Identifying norovirus exclusion zones in shellfish production areas through microbiological monitoring, drogue tracking and sewage effluent tracing: studies in a shallow estuary and a deep coastal embayment

The establishment of exclusion zones to provide separation between sewage discharges and shellfish production areas (SPAs) can reduce the risk of shellfish-related illness, particularly for norovirus (NoV). This study... [ view full abstract ]

The establishment of exclusion zones to provide separation between sewage discharges and shellfish production areas (SPAs) can reduce the risk of shellfish-related illness, particularly for norovirus (NoV). This study developed a sewage dilution-based approach to buffer zones for SPAs with different oceanographic characteristics and shellfish species. Cages with mussels and oysters were placed at different distances from sewage discharges representing varying dilutions in a shallow estuary and in a coastal embayment on the coast of England. Microbiological pollution in the sites was characterised through a monitoring programme for NoV (genogroups I and II) and E. coli. Fluorometers were attached to the shellfish cages to record levels of dye tagged effluent and determine the residence time and dilutions at steady state. Satellite drogue tracking as well as dye tracing by boat were conducted to determine the dispersion of dye tagged effluent within the SPAs. High percentages of samples were NoV-positive during the sampling periods. Effluent circulation patterns and dilutions were compared with NoV and E. coli levels. Linear associations were found between effluent dilution and NoV (R2 = 74%) and E. coli (R2 = 54%) contamination in oysters during the period October–March. The NoV (GI + GII) concentrations predicted by the model at 300:1, 1,000:1 and 5,000:1 dilutions of estuarine water to sewage effluent were 1,200; 600; and 200 copies/g, respectively. The approach used in this study enables understanding of the factors that influence exclusion zones and can be applied with confidence as a control measure for NoV in SPAs.