Giardia duodenalis is the most prevalent intestinal protozoan infection especially in children. In Portugal scarce data are available relative to this infection in preschoolers. The present study was conducted from April to July 2009 in public preschools in Lisbon enrolling 316 children. Stool examination was performed through microscopy. Molecular analysis was conducted in all positive samples for G. duodenalis in order to determine the assemblage and subassemblage of this parasite. Eight of the preschoolers studied children (2.5%, 8/316) were infected with G. duodenalis. Additionally the brother of one of the infected children was also infected. Genotyping analysis targeting ssu-rRNA and β-giardin loci revealed six infections with assemblage A and 3 with assemblage B. Sub-assemblage determination was possible in four of the samples, with three A2 and one A3. The limited number of cases precluded an association of a determined symptom with an assemblage. The data presented here show the relevance of considering G. duodenalis analysis in children with intestinal complaints even in developed countries. 1. Introduction Giardiasis is a widespread intestinal disease caused by Giardia duodenalis. This protozoan parasite has a global distribution, infecting humans and a wide range of mammalian hosts [1]. The prevalence of giardiasis in humans in developed countries is 2–7% [2], while it may vary between 20 and 30% in developing countries [3]. The spectrum of clinical manifestations in human giardiasis is relatively variable, ranging from the absence of symptoms to acute or chronic diarrhea, dehydration, abdominal pain, nausea, vomiting, and weight loss [4]. Children are especially affected, with more severe consequences than adults. However, the impact of Giardia infection in children development is not clear. Some studies showed detrimental effects on nutritional status and poorer cognitive function on children with giardiasis [5–8], while others showed that giardiasis did not affect childhood growth [9]. Host factors, such as immune status, nutritional status, and age, are recognized as important determinants for the severity of infection [10]. However, studies on the possible association between G. duodenalis assemblages and the severity of the disease have proved thus far to be inconsistent [11]. Molecular tools have demonstrated that G. duodenalis is a species complex comprising at least eight assemblages (A–H), among which only A and B were found infecting humans [12]. Previous studies focused on the prevalence of giardiasis in preschool children (3-4%)
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