National survey of foodborne viruses in Australian oysters at production

Highlights

• Norovirus and hepatitis A virus were not detected in oysters during the survey.

• The estimated prevalence of norovirus and hepatitis A virus was <2%.

• No oyster-related foodborne viral illnesses were reported during the survey period.

• Industry produced a low risk product with respect to viral contamination.

• No need for end-product viral limit adoption in the Australian context.

Abstract

Internationally human enteric viruses, such as norovirus (NoV) and hepatitis A virus (HAV), are frequently associated with shellfish related foodborne disease outbreaks, and it has been suggested that acceptable NoV limits based on end-point testing be established for this high risk food group. Currently, shellfish safety is generally managed through the use of indicators of faecal contamination. Between July 2014 and August 2015, a national prevalence survey for NoV and HAV was done in Australian oysters suitable for harvest. Two sampling rounds were undertaken to determine baseline levels of these viruses. Commercial Australian growing areas, represented by 33 oyster production regions in New South Wales, South Australia, Tasmania and Queensland, were included in the survey. A total of 149 and 148 samples were collected during round one and two of sampling, respectively, and tested for NoV and HAV by quantitative RT-PCR. NoV and HAV were not detected in oysters collected in either sampling round, indicating an estimated prevalence for these viruses in Australian oysters of <2% with a 95% confidence interval based on the survey design. The low estimated prevalence of foodborne viruses in Australian oysters was consistent with epidemiological evidence, with no oyster-related foodborne viral illness reported during the survey period.

Introduction

Human enteric viruses are increasingly recognised as important causes of foodborne disease globally, based on the incidence of reported foodborne disease and the severity of disease (including mortality) (FAO/WHO, 2008, FAO/WHO, 2012). International estimates of the proportion of enteric virus illnesses attributed to food are in the range of approximately 5% for hepatitis A virus (HAV) and 12–47% for norovirus (NoV). The virus-commodity combinations of greatest public health concern are NoV and HAV in bivalve molluscs, fresh produce and prepared (ready-to-eat) foods (FAO/WHO, 2008, FAO/WHO, 2012). A systematic review of global shellfish related viral foodborne outbreaks between 1980 and 2012 reported NoV (83.7%) and HAV (12.8%) as the most common viral pathogens and oysters (58.4%) as the most frequently consumed shellfish associated with outbreaks (Bellou et al., 2013). The majority of the reported outbreaks have been located in East Asia, followed by Europe, America, Oceania, Australia and Africa (Bellou et al., 2013). In Australia, between 2001 and 2010, seventeen suspected foodborne outbreaks of NoV or unknown aetiology were associated with consumption of bivalve shellfish, which included imported product (OzFoodNet Reports). A recent oyster related outbreak of NoV occurred in 2013 with 525 people affected nationally following consumption of contaminated oysters from Tasmania (Lodo et al., 2014).

As there are currently no effective control measures available to eliminate these viruses from food without changing the characteristics of the product, the most effective risk management strategy for NoV and HAV in bivalve shellfish is to prevent contamination in production areas. Freezing of shellfish does not deactivate foodborne viruses, but rather preserves them (EFSA, 2012). High-risk factors for contamination of oysters with enteric viruses include low water temperatures (allowing greater persistence of the viruses), elevated prevalence of enteric illness within the community and high rainfall leading to sewage system overflows (CEFAS, 2011).

In 2012, the Codex Alimentarius commission released guidelines on general principles of food hygiene to control viruses in food, with Annex I specifically focusing on control of HAV and NoV in bivalve molluscs (FAO/WHO, 2008). It recommended that countries monitor for NoV and HAV in bivalves following shellfish-related foodborne outbreaks and high-risk pollution events (heavy rainfall and overflow from sewage treatment plants). The EU legislation on the microbiological criteria for foodstuff has suggested that “criteria for pathogenic viruses in live bivalve molluscs should be established when the analytical methods are developed sufficiently” (EC, 2005). With the development of the ISO/TS 15216 method “Microbiology of food and animal feed - horizontal method for the determination of hepatitis A virus and norovirus in food using real-time RT-PCR” (ISO/CEN, 2013), virus methods have become available that may be considered suitable for use in legislation. Hence, consideration is being given to establishing virus limits for high-risk live bivalve molluscs. The EFSA Scientific Opinion on NoV in oysters recommended: the establishment of an acceptable limit for NoV in oysters to be harvested and placed on the market; NoV testing of oysters to verify compliance with the acceptable NoV limits established; and for food businesses to verify their Hazard Analysis and Critical Control Points plans and demonstrate compliance with acceptable levels (EFSA, 2012). In 2012, the EU Community Reference Laboratory recommended that if virus standards are introduced, then standards for NoV should be quantitative (i.e. a maximum acceptable level) and standards for HAV be qualitative (i.e. presence/absence) (CEFAS, 2013). It also considered and made recommendations on possible levels for a NoV standard in the context of both end-product and production area monitoring applications (CEFAS, 2013). The EU is current undertaking a two year survey to establish European prevalence of NoV contaminated oysters at the production area and dispatch centre levels (EFSA, 2016). Following this survey, the European Commission will appraise the results and decide whether microbiological criteria for NoV are appropriate.

The prevalence of NoV in oysters internationally has been reported to range from 2.4% to 76.2% (Lowther et al., 2012, Pavoni et al., 2013, Suffredini et al., 2014). Information on the prevalence of NoV in Australian oysters is limited, but suggests a low prevalence. A study of oysters from growing areas at risk of contamination, over a range of environmental conditions, found NoV in 1.7% of oysters sampled (Brake et al., 2014). As a response to the impending international regulations (noting that some nations already require NoV testing on imported products e.g. Singapore), the Australian oyster industry desired a more comprehensive evaluation of the prevalence of enteric foodborne viruses in Australian oysters at production. Similar surveys have been undertaken worldwide, and have found that the prevalence of foodborne viruses in oysters obtained in market products were comparable to those observed in commercial harvesting areas (EFSA, 2012). The current study aimed to estimate the national prevalence of NoV and HAV in Australian oysters suitable for harvest. The survey used the ISO/TS 15216 standard testing methodology for foodborne viruses in shellfish and a robust statistical sampling plan conducted over two rounds of sampling between July 2014 and August 2015.