Dientamoeba fragilis (there are no synonyms) is a little studied parasite of the large intestine, residing on the cecum and ascending colon. “Since its discovery by Weynon in 1909, and its description by Jepps and Dobell in 1918, the parasite has been found in practically all regions of the world.” (10) Once believed to be commensal, experts now believe that it might be one of the chief causes of gastrointestinal disease. However, most doctors are unfamiliar with D. fragilis and many labs do not routinely test for it.
A Line drawing of a Dientamoeba fragilis trophozoite (Courtesy of DPDx) 4
Dientamoeba fragilis was originally classified as an ameba, even though it is commonly binucleate and lacks a cyst stage. Protozoologists recently classify D. fragilis as a flagellate within the genera Trichomonas even though it was thought to lack flagella. However, the latest electron microscope studies have revealed that D. fragilis organisms do in fact have flagella.
D. fragilis generally range in diameter from 7 to 12 μm, but can be anywhere from 3 to 18 μm. “Pseudopodia are hyaline, broad, and leaflike in appearance, with characteristic serrated margins. Motility is progressive, and organisms are quite active in freshly passed stools but round up quickly on cooling.” (7) The majority of organisms, as many as 80 percent, are binucleate. The remaining 20 percent are primarily mononucleate, but can also include forms with three or four nuclei.
A study published by Johnson and Clark (5) in 2000 revealed evidence for two genetically distinct forms of D. fragilis. The study examined organisms found in fecal samples of both symptomatic and asymptomatic individuals. Based on the results, Johnson and Clark hypothesized that the uncertainty as to its role in human disease results from the existence of pathonogenic and nonpathonogenic forms. Future studies will need to take this genetic diversity into account.