Dientamoeba fragilis

Disease / Pathogenesis

The flagellate infects the mucosal crypts of the large intestine which are located close to the mucosal epithelium cells. The organism is not considered to be invasive or produce any type of cellular damage. However it does cause surface irritation that initiates an eosinophilic inflammatory response. Symptoms vary but commonly include abdominal pain and variable diarrhea. Stool appearance can be greenish-brown, mucoid, but not usually bloody. Disease is produced regardless of the host’s immune status.

Most common in children ages 5-10 years. Other symptoms that patients may have include flatulence, bloating, constipation, anorexia including weight loss. Less common symptoms may include irritability, pruritus, headache and fever. These symptoms may be the result of co-infections with helminths.

Location in Host

The location of the parasite is the large intestine (cecum and illeum)

Geographic Distribution

Cosmopolitan in distribution


There is no definitive vector however other helminths are thought to be mechanical vectors of the flagellate to the host. An example is Enterobius vermicularis or pinworm. This is due to Dientamoeba fragilis having no known cyst form which would more efficiently act as a transmissible form. Only the trophozoite form of the flagellate can be detected in ova and parasite examinations. Hand/mouth transmission is also thought to be a viable mode though limited by the frailty of the trophozoite.

Life Cycle

Image courtesy of the CDC

Morphology & Diagnosis

The organism as noted previously does not have a known cyst stage. The diagnostic stage is the trophozoite stage which is characteristically binucleated and stains lightly. The light staining may make it difficult for the inexperienced to detect it in stained preparations as the trophozoite may blend in with background material. The size of the trophozoite is approximately 5 to 15 um which is particularly small and therefore makes it even more difficult to detect.


Dientamoeba fragilis trophozoite in trichrome stain – delicate staining and binucleate
Dientamoeba fragilis, note the binucleate and light staining appearance of the trophozoites – photo by W. Vientos
Close-up of Dientamoeba fragilis trophs with nuclei pointed out – photos by W. Vientos
Dientamoeba fragilis in trichrome stain – delicate staining and binucleate