Dientamoeba fragilis: A harmless commensal or a mild pathogen?

Dientamoeba fragilis is a flagellated protozoan which, unlike other intestinal protozoan parasites, does not have a cyst stage. Neither its epidemiology nor its route of transmission is completely known. It is infrequently found in association with other parasitic intestinal pathogens with the exception of pinworm infestations, where the prevalence of infections with both organisms is ninefold greater than would be expected based on just a random association (1).

D fragilis is believed to be transmitted between human hosts inside helminth eggs or larvae, particularly those of Enterobius vermicularis. The evidence for this is the frequent association between the two organisms and the presence of bodies inside the helminth eggs, which resemble D fragilis (1,2). The prevalence of the parasite in stools varies from 0.2% to more than 19%, depending upon the population studied. The prevalence was 4.2% in one large study, involving approximately 43,000 diagnostic specimens submitted to the parasitology laboratory in Toronto from 1970 through 1974 (1). In another study in Calgary involving 2652 diagnostic stool specimens submitted from 1532 children hospitalized or attending the general pediatric or gastroenterology out-patient clinics from July 1989 through June 1992, D fragilis was only detected in 33 (0.2%) of children (3). A similar analysis was done at the Children’s Hospital of Eastern Ontario in Ottawa from 1989 to 1993, and D fragilis was found in 2.6% of submitted specimens (4). However, in a study conducted in Toronto day cares in 1981, the prevalence of D fragilis, based on examination of a single stool specimen, was 8.6% among 900 attendees and 4% among 146 adult employees (5). In this study, there was no apparent association with any symptoms. Finally, Chan et al (4) recently reported that 91% of healthy children, ranging from six months to age 19 years, had specific antibodies against D fragilis, implying an almost universal exposure to the parasite over time.

THE DISEASE

There continues to be debate concerning whether the organism is a harmless commensal or is pathogenic. D fragilis infests the large intestine and, while its presence can be observed in the intestinal crypts on biopsy, it does not actively invade the gastrointestinal tract. In a number of small or uncontrolled studies, it has been associated with vague abdominal pain, bloating, diarrhea, fatigue, weight loss and anorexia (6–9). In one study, eosinophilia was noted among 50% of infected children (7). Because it does not penetrate the host tissue, these symptoms may be due to irritation stimulating colonic motility. The eosinophilia may be related, in some instances, to pinworm and other parasite co-infections.

DIAGNOSIS

Diagnosis is by microscopic examination of fresh stool or stool that has been preserved specifically for parasitic examination. The organism can be difficult to detect and three or more samples may be necessary in order to establish the diagnosis of infection (8). Generally, sensitivity is improved if stained, preserved stool specimens are used.

TREATMENT

When this parasite is detected, a search for co-infection with E vermicularis should also be completed, in view of the frequent association between the two parasites. The decision concerning whether to offer specific treatment depends upon the clinical status of the patient. If the child is symptomatic, most experts would recommend a treatment trial to determine whether symptoms can be relieved. If they are not, another diagnosis for the child’s problem should be explored. If the child is asymptomatic, treatment is likely not necessary. If co-infection with E vermicularis is also demonstrated, specific therapy for this parasite is recommended for both the child and the family.

If treatment is given, the regimens outlined in the Table 1 have been recommended (7,9,10). For iodoquinal, important side effects include nausea, abdominal cramping and rash. The drug may interfere with thyroid function tests and should be used with caution in patients with thyroid diseases. Optic atrophy can occur with prolonged usage. The medication is contraindicated in patients with hepatic dysfunction or with pre-existing optic atrophy. Tetracycline and doxycycline should not be administered to children under age eight years. Paromomycin, an aminoglycoside, is not well absorbed from the gastrointestinal tract. Its use can result in overgrowth of resistant bacteria and has been associated with nausea, abdominal cramping and bloating, vertigo, headaches and rashes. While metronidazole may relieve symptoms, it does not necessarily eliminate the parasite.

TABLE 1:

Treatment recommendations

Drug	Dose
Drug of choice
Iodoquinol	40 mg/kg/day divided tid (max 650 mg tid) oral for 20 days (available in 210 or 650 mg tablets)
Alternate therapies
Tetracycline	500 mg oral qid for 10 days
Doxycycline	100 mg oral bid for 10 days
Paromomycin	8 to 12 mg (max 500 mg) oral tid for 7 days
Metronidazole	35 to 50 mg/kg oral divided tid for 5 days

SUMMARY

Although D fragilis is a relatively common finding in stools from children, its clinical importance is still unclear. While treatment of symptomatic children can be considered, treatment of asymptomatic colonization is not warranted. Due to its noninvasive nature, it is not a cause of medical complications for children who harbour it, and chronic carriage has not been associated with any ill effects.