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Dientamoeba fragilis is a nonflagellate trichomonad parasite that inhabits the human colon and has been associated with acute and chronic gastrointestinal symptoms. Humans are likely the natural host for D fragilis. Unlike most other intestinal protozoa, D fragilis has no known cyst form and has not been found to invade tissues.1 The organism is usually 7 to 12 μm in diameter and demonstrates pointed or leaf-shaped pseudopodia (see Fig. 347-1). While moving actively in fresh fecal specimens, the organism quickly becomes rounded and granular and is therefore less identifiable in stored specimens. When suitably stained, most D fragilis reveal two characteristic nuclei that each contains a large karysome with granules. Some large uninucleate forms may also be found. The organism feeds on bacteria and will ingest human red blood cells.1 Advances in culturing techniques have improved the ability to detect the organism2-4 but are limited, since cultures cannot be done on fixed fecal samples. Because of its small size, this organism may be overlooked by inexperienced laboratory personnel or can be lost if not preserved properly.

Figure 347-1.

Dientamoeba fragilis. Note the two nuclei.

(From Centers for Disease Control and Prevention at Accessed Sept 19, 2010.)

The mode of transmission of D fragilis is unknown; however, several investigators have noted a high frequency of concomitant infection with D fragilis and the pinworm Enterobius vermicularis.1,4 It is thought that this organism infects the human host by entering and passing in pinworm eggs. Amoeboid bodies resembling D fragilis have been described in eggs of E vermicularis.5 However, attempts to culture D fragilis from pinworm eggs or larvae have been consistently unsuccessful, and polymerase chain reaction of nucleic acid from E vermicularis eggs did not identify Dientamoeba DNA in coinfected individuals.6 Other investigators have postulated that transmission occurs by the fecal-oral route with other organisms.1

D fragilis has been found in most parts of the world. Prevalence varies widely from 1.4% to 38% in selected populations.1In Manitoba, Canada, D fragilis was second only to Blastocystis hominis on stool examinations for parasites and was far more common than Giardia,Cryptosporidium, and so on.5 Increased prevalence is seen in persons residing in crowded living conditions, such as those living in institutions and communes and those traveling outside the United States. A serological survey has indicated high prevalence of antibodies to D fragilis in healthy children, suggesting infection occurs in most children, many during infancy.7

Organisms infect mucosal crypts of the large intestine, from the cecum to the rectum.1The parasite is not invasive and does not cause cellular damage. Colonization may occur without disease development, especially in adults. Symptoms come from irritation of the colonic mucosa similar to other parasites. Like Cryptosporidium parvum, D fragilis can cause disease regardless of ...

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