Norovirus contamination and genotypes in shellfish in Shanghai

Background Norovirus (NoV) is a major cause of foodborne diseases. Shellfish as an important vector can bioconcentrate viruses from the environment.

Objective This study investigates NoV contamination in retail shellfish in Shanghai and genotypes of positive strains, aiming to provide evidence to prevent foodborne diseases and make relevant preventive measures.

Methods Real-time reverse transcription polymerase chain reaction (RT-PCR) was used for screening NoV in 580 shellfish samples collected from Shanghai markets from 2017 to 2019. Reverse transcription nested polymerase chain reaction (RT-Nest-PCR) was used for amplification of partial viral protein 1 (VP1) fragments and genotyping.

Results Among a total of 580 shellfish samples, the positive rate of NoV was 8.97%. GⅡ was the main genotypes (5.86%, 34/580), followed by GⅠ (2.24%, 13/580) and GⅠ+GⅡ dural infection (0.86%, 5/580). The positive rates of NoV in shellfish samples were different among different seasons (χ²=8.132, P=0.043), higher in winter and spring. The positive rates in different shellfishes were also different (χ²=14.152, P=0.049), higher in Unionidae and Ostreidae. The phylogenetic analysis results for the partial VP1 showed that GⅡ.2, 3c, 13, and 17 were positive in the shellfish samples. The identity of nucleotide sequences between one GⅡ.2 strain and the reference X81879 was 88.8%; between two GⅡ.3c strains, 91.6% identity, and between the two GⅡ.3c strains and the reference AB385626, 87.1%-92.0% identity; between one GⅡ.13 strain and the reference AY113106, 93.6% identity; between one GⅡ.7 strain with the reference LC037415, Nanjing GⅡ.17 strain KU757049, and Shanghai GⅡ.17 strain (KT380915), the identities were 98.6%, 97.8%, and 97.4%, respectively.

Conclusion NoV contamination is prevalent in retail shellfish in Shanghai, and displays seasonality with a winter/spring peak. NoV GⅡ remain the predominant genotype.