A quantitative exposure assessment model for norovirus in oysters harvested from a classified production area

Norovirus is a significant hazard to consumers of shellfish, in particular oysters. Oysters grown in waters contaminated with wastewater filter and accumulate norovirus particles, causing infection in humans when the product is consumed raw or lightly cooked. In the European Union (EU) and the United States, bacterial detection criteria are used to assess and manage microbial risk in shellfish. This regulatory framework is effective in managing the bacterial risks associated with microbiological contamination of bivalve shellfish but not for viruses. Although a standard detection method for norovirus in oyster exists (ISO 15,216–1:2017), no quantitative microbial risk assessment has been published that links the concentration of norovirus in oysters from a classified production area with consumer exposure. This study shows the successful development of a two-dimensional Monte Carlo exposure assessment model, taking an ISO 15,216–1:2017 detection result and producing an estimate of the resulting per-serving consumer exposure distribution. In contrast to previous oyster virus risk assessments, consumption is modelled using individual oysters as the unit, rather than total flesh weight. The variation in copies per oyster is modelled using a Poisson-lognormal distribution. The results show the boundaries for potential exposure following a given ISO detection result, and the relative importance of mean concentration, serving size, and oyster grade. This is directly relevant to potential regulatory thresholds being considered in the EU.