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Development of Procedures for Improved Viral Reduction in Oysters During Commercial Depuration

Institutions
Centre for Environment, Fisheries and Aquaculture Sciences (CEFAS)
Start date
2000
End date
2003
Objective
This research study aims to develop procedures that will improve viral reduction during commercial purification of oysters.

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.

More information
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.

Find more about this project and other FSA food safety-related projects at the Food Standards Agency Research webpage.

Funding Source
Food Standards Agency
Project number
B04002
Categories
Bacterial Pathogens
Risk Assessment, Management, and Communication
Commodities
Seafood