Outbreaks of Gastroenteritis Caused by Norovirus in Hokkaido, 2003/04 Season

There were 76 outbreaks of acute gastroenteritis in Hokkaido from November 2003 to June 2004 and viral tests for these outbreaks were carried out. In 65 of these outbreaks, the genome of Norovirus (NV) was detected from patient samples by reverse transcription-polymerase chain reaction (RT-PCR). Of the 65 outbreaks, only 8 (12.3%) appeared to be due to food-borne infection, and out of them, 3 were associated with the consumption of oysters. Most of outbreaks (87.7%) appeared to be due to infection by person-to- person transmission. In 2003/04 season, very few outbreaks occurred in restaurants (3.1%, 2 of 65), and more than half of outbreaks occurred in facilities with closed environment, such as nursing homes (21.5%), nursery schools and kindergartens (16.9%), welfare facilities (12.3%), and hospitals (9.2%). Outbreaks in nursing homes, welfare facil- ities, and hospitals occurred mainly from February to April, while outbreaks in elementary schools and junior high schools occurred until June. Based on the genetic analysis using nucleotide sequences of a part of the capsid gene, it was found that most of outbreaks unassociated with oyster consumption (85.5%, 53 of 62) were caused by NV strains belonging to genogroup II, and 41 of them were associated with NV strains belonging to a specific cluster including the Bristol strain. Moreover, 85% of outbreaks in hospitals and nursing homes, which were the most common settings of outbreaks in 2003/04 season, were associated with NV strains belonging to this cluster. These results suggest that a remarkable increase of outbreaks by person-to-person transmission in 2003/04 season, might be caused by trends of NV strains closely related to the Bristol strain