Norovirus genotypes implicated in two oyster-related illness outbreaks in Ireland

Abstract

We investigated norovirus (NoV) concentrations and genotypes in oyster and faecal samples associated with two separate oyster-related outbreaks of gastroenteritis in Ireland. Quantitative analysis was performed using real-time quantitative reverse transcription polymerase chain reaction and phylogenetic analysis was conducted to establish the NoV genotypes present. For both outbreaks, the NoV concentration in oysters was >1000 genome copies/g digestive tissue and multiple genotypes were identified. In faecal samples, GII.13 was the only genotype detected for outbreak 1, whereas multiple genotypes were detected in outbreak 2 following the application of cloning procedures. While various genotypes were identified in oyster samples, not all were successful in causing infection in consumers. In outbreak 2 NoV GII.1 was identified in all four faecal samples analysed and NoV GII concentrations in faecal samples were >108 copies/g. This study demonstrates that a range of NoV genotypes can be present in highly contaminated oysters responsible for gastroenteritis outbreaks.

Fig. 1.

Maximum-likelihood tree based on capsid N/S domain (285 bp) of the NoV GI sequence alignment. Bootstrap analysis was performed for 1000 replicates of the dataset and values of >70% are indicated by the black dots beside the appropriate branch. NoV GI sequences detected during outbreaks 1 and 2 are preceded by ‘>>2010’ and ‘>>2012’, respectively. The lower scale represents genetic distances in nucleotide substitutions per site.

Fig. 2.

Maximum-likelihood tree based on capsid N/S domain (294 bp) of the NoV GII sequence alignment. Bootstrap analysis was performed for 1000 replicates of the dataset and values of >70% are indicated by the black dots beside the appropriate branch. NoV GII sequences detected during outbreaks 1 and 2 are preceded by ‘>>2010’ and ‘>>2012’, respectively. The lower scale represents genetic distances in nucleotide substitutions per site.