Scientists at the Food Standards Agency (FSA) have looked at the key topics to consider when dealing with norovirus outbreaks linked to oysters.
Microbiological experts at the FSA assessed the public health risk of raw oysters, to help with development of risk management options during outbreaks.
Work was done in response to recurring norovirus outbreaks linked to consumption of raw oysters. In England, between 2013 and 2022, there were 1,307 cases of norovirus linked to oysters. In Scotland, from 2017 to 2023, there were 259 reported cases. In the same periods, there were 28 outbreaks in England and eight in Scotland caused by oysters.
Earlier this month, two incidents of norovirus in oysters from France were reported to have sickened six people in Norway and 14 in Sweden.
Request for guidance and risk management options
Testing options for norovirus are limited and unreliable, said the FSA. The virus can be detected and quantified in foods including oysters, but tests can’t distinguish between infectious virus and damaged virus, which is unable to cause infection.
The current FSA and FSS positions are that testing oysters from batches epidemiologically linked to outbreaks, where cases have symptoms typical of norovirus, cannot determine infectivity. However, there is merit in norovirus testing as a preventative tool in the case of adverse weather conditions that may lead to contamination of oyster beds or to determine the effectiveness of interventions.
Norovirus contamination in oysters largely occurs due to human sewage releases close to oyster beds. Oysters are filter feeders who take up norovirus as they filter seawater. The levels of norovirus vary widely depending on season, with higher amounts in winter months.
Local authorities, food businesses and the UK Health Security Agency (UKHSA) want guidance on dealing with norovirus outbreaks as there are no limits in regulation. In France, actions after an outbreak include oyster bed closures for 28 days and weekly norovirus testing until the production area tests negative.
Managing the risks from oysters during norovirus outbreaks is difficult – closing a shellfish bed and ceasing harvesting for a long time is economically damaging to the business. However, reopening too quickly can lead to further cases. Conflicting results from different labs can also complicate the ability to issue advice, according to the report.
The FSA had 110 incidents associated with eating oysters and potentially linked to norovirus between 2000 and 2022. FSS recorded 16 incidents associated with oysters grown or eaten in Scotland and linked to norovirus between 2017 and February 2023.
Risk level varies
Oysters from class A production areas can be sold for direct human consumption. Those from class B sites must undergo depuration or relaying before sale to the public.
The analysis found the UK appears to have a higher prevalence of norovirus in oysters compared to other countries, likely due to poorer sanitary quality of its waters. There is uncertainty about the levels of oyster consumption in the UK but it is thought to be low.
Scientists compared norovirus levels in oysters at retail to levels in oyster batches linked to outbreaks and found that outbreak batches had significantly higher levels.
Food businesses need to consider environmental factors that may affect coastal sites where oyster beds are located, such as rainfall levels, average wind speed and direction, and nearby sewage release points in their food safety management systems.
“We conclude that if oysters are eaten raw and there is potential human wastewater contamination from sewage spills or if the oyster batch is linked to outbreaks, there is a risk of illness from norovirus. The risk varies from low to very high, depending on the levels of norovirus in the oyster batch,” said scientists.
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