Twenty-nine norovirus (NV) outbreaks that occurred in daycare centers in Suginami City, Tokyo between November 2004 and December 2005 were investigated. In total, 823 of 2,717 children developed symptoms of NV infection, with an average incidence rate of ca. 30%. Children younger than 2 years old showed higher a incidence of ca. 50%. Among the symptoms, vomiting was seen most frequently in 79% of children, while diarrhea and fever were observed in 43% and 14% of the children, respectively. Detection rates for NV were investigated using the stool specimens of 138 children obtained days after the onset of illness. High detection rates of around 90% were observed on the third to fourth day of illness, followed by a decrease in the rate during later periods. However, NV was detected in more than 50% of the patients even 10 days after the onset of illness. Outbreaks of NV GIIoccurred 3 times in a single daycare center within approximately 7 months. In these outbreaks, 3 children became ill 3 times, and 5 children became ill twice. Genotypes of causative viruses for each of the 3 outbreaks were GII/4, GII/6, and GII/2, respectively. All of the food handlers in the daycare centers where the outbreaks took place were tested for NV. NV was detected in 12 (12%) of 104 food handlers, who worked in 8 (28%) of 29 facilities. Of the 12 NV-positive cases, only 3 cases were symptomatic, whereas 9 cases were asymptomatic.
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