Use of Mytilus edulis biosentinels to investigate spatial patterns of norovirus and faecal indicator organism contamination around coastal sewage discharges

Bivalve shellfish have the capacity to accumulate norovirus (NoV) from waters contaminated with human sewage. Consequently, shellfish represent a major vector for NoV entry into the human food chain, leading to gastrointestinal illness. Identification of areas suitable for the safe cultivation of shellfish requires an understanding of NoV behaviour upon discharge of municipal-derived sewage into coastal waters. This study exploited the potential of edible mussels (Mytilus edulis) to accumulate NoV and employed the ISO method for quantification of NoV within mussel digestive tissues. To evaluate the spatial spread of NoV from an offshore sewage discharge pipe, mesh cages of mussels were suspended from moorings deployed in a 9 km ² grid array around the outfall. Caged mussels were retrieved after 30 days and NoV (GI and GII), total coliforms and E. coli enumerated. The experimentally-derived levels of NoV GI and GII in mussels were similar with total NoV levels ranging from 7 × 10 ¹ to 1.6 × 10 ⁴ genome copies g ⁻¹ shellfish digestive gland (ΣGI + GII). NoV spread from the outfall showed a distinct plume which matched very closely to predictions from the tidally-driven effluent dispersal model MIKE21. A contrasting spatial pattern was observed for coliforms (range 1.7 × 10 ² to 2.1 × 10 ⁴  CFU 100 g ⁻¹ shellfish tissue) and E. coli (range 0–1.2 × 10 ³  CFU 100 g ⁻¹ shellfish tissue). These data demonstrate that hydrodynamic models may help inform effective exclusion zones for bivalve harvesting, whilst coliform/E. coli concentrations do not accurately reflect viral dispersal in marine waters and contamination of shellfish by sewage-derived viral pathogens.