Shellfish production areas can become contaminated with faecal material from sewage effluent discharges and surface water run-off from agricultural land. This may contaminate oysters and other shellfish with bacteria and viruses known to cause human illness. Oysters filter large volumes of water to get their food and any pathogens in the water can build up within the oyster.
Controls before and after commercial harvesting of oysters, such as relaying and depuration (more about these processes can be found below), provide good protection against harmful bacteria, but are less effective at removing viruses from live shellfish.
Although norovirus contamination can be an issue for a range of foods, the presence of norovirus in oysters can present a particular risk to consumer safety as oysters are generally consumed as a raw product, so the virus is not destroyed before consumption by heat processes such as cooking.
A purification process used to treat bivalve shellfish, but is not commonly used in the UK. 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, generally for a period of at least two months.
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, treated by UV irradiation, and allowed to purge their contaminants over several days. In the UK a minimum purification time of 42 hours is currently required.