Genotypic prevalence of norovirus GII in gastroenteritis outpatients in Shanghai from 2016 to 2018

BACKGROUND: With the help of an existing citywide comprehensive surveillance on gastroenteritis outpatients, although norovirus genogroup II (NoV GII) was tested routinely, its genotypes were never investigated systematically on a municipal level. This study aimed to understand the prevalence, major genotypes and evolutional trends of NoV GII in Shanghai during the period of 2016-2018, and to provide molecular bases for early warning for any potential NoV outbreaks. METHODS: 27 sentinel hospitals from all 16 districts were recruited by stratified probability proportional to size (PPS) method in Shanghai comprehensive diarrhea surveillance programme. Stool samples were collected and screened for NoV GII by real-time reverse transcription polymerase chain reaction (qRT-PCR). For samples that were positive in qRT-PCR, conventional RT-PCR was performed to amplify the ORF1-ORF2 junction of NoV GII gene. Generated sequences were typed by RIVM online genotyping tool, and then strains of interest were analyzed phylogenetically using MEGA 6.0. RESULTS: A total of 7883 stool samples were collected from diarrhea outpatients, among which 6474 were from adults and 1409 were from children. 13.66% (1077 cases) were screened positive in qRT-PCR for NoV GII, from which 71.96% (775 cases) were sequenced successfully. The top three genotypes were GII.Pe/GII.4 (37%), GII.P17/GII.17 (26%) and GII.P16/GII.2 (17%). While GII.Pe/GII.4 detection rate decreased significantly over the 3 years (from 48.4 to 20.9%); GII.P16/GII.2 appeared for the first time in October 2016 and rose rapidly to 27.0% in 2017, but fell back to 23.4% in 2018. Meanwhile there was a significant increase for both GII.P12/GII.3 and GII.P7/GII.6 recombinant genotypes detected in adult population in 2018. Phylogenic analysis revealed the existence of multiple gene clusters within both of these recombinant genotypes. CONCLUSION: Unlike the alternating circulation of GII.4 and non-GII.4 NoV observed in 2016 or 2017, the genotype profile of NoV GII in 2018 was characterized by the co-prevalence of multiple recombinant genotypes. A recent increase in detection rate in less reported recombinant genotypes such as GII.P12/GII.3 and GII.P7/GII.6 among adult population calls for a continuing close monitoring on NoV GII genotypes in case of potential local outbreaks.