Introduction: Dientamoeba fragilis (DF) is a commonly occurring intestinal protozoan that is considered a possible cause of infectious gastrointestinal disease in adults and children. DF has a particular high prevalence in children, and it is suspected that children present more symptoms in infection than adults. However, evidence of causality is lacking, treatment regimens are largely untested in controlled trials, and the most commonly used antibiotic against DF in Denmark, metronidazole, has never been tested against placebo.

Main objective: To determine the clinical effect of metronidazole in DF-infected children with gastrointestinal complaints, where no other aetiology is known and no other gastrointestinal pathogens could be shown.

Inclusion Criteria:

• Patients with samples investigated at Statens Serum Institut.
• Faecal sample (index) positive for Dientamoeba fragilis (DF) by realtime PCR, within < 7 days.
• No faecal samples positive for DF within the period: 3 months prior to and up to index-sample.
• Telephone interview to parents no later then 14 days after result from index-sample.
• Age 3-12 years old.
• Place of residence: Island of Zealand, incl. capital region.
• Symptoms consistent with gastrointestinal infection of DF: Either 1) ≥ 2 episodes of diarrhea per week or 2) ≥ 2 episodes of stomach ache per week or 3) ≥ 2 of the following symptoms: Anorexia, Failure to thrive, Anal itching, excessive flatulence, other change in bowel movements.

Exclusion Criteria:

• Expected non-compliance.
• Objection to subject participation from referring physician.
• Underlying illness or comorbidity, incl. known gastrointestinal illness (both infectious and non-infectious), but excluding constipation.
• Known liver disease or intolerance/allergy to metronidazole.
• Positive screening for other intestinal pathogens, which may explain subject symptoms.
• Treatment with metronidazole outside of study within study period.
• Weight > 50 kg