Foodborne outbreak of norovirus GII.17 due to water contamination caused by backflow from a malfunctioning sewage tank.

Food poisoning caused by human norovirus genotype GII.17 affecting 73 individuals occurred in a single commercial building in Sapporo, Hokkaido, Japan, from late December 2024 to early January 2025. An epidemiological and environmental investigation was carried out and it was identified that the potable water contaminated with the virus was the sole factor for patients with gastrointestinal symptoms. Furthermore, it was revealed that the building's water distribution system was tainted with the virus because the potable water storage tank for supply in the building's underground system was contaminated by sewage which flowed from the wastewater tank. A malfunction of the tank drainage system caused an overflow of sewage containing norovirus into the potable water storage tank, contaminating the building's water supply. Although the mechanism causing this norovirus outbreak is infrequent, ageing building water infrastructure may increase such risks in future, unless appropriate maintenance is regularly performed.