A waterborne gastroenteritis outbreak caused by norovirus GII.9[P7] in Guangdong, China

Noroviruses are a predominant cause of acute gastroenteritis (AGE) outbreaks globally, many outbreaks are associated with waterborne transmission. However, waterborne AGE outbreaks caused by the GII.9[P7] strain are relatively rare. In April 2024, an AGE outbreak occurred among high school students on an educational excursion in Guangdong, China. Feces or anal swabs from clinical cases and asymptomatic canteen staffs, water samples from septic tank and tap water, along with food samples were collected for pathogen detection by real-time RT-PCR, and positive samples were subsequently characterized through gene sequencing analysis. From 12 April to 14 April 2024, a total of 84 individuals met the case definitions. The cases occurred continuously throughout the excursion without a distinct epidemic peak and the number of cases decreased significantly after the students left on April 13. Norovirus GII was detected in 12 symptomatic cases (12/24) and an asymptomatic food handler (co-infected with rotavirus A,1/7) and all water samples (7/7). The norovirus strain was identified as GII.9[P7] based on phylogenetic analysis, with 100% nucleotide sequence identity among the clinical cases and water samples, implying that the causative agent of the outbreak originates from contaminated drinking water. This study identified GII.9[P7] norovirus as the causative agent of this outbreak. This was the first reported waterborne outbreak of GII.9[P7] norovirus in China. Our study highlights the necessity of an integrated environmental and clinical case surveillance system for prevention and control of norovirus-associated gastroenteritis outbreak.