Scientists from the Food Standards Agency (FSA) have looked at the key topics to consider when dealing with norovirus outbreaks linked to oysters.
Microbiological experts from the FSA assessed the public health risk from raw oysters to help develop risk management options during outbreaks.
Work was done in response to recurrent outbreaks of norovirus associated with the 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, 259 cases were reported. During the same periods there were 28 outbreaks in England and eight in Scotland caused by oysters.
Earlier this month, it was reported that two cases of norovirus in oysters from France sickened six people in Norway and 14 in Sweden.
Request for guidance and risk management options
Testing options for norovirus are limited and unreliable, the FSA said. The virus can be detected and quantified in foods, including oysters, but the tests cannot distinguish infectious virus from damaged virus that cannot cause infection.
The current positions of the FSA and FSS 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 preventive tool in the event of adverse weather conditions that may lead to contamination of oyster beds or to determine the effectiveness of interventions.
Norovirus contamination in oysters is largely due to discharge of human sewage near oyster beds. Oysters are filter feeders that take in norovirus as they filter seawater. Norovirus levels vary greatly by season, with higher amounts in the winter months.
Local authorities, food businesses and the UK Health Safety Agency (UKHSA) are asking for guidance on dealing with norovirus outbreaks as there are no restrictions in the regulations. In France, post-outbreak actions include closing oyster beds for 28 days and weekly testing for norovirus until the production area tests negative.
Managing oyster risks during norovirus outbreaks is difficult—closing oyster beds and stopping harvesting for long periods of time is economically detrimental to business. However, reopening too quickly could lead to new cases. Conflicting results from different labs can also complicate the ability to issue advice, according to the report.
The FSA had 110 incidents involving eating oysters potentially linked to norovirus between 2000 and 2022. The FSS recorded 16 incidents involving oysters grown or eaten in Scotland linked to norovirus between 2017 and February 2023.
The level of risk varies
Oysters from Class A production areas may be sold for direct human consumption. Those from Class B sites must undergo cleaning or resurfacing before sale to the public.
The analysis found that the UK appears to have a higher prevalence of norovirus in oysters than other countries, possibly due to poorer sanitary water quality. There is uncertainty about oyster consumption levels in the UK, but they are believed to be low.
The scientists compared norovirus levels in retail oysters to levels in batches of oysters linked to outbreaks and found that batches with outbreaks had significantly higher levels.
Food establishments should consider environmental factors that may affect coastal locations where oysters are found, such as rainfall levels, average wind speed and direction, and nearby effluent discharge points in their safety management systems of foods.
“We conclude that if oysters are consumed raw and there is potential contamination of human wastewater from sewage spills, or if the batch of oysters is associated with outbreaks, there is a risk of norovirus disease.” The risk ranges from low to very high, depending on the levels of norovirus in the batch of oysters,” the scientists said.
(To sign up for a free food safety news subscription, Press here.)