Survey of Norovirus Genogroup II Contamination in Commercial Oysters in Guangzhou from 2020 to 2021

Filter-feeding oyster is an important food transmission vector for the foodborne norovirus. To investigate the contamination level and genetic diversity of norovirus in commercial oysters in Guangzhou and better assess the risk of consumption, oyster samples were collected from local aquatic market from June 2020 to May 2021 in this study. The contamination content and genotype distribution of virus in shellfish were detected by protease K treatment coupled with polyethylene glycol precipitation small system previously established in the laboratory, including the real-time reverse transcription polymerase chain reaction and the nested RT-PCR. 110 oyster samples were detected, the positive rate of norovirus genogroup II(GII) was 52.7% (58/110), and the contents of virus was 1.56×10³~1.09×10⁶ copies/g (digestive gland). The positive rate of norovirus in oysters collected in spring and summer (March to August) was 35.7% (20/56), which was lower than the positive rate of 70.4% (38/54) in autumn and winter (September to February). However, there was no significant difference in content of virus detected in all seasons, in spring, content of virus was (2.69±1.46)×10⁵ copies/g (digestive gland), in summer, content of virus was (1.97±2.16)×10⁵ copies/g (digestive gland), in autumn, content of virus was (6.91±6.16)×10⁴ copies/g (digestive gland), and winter was (4.83±2.99)×10⁴ copies/g (digestive gland). Positive samples were sequenced and analyzed, the result showed that except for one was GII.17 genotype, the rest were all GII.4 genotypes (n=13), which was consistent with the local clinical epidemic genotypes. The results of this study showed that the level of norovirus contamination in commercial oysters of Guangzhou was severe, it is necessary to strengthen control work of prevention for virus, especially to remind consumers to fully process when eating oysters.