Dientamoeba fragilis

Dientamoeba fragilis is an amoeboflagellate and, though forming no flagella, is most closely allied with the flagellates. It is currently placed within the family Monocercomonadidae. Trophozoites are small, 5-12 micrometers in length, and 60-80% are binucleate. Good staining reveals a mitotic spindle interconnecting the two nuclei. No cysts are formed and the parasite is unable to survive passage through the stomach. Like most intestinal amoebae (or in this case amoebae-like organism), Dientamoeba fragilis can cause mild to moderate diarrhea and abdominal discomfort. However, it is not known to be invasive like Entamoeba histolytica. Studies have shown that D. fragilis can survive in the eggs of the human pinworm, Enterobius vermicularis (1956, Am. J. Trop. Med. Hyg. 5: 258-265; 1977, Am. J. Trop. Med. Hyg. 26: 16-22). Thus, like the closely related Histomonas meleagridis, it is likely that transmission of this protozoan is also through ingestion of pinworm ova. One recent study (2000, J Clin Microbiol 38: 4653-4654) has revealed two distinct genetic variants. These may or may not represent distinct, morphologically similar species.