Dennis L. Murray
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. 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 http://www.dpd.cdc.gov/dpdx/html/imagelibrary/A-F/Dientamoeba/body_Dientamoeba_il1.htm. 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. 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
D fragilis has been found in most parts of the world. Prevalence varies widely from 1.4% to 38% in selected populations.1 In 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.1 The 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 the patient’s immune status.
Both acute and chronic illnesses have been associated with infection due to D fragilis. Infected children and adults may present with acute watery diarrhea, accompanied by abdominal pain, cramping, nausea, and vomiting. Fever, fatigue, weight loss, and weakness are less common but can occur. Stools are usually mushy or sticky and at times may contain mucous or, rarely, blood.1D fragilis infection is most often associated with chronic abdominal pain lasting months to years. Patients may present having alternating diarrhea and constipation, flatulence, and fatigue.1,4,8 Some patients with chronic infection have been reported with low-grade eosinophilia.8,9 Many individuals harbor this protozoan and have no symptoms.1
This organism should be suspected in patients who have had abdominal pain and/or diarrhea for an extended period of time, particularly those patients who reside in institutions, patients who recently traveled outside the United States, or those who are infected with pin-worms.1,5,8,10 On physical examination, usually no specific findings are evident. Stool specimens should be collected on alternate days for a total of at least three specimens.1 Specimens should be placed immediately in a stool preservative, such as polyvinyl alcohol. Trichrome staining of stool after sedimentation concentration technique may produce the best yield of D fragilis trophozoites. In unpreserved feces, the morphology of the trophozoites deteriorates within 15 to 20 minutes.11 Single stool specimens are diagnostic only slightly greater than 50%. Three stool specimens increase the diagnostic yield to over 70%. Six stool specimens increase the diagnostic yield to over 90%.11 Interference with the detection of protozoa may occur with barium, so stool samples should be collected before radiological studies with barium are performed. Interference with detection can also occur with mineral oil, bismuth-containing preparations, and certain other medications.1No serological test is commercially available.
Treatment is recommended only for symptomatic infection. Iodoquinol, tetracycline, and paromomycin are recommended treatments.1,11 However, tetracycline and paromomycin are investigational due to a lack of clinical studies.1Metronidazole may be effective. Adults should be treated with iodoquinol at a dose of 650 mg tid for 20 days1,11,13 and children at a dose of 40 mg/kg per day (maximum 2 gm/day) divided tid for 20 days.1,14 Iodoquinol should be taken with food. Alternatively, paromomycin at a dose of 500 mg tid in adults1,11,13 and 25 to 35 mg/kg per day in divided doses for 7 days in children1,14 may be more effective than iodoquinol but may not be readily available. The syrup form of paromomycin is no longer available in the United States. Tetracycline may be used in adults (500 mg qid for 10 days) and in children at least 8 years of age (10 mg/kg qid, for a maximum of 2 gm/day for 10 days), given on an empty stomach.1,14 Metronidazole dosing includes 500 to 750 mg tid for 10 days in adults1,11 and 20 to 40 mg/kg per day divided tid for 10 days (maximum 2 gm/day) in children.1,14 Due to several potential drug interactions and many adverse reactions from these medications, the need for therapy should be evaluated on a case by case basis, and treatment options should be discussed with the patient and/or family.1
Nitazoxanide, a thiazolide derivative, which has a broad spectrum of antiprotozoal and antihelminthic activity, may have some value in treating children whose persistent diarrhea could be due to agents like Blastocystis hominis, D fragilis, and other protozoa. Treating D fragilis with a combination of agents, including furazolidone, nitazoxanide, and secnidazole, has been recommended by a group of physicians in Australia.17
Educating patients and families about judicious hand-washing and improved personal hygiene is essential, as well as disinfecting surfaces and equipment handled by infected persons. In childcare centers, diapering areas should be separated from the food-preparation site.