B04002: Development of procedures for improved viral reduction in oysters during commercial depuration

Study Duration: May 2000 to April 2003

Contractor: Centre for Environment, Fisheries and Aquaculture Sciences (CEFAS)

The most frequent cause of gastroenteritis associated with the consumption of oysters is from Norovirus (NV) as they tend to be eaten raw or lightly cooked. Bivalve molluscan shellfish accumulate human pathogenic micro-organisms when grown in sewage-polluted waters. To reduce the risk associated with such oysters, they are purified prior to consumption, by a process called depuration. Depuration involves submerging the oysters in a tank of clean seawater, which removes sewage contaminants. Despite worldwide use of depuration, outbreaks of illness following the consumption of purified oysters continue to occur.

Male-specific RNA (FRNA) bacteriophages have similar physical and genomic characteristics to the NVs and their ease of enumeration makes them an attractive indicator for modelling the removal of these viruses during depuration. FRNA bacteriophage has therefore been proposed as an index of virus removal during depuration. It is considered that the use of depuration procedures, which produce oysters free from FRNA bacteriophage, would be likely to remove NVs and other human viruses.

The overall objective of this study was to develop procedures involving the use of elevated temperature for extended periods that would allow increased viral reduction during commercial depuration of oysters.

The work was carried out in three major phases:

Application of elevated temperatures to commercial depuration
The use of elevated temperatures (17-20°C) for extended periods in depuration systems was investigated. Both Pacific and native oysters from two category B commercial harvesting areas were investigated. Depuration trials were carried out in commercial scale depuration systems on a monthly basis for the period of a year to incorporate potential seasonal differences. Virus removal was investigated using FRNA bacteriophage as an index organism. The effect of temperature/time on product quality was investigated and the cost of heating the seawater in the depuration systems to elevated temperatures was calculated.

Validation of FRNA bacteriophage as an index of Norovirus elimination during depuration
Although FRNA bacteriophage has been proposed as an index of NV behaviour in shellfish during depuration its use in this role has not been validated previously. Samples from representative monthly depuration cycles described above were analysed for NVs using a recently developed quantitative TaqMan PCR assay (developed at CEFAS under FSA project B04001) and compared with FRNA bacteriophage levels.

Investigations to evaluate the developed depuration procedures
The procedures developed to increase FRNA bacteriophage elimination on the basis of the results obtained from the monthly experiments were applied to shellfish gathered from six different commercial category B harvesting areas. Shellfish were analysed for FRNA bacteriophage and NV content before and after depuration. The effect of applying the new procedures on virus removal was assessed. Shelf life and meat yield studies were assessed the impact of the new procedures on product quality.

This study indicated that FRNA bacteriophage is a suitable index of NV elimination during depuration. Therefore it can be assumed that procedures developed to increase FRNA bacteriophage elimination during depuration will increase NV elimination.

Project results indicated that depuration at elevated temperatures (17°C or more) for prolonged periods (up to 5 days) would have an impact on virus removal and can be applied to oysters. Indeed, elimination of FRNA bacteriophage under these conditions was greater for native and Pacific oysters compared with depuration at ambient temperatures. Whilst depuration at elevated temperatures and for extended periods significantly increased virus reduction, it was not possible to achieve complete elimination.

Prolonged depuration at elevated temperatures did not affect the quality of Pacific oysters, however a slight reduction in shelf life was detected for native oysters, especially during winter months. High mortality rates were observed for native oysters in the summer months and this tends to suggest that depuration, under these conditions, would be more beneficial for Pacific oysters than for native oysters.

The final report is available from the Agency's Information Centre.

To obtain a copy, please contact the Enquiry Desk, Information Services, Food Standards Agency (tel: 020 7276 8181/8182 or email: infocentre@foodstandards.gsi.gov.uk

Contact: For any enquiries concerning this research project, please contact the relevant Programme contact or email: science@foodstandards.gsi.gov.uk