General Characteristics

D. fragilis was described in 1918. It has a worldwide distribution, and surveys report incidence rates of 1.4% to 19%. Much higher incidence figures have been reported for patients in mental institutions, missionaries, and Native Americans in Arizona. D. fragilis tends to be common in some pediatric populations, and the incidence is higher for patients under 20 years of age in some studies. Some speculate that D. fragilis may be infrequently recovered and identified; a low incidence or absence from survey studies may be due to poor laboratory techniques and a general lack of knowledge about the organism.

The D. fragilis trophozoite is characterized as having one nucleus (20% to 40%) or two nuclei (60% to 80%). The nuclear chromatin usually is fragmented into three to five granules, and normally no peripheral chromatin is seen on the nuclear membrane. In some organisms the nuclear chromatin tends to mimic that of E. nana, E. hartmanni, or even C. mesnili, particularly if the organisms are overstained with trichrome or iron hematoxylin stain. The cytoplasm is usually vacuolated and may contain ingested debris and some large, uniform granules. The cytoplasm can also appear uniform and clean with few inclusions. Size and shape vary considerably among organisms, even on a single smear.

Epidemiology

The life cycle and mode of transmission of D. fragilis are not known, although transmission in helminth eggs (e.g., Ascaris and Enterobius spp.) has been postulated (see Figures 1 to 3). The cyst stage has not been confirmed to date.

Fig1. Trophozoites of Dientamoeba fragilis.

Fig2.  A and B, Trophozoites of Dientamoeba fragilis. 

Fig3. A, Dientamoeba fragilis, two nuclei. B, D. fragilis, one  nucleus. 

Pathogenesis and Spectrum of Disease

D. fragilis has been associated with a wide range of symptoms. Case reports of children infected with D. fragilis reveal a number of symptoms, including intermittent diarrhea, abdominal pain, nausea, anorexia, malaise, fatigue, poor weight gain, and unexplained eosinophilia. The most common symptoms in patients infected with this parasite appear to be intermittent diarrhea and fatigue. In some patients, both the organism and the symptoms persist or reappear until appropriate treatment is initiated.

Laboratory Diagnosis

Routine Methods. Diagnosis of D. fragilis infections depends on proper collection and processing techniques (a minimum of three fecal specimens). Although the survival time for this parasite has been reported as 24 to 48 hours in the trophozoite form, the survival time in terms of morphology is limited, and stool specimens must be examined immediately or preserved in a suitable fixative soon after defecation. It is particularly important to examine permanent stained smears of stool with an oil immersion objective (×100).These trophozoites have been recovered in formed stool; therefore, a permanent stained smear must be prepared for every stool sample submitted for examination. Organisms seen in direct wet mounts may appear as refractile, round forms; the nuclear structure cannot be seen without examination of the permanent stained smear.

Antigen Detection. Although fecal immunoassays for antigen detection are not yet available commercially, they have been developed using several test formats. Detection of DNA from feces also is being used in some laboratories.

Antibody Detection. On indirect immunofluorescence assay, serum samples from patients with confirmed D. fragilis infections showed positive titers, and all matched controls had positive titers ranging from 20 to 160. However, these tests are not routinely used, nor are the reagents commercially available.

Therapy

Clinical improvement has been seen in adults receiving tetracycline, and symptomatic relief has been observed in children receiving diiodohydroxyquin, metronidazole, or tetracycline. Current recommendations include iodoquinol, paromomycin, or tetracycline. Although limited studies have been undertaken on the efficacy of various therapies, information continues to support the finding that elimination of this organism from symptomatic patients leads to clinical improvement. Treatment of D. fragilis infection with iodoquinol, paromomycin, or combination therapy results in eradication of the para site and complete resolution of symptoms.

Prevention

 Fecal-oral transmission has not been documented; therefore, it is difficult to speculate about preventive measures. However, if transmission does occur from ingestion of certain helminth eggs, the appropriate hygiene and sanitary measures to prevent contamination with fecal material are appropriate.

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