Norovirus contamination in retail oysters from Beijing and Qingdao, China

Abstract

The consumption of raw oyster has been linked to numerous foodborne gastroenteritis outbreaks caused by norovirus (NoV) in different countries of the world, including in China. This study investigated the prevalence of NoV and contamination levels in retail oysters taken from markets in Beijing and Qingdao in China. The oysters were collected monthly from seafood markets between September 2015 and September 2016 (13 months) in both cities. The digestive glands of these oysters were dissected and NoV particles were extracted. Viral RNA was detected using a TaqMan-based real time one-step reverse transcription-PCR (qRT-PCR) protocol. Of total 672 oyster samples assessed, 652 were considered to be valid for inclusion in the study. The prevalence of NoV was 20.71% (135/652), and this included 21.48% (29/135) of the samples that were positive for the GI strain alone, 62.96% (85/135) contained the GII strain alone and 15.56% (21/135) were mixed with GI and GII, respectively. A total of 68 NoV-positive samples were quantified by qRT-PCR and values obtained ranged from 3.55 × 103 to 1.45 × 106 genomic copies per g digestive tissue. The NoV contamination in retail oysters fluctuated with the sampling month and peaks of contamination occurred in February (49.12%) and March (55.36%) 2016, respectively. Considering some of the oysters were consumed as raw or half-cooked in China, as a risk reduction measure these oysters should be well heated prior to consumption. Furthermore, successive and region-extended monitoring in retail oysters for NoV as well as risk communication is recommended.