First-time systematic study into norovirus in oysters published

First-time systematic study into norovirus in oysters published

The first published systematic study of norovirus in oyster-harvesting areas using the most up-to-date quantitative methodology was published today.

At present the level of norovirus in oysters that might lead to actual illness is not yet well defined. There are also still significant knowledge gaps about the transmission of norovirus.

Cefas’ study, funded by the Food Standards Agency (FSA), contributes important evidence to inform UK views about norovirus prevalence in oysters across the UK.

Cefas has a long-standing capability in research. In addition, it chairs a working group of the European Committee on Normalisation (CEN) charged with developing a standard method for detecting norovirus in foodstuffs (including bivalve shellfish, such as oysters).

Via its commercial arm, Cefas Technology Ltd, the agency also provides a shellfish testing service to food producers.

David Lees, the lead investigator at Cefas, said: “We wanted to collect data on norovirus in oysters from a representative selection of production areas around the UK. Our aim was to build up an accurate picture of seasonal variation of the levels and prevalence of norovirus in shellfish beds.

“In addition, we investigated relationships between the levels of norovirus and E. coli, and looked at other risk factors such as temperature and population.”

Lees continues: “We were fortunate to have excellent co-operation from the oyster producers and from local authority officers in conducting this study. Norovirus is a recognised problem for the sector, and this study provides important baseline data to help the industry and regulators to focus on the key risks.”

Over the last decade considerable progress has been made towards the development of methods for norovirus in bivalve shellfish.

All available methods are based on detection of the virus genome using a molecular technique called RT-PCR (reverse transcriptase ). This is because growing norovirus using routine virological techniques is not possible and immunological methods – used in clinical diagnostics – are not sensitive enough for application in foodstuffs.

One limitation of PCR-based approaches is that the data generated do not differentiate between viable and potentially non-viable viruses. However, there is a growing body of evidence indicating that there is an increased public health risk associated with higher detected virus levels in shellfish.

Cefas used a standardised method derived from the CEN draft procedure. This uses magnetic extraction to remove viral RNA from shellfish tissues, followed by quantitative RT-PCR. The assay includes a comprehensive suite of controls and has been accredited to ISO 17025 by UKAS.

Sampling technique

Two species of oysters grown in UK waters – the Pacific oyster (Crassostrea gigas) and native oyster (Ostrea edulis) – were sampled.

Samples were collected by the relevant local authority sampling officers in parallel to samples collected for the statutory classification monitoring programme.

Monthly sampling took place between May 2009 and April 2011. A range of environmental factors that may influence the results – including tidal states, rainfall and wind – were also recorded.

Sampling was undertaken according to the Protocol for the Collection of Shellfish under the Microbiological Classification Monitoring Programme.

Almost 850 samples (each sample consisting of 10 oysters) were tested, covering 39 oyster harvesting areas across the UK: 31 English and Welsh sites, 6 sites in Scotland and 2 sites in Northern Ireland. A risk matrix incorporating various weighted factors (population density, E. coli scores, outbreak history, etc.) was developed to aid site selection and ensure representative sampling in this study.



Once the data were analysed, the prevalence of norovirus in the two oyster species tested was similar, though the techniques used were unable to differentiate between infectious and non-infectious norovirus material within the oysters.

The levels of norovirus varied seasonally, with higher levels and prevalence in samples collected between October and March compared to those obtained between April and September. Several factors may contribute towards the higher levels of norovirus seen during the winter months. This result is consistent with previous studies carried out at Cefas and at other laboratories.

The study found a statistically significant and predictive correlation between E. coli and norovirus levels, and a strong correlation between norovirus contamination and environmental temperatures.

The Cefas team developed a risk scoring system based on existing and recent harvesting area classifications, E. coli contamination profiles, population densities in adjacent areas and reported shellfish-associated outbreaks. The observed correlation between norovirus levels and identified risk factors could improve predictions about norovirus for the future.

Andrew Wadge, Chief Scientist at the FSA, said: “This research is the first of its kind in the UK. The results, along with data from other research, will help us work with producers to find ways to reduce the levels of norovirus in shellfish, and work within Europe to establish safe levels.”

Fact file

Oysters filter large volumes of water to obtain their food, and any bacteria and viruses that may be in the water can build up within oysters. Good protection against harmful bacteria is offered through controls before and after the commercial harvesting of oysters. However, it can be difficult to remove viruses from live shellfish.

Re-laying is a purification process used to treat bivalve shellfish. Shellfish are harvested from a contaminated area and moved to clean areas where they are placed on the ocean floor or into containers laid on the ocean floor, or suspended in racks. Re-laying will generally take place for at least two months.

Depuration is a purification process used commercially and regulated by the FSA. It is commonly used by producers to reduce or eliminate microbiological contamination in oysters and other shellfish. Shellfish are placed in tanks of clean re-circulating seawater and allowed to purge their contaminants over several days (in the UK a minimum purification time of 42 hours is required).

Norovirus illness can be transmitted from a number of different of sources. This includes person-to-person spread (e.g. via affected food handlers) or from contaminated surfaces and foods such as oysters, soft fruit and produce. Most norovirus infections are thought to be spread from person-to-person rather than from contaminated food.

High cooking temperatures, such as 90ºC for 90 seconds, are needed to destroy norovirus. It is also rapidly inactivated by chlorine-based , so thorough hand washing with soap and water is recommended.

Original article.  © Crown copyright, 2011

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About Norovirus

    Noroviruses are highly contagious and are transmitted via the faeces and vomit of infected people, either through direct contact or throught contaminated objects or via food and water. The incubation period is one to three days.



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