chromatin granules connected to the nuclear membrane by delicate strands. No cysts are known. Dobell (1940) considered that this genus might be an aberrant flagellate closely related to Histomonas.
This species occurs in the cecum and colon of man and also of some monkeys. According to Belding (1952), it was found in 4.2% of 7120 persons in 14 surveys thruout the world. Hegner and Chu (1930) found D. fragilis in 2 out of 44 Macaca philippinensis in the Philippines, and Knowles and Das Gupta (1936) found it in 1 out of 4 M. irus in India. In addition, Noble and Noble (1952) mentioned finding a Dientamoeba in sheep feces in California.
Only trophozoites are known for this species. They are very sensitive to environmental conditions, bursting in water and becoming degenerate in older fecal samples. In order to identify them, smears of fresh feces should be fixed in Schaudinn's fluid containing 10 to 20% acetic acid or in Bouin's fluid and stained with iron hematoxylin. Their morphology has been studied by Wenrich (1936, 1939, 1944) and Dobell (1940), whose accounts do not always agree.
The trophozoites range from 3 to 22 u but are usually 6 to 12 u in diameter. The ectoplasm is distinct from the endoplasm, which contains food vacuoles filled with bacteria, yeasts, starch granules, and parts of cells. In fresh feces there may be a single clear, broad pseudopod. About 3/5 of the protozoa contain 2 nuclei which are connected by a filament or desmose. This appears to be one of the first structures to disappear during degeneration. Each nucleus is vesicular and has an endosome composed of 4 to 8 granules from which a few delicate fibers radiate to the nuclear membrane. There is no peripheral chromatin. Reproduction is by binary fission. There are 4 chromosomes.
At one time D. fragilis was thought to be non-pathogenic, and this is true in most cases. However, in some persons it causes a mucous diarrhea and gastrointestinal symptoms. It does not invade the tissues, but may cause low-grade irritation of the intestinal mucosa, excess mucus secretion and hypermotility of the bowel. There may be mild to moderate abdominal pain and tenderness or discomfort. There may also be an increase in eosinophiles.
The mode of transmission of D. fragilis is not clear, since there are no cysts and the trophozoites are so delicate. Dobell (1940) was unable to infect himself by mouth or 2 monkeys by mouth or rectally and suggested, by analogy with Histomonas, that D.fragilis might possibly be transmitted by an intestinal nematode such as Trichuris. This idea has been partially confirmed by Burrows and Swerdlow (1956), who found small, amoeboid organisms resembling D. fragilis in the eggs of Enterobius vermicularis and suggested that the pinworm might be the vector.
D. fragilis can be readily cultivated in the usual culture media. It is sensitive to most amoebicidal drugs, including carbarsone, diodoquin and erythromycin.