Noroviruses in oysters from local markets and oyster farms in Southern Thailand

Abstract

One hundred and eighteen oyster samples collected from local markets and oyster farms in southern Thailand were examined for noroviruses (NoVs) and bacterial indicators of fecal contamination (fecal coliforms and Escherichia coli). Using a virus concentration procedure followed by RT-nested PCR, NoVs were detected in 38% of the samples. Oysters collected from oyster farms were found with NoVs at a higher detection rate (25/53 samples) than oysters from local markets (20/65 samples). Of the 45 NoV-positive oyster samples, 67% belonged to NoV genogroup I (GI), 15% to GII, and 18% to both GI and GII. DNA sequencing showed that 2 NoVs belonged to NoV GI-2 genotype. Fecal coliforms in NoV-positive oyster samples were in the range of < 3.0 to 1.5 × 10 4 most probable number (MPN)/g and 33% of NoV-positive oyster samples contained fecal coliforms within the standard acceptable level of raw shellfish ( < 20 MPN/g). E. coli was found in the range of < 3.0 to 1.5 × 10 4 MPN/g and 9% of NoV-positive oyster samples were within acceptable levels of E. coli contamination ( < 3 MPN/g). These findings indicate that NoV contamination in oysters obtained from both markets and oyster farms might pose a potential risk of acute gastroenteritis associated with raw oyster consumption. Examination for both fecal bacterial indicators and enteric viruses should be conducted for microbiological food safety of shellfish.