Norovirus contamination found in oysters worldwide

Noroviruses (Norwalk-like viruses) are recognized as major causes of acute gastroenteritis worldwide. Outbreaks of norovirus gastroenteritis are often associated with consumption of oysters. In this study, oysters imported into Hong Kong from 11 countries over a 3-year period were screened by RT-PCR. Overall, 53 out of 507 (10.5%) samples were positive for norovirus-RNA, and a wide variety of strains were found. Two novel genetic clusters were detected, which could indicate novel human or animal norovirus strains. However, whether these two new clusters are of human or animal origin is not known. Thirteen outbreaks, in which oysters were implicated as the source of infection were investigated: Norovirus RNA sequences could be detected in oysters from six outbreaks, but only in one outbreak the strains isolated from patients and oysters matched (>98% homology). Therefore, RT-PCR was of use in detecting norovirus contamination of oysters implicated in an outbreak, but was less useful in demonstrating an actual molecular epidemiological link with human cases. It was shown that contamination by noroviruses could be demonstrated in oysters worldwide, and therefore oysters may serve as an important vehicle for introducing novel norovirus strains.