The Impact of Winter Relocation and Depuration on Norovirus Concentrations in Pacific Oysters Harvested from a Commercial Production Site

Abstract

Oysters contaminated with norovirus present a significant public health risk when consumed raw. In this study, norovirus genome copy concentrations were determined in Pacific oysters (Magallana gigas) harvested from a sewage-impacted production site and then subjected to site-specific management procedures. These procedures consisted of relocation of oysters to an alternative production area during the norovirus high-risk winter periods (November to March) followed by an extended depuration (self-purification) under controlled temperature conditions. Significant differences in norovirus RNA concentrations were demonstrated at each point in the management process. Thirty-one percent of oyster samples from the main harvest area (Site 1) contained norovirus concentrations > 500 genome copies/g and 29% contained norovirus concentrations < 100 genome copies/g. By contrast, no oyster sample from the alternative harvest area (Site 2) or following depuration contained norovirus concentrations > 500 genome copies/g. In addition, 60 and 88% of oysters samples contained norovirus concentrations < 100 genome copies/g in oysters sampled from Site 2 and following depuration, respectively. These data demonstrate that site-specific management processes, supported by norovirus monitoring, can be an effective strategy to reduce, but not eliminate, consumer exposure to norovirus genome copies.

Norovirus Persistence in Oysters to Prolonged Commercial Purification

“…These results are consistent with both laboratory-based depuration studies and commercial-based depuration studies, which evidenced a two-phase depuration kinetics in many mollusk species, although with different timing in the different species [ 10 , 16 , 31 ], probably due to the variability among species and the capability of NoV to resist to suboptimal conditions during depuration process, as suggested by other authors [ 14 , 16 ]. In several studies on oysters, indeed, a maximum viral reduction was obtained in the first 3–4 days of depuration, while no further significative reduction was observed by extending the time [ 26 , 32 ]. Some authors suggest that the more rapid viral reduction in the first phase could be related to extracellular digestion and purging of the digestive tract, while in the second phase, viral reduction is more difficult and slower because NoV is specifically attached to ligands present on oysters’ gastrointestinal cells or in other parts outside the digestive tract lumen, like hemocytes [ 17 , 20 , 23 , 33 , 34 ].…”

Evaluation of Norovirus Reduction in Environmentally Contaminated Pacific Oysters During Laboratory Controlled and Commercial Depuration

“…Therefore, oyster producers need effective virus reduction measures in order to meet any such microbiological criterion for norovirus in the future. Previously, we demonstrated the potential of risk management procedures to reduce the risk of norovirus contamination in a production area as well as the reduction of norovirus concentrations in end-product by depuration (Rupnik et al 2018 ). In that particular study extended depuration periods of up to 9 days were applied during the high-risk winter season.…”

“…However, some studies have indicated that depuration time and seawater temperature are both factors that may influence virus reduction during bivalve shellfish depuration (Lees et al 2010). The potential for norovirus reduction in oysters during enhanced depuration procedures was identified previously by this group (Rupnik et al 2018) prompting this study to investigate norovirus reduction in oysters under controlled laboratory conditions.…”

“…Oysters present additional risks due to a number of factors such as being grown in intertidal areas often impacted by sewage and being consumed raw or only lightly cooked. This has resulted in an increasing number of commercial oyster producers in Ireland and elsewhere that harvest from A classified production areas to include depuration as an extra step in their HACCP (Hazard Analysis and Critical Control Point) or risk management procedures (Rupnik et al 2018 ).…”

“…The introduction of quantitative real-time PCR for the detection and quantification of norovirus in bivalve shellfish, particularly the method described in ISO 15216-1:2017 (Anonymous 2017 ), has been an important step in the area of risk management of norovirus. This tool has allowed better understanding of the relative differences in norovirus concentrations when applying risk management procedures (Rupnik et al 2018 ), as well as understanding the concentrations of norovirus found in classified production areas across the EU (EFSA 2019 ).…”