1. *Hospital for Children and Adolescents, University of Helsinki, PO Box 280, FIN-00290 Helsinki, †Laboratory of Enteric Pathogens, National Public Health Institute (KTL), Mannerheimintie 166, FIN-00280 Helsinki and ‡Department of Pediatrics, Tampere University Hospital, PO Box 2000, FIN-33521 Tampere, Finland