This study investigates the efficacy and safety of rectally administered metronidazole for pediatric Giardia lamblia infection, as well as the acceptance of this treatment regimen by patients/caregivers. The investigators hypothesize that rectal metronidazole is non-inferior to single-dose of oral tinidazole and will cure at least 72% of patients, a well as will be tolerated as good as tinidazole.