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

Abstract

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 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 to 1.6 × 10 genome copies g 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 to 2.1 × 10 CFU 100 g shellfish tissue) and E. coli (range 0-1.2 × 10 CFU 100 g 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.

Critical Review on the Public Health Impact of Norovirus Contamination in Shellfish and the Environment: A UK Perspective

“…Intermittent discharges offer a greater risk in areas with high average rainfall/runoff or subject to flashy storm events (Riou et al 2007). Viral particles can persist in the marine environment and be transported (>10 km) representing a possible risk to shellfish production areas (Flannery et al 2013; Winterbourn et al 2016). WTWWs are currently not designed to be effective at removing viruses such as NoV.…”

“…NoV prevalence is also positively correlated with sites close to inputs of human faecal matter (Ueki et al 2005; Victoria et al 2010b; Wyn-Jones et al 2011) with a subsequent reduction due to dilution with increasing distance from the source (Kitajima et al 2010). However, NoV has been observed up to 10 km from wastewater discharge points, indicating high survival rates of NoV, persistence in the environment and widespread contamination of the coastal zone (Aquatic Water Services Ltd. 2014; Winterbourn et al 2016). …”

“…Proximity-based zoning has been used to exclude harvesting in areas ranging from 50 to 1500 m from discharges or other inputs (Aquatic Water Services Limited 2014). However, the efficacy of geographical zoning for reducing NoV risk is strongly dependent on prevailing hydrodynamics (Winterbourn et al 2016). Norovirus load in shellfish did not significantly decrease in ≤7 km from a point source discharge, despite E. coli shellfish concentrations meeting class A criteria (Campos et al 2015).…”

Barriers and facilitators to shellfish cultivation

“…), while flooding events have increased pressure on sewage structures and resulted in increased microbial load of coastal and harvesting waters (Winterbourn et al . ). In particular, bivalve larvae naturally exhibit high mortality rate (>98% in the wild) during their transitional phase from free swimming larvae juveniles.…”

“…Other water users pose the greatest threat to water quality (Winterbourn et al . ). This is evident during summer when there is greater effluent loading into water bodies located near popular tourist destinations (Dabrowski et al .…”