First-time systematic study into norovirus in oysters published
29 November 2011
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 food
safety 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 sensitive detection 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
polymerase chain reaction). 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.
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Results
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 disinfectants, so thorough hand washing with soap
and water is recommended.