title: Comparison of norovirus RNA levels in outbreak-related oysters with background environmental levels.
abstract: Norovirus is the principal agent of bivalve shellfish-associated gastroenteric illness worldwide. Numerous studies using PCR have demonstrated norovirus contamination in a significant proportion of both oyster and other bivalve shellfish production areas and ready-to-eat products.  By comparison, the number of epidemiologically confirmed shellfish-associated outbreaks is relatively low. This suggests that factors other than the simple presence or absence of virus RNA are important contributors to the amount of illness reported. This study compares norovirus RNA levels  in oyster samples strongly linked to norovirus or norovirus-type illness with the levels typically found in commercial production areas (non-outbreak-related samples). A statistically significant difference between norovirus levels in the two sets of samples was observed. The geometric  mean of the levels in outbreak samples (1,048 copies per g) was almost one order of magnitude higher than for positive non-outbreak-related samples (121 copies per g). Further, while none of the outbreak-related samples contained fewer than 152 copies per g, the majority of positive  results for non-outbreak-related samples was below this level. These observations support the concept of a dose-response for norovirus RNA levels in shellfish and could help inform the establishment of threshold criteria for risk management.
authors.affiliation: Centre for Environment, Fisheries and Aquaculture Science, European Union Reference Laboratory for Monitoring Bacteriological and Viral Contamination of Bivalve Molluscs, Weymouth DT4 8UB, UK. james.lowther@cefas.co.uk
authors.forename: James A
authors.initials: JA
authors.lastname: Lowther
authors.forename: Nicole E
authors.initials: NE
authors.lastname: Gustar
authors.forename: Rachel E
authors.initials: RE
authors.lastname: Hartnell
authors.forename: David N
authors.initials: DN
authors.lastname: Lees
mesh_terms: D000818:Animals; D003257:Consumer Product Safety; D004196:Disease Outbreaks; D005506:Food Contamination / Q000032:analysis*; D005516:Food Microbiology; D005759:Gastroenteritis / Q000453:epidemiology / Q000821:virology; D006801:Humans; D029322:Norovirus / Q000302:isolation & purification*; D010125:Ostreidae / Q000821:virology*; D012367:RNA, Viral / Q000032:analysis*; D012758:Shellfish / Q000821:virology*; D019562:Viral Load
chemical_list: D012367:RNA, Viral