Experts have assessed the main risks of importing oysters into the UK and measures to help mitigate possible issues.

The Food Standards Agency (FSA) was commissioned by the UK Office for Sanitary and Phytosanitary Trade Assurance to profile the potential risks of importing oysters into the United Kingdom. It is intended to provide information for auditors and risk managers involved in market access requests.

Hazard identification was done via a literature review. Two categories were identified: chemical and microbiological hazards. They included heavy metals, natural biotoxins, viral and bacterial pathogens, and persistent organic chemicals. Allergens and physical hazards were excluded.

Earlier this year, specialists at the FSA assessed the public health risk of raw oysters to help with development of risk management options during norovirus outbreaks.

A high-risk product

The UK imports 350,000 kilograms of oysters per year, and they are commonly processed. The main exporters to the UK include Korea, France, and New Zealand. Consumption surveys indicate that the general population rarely eats oysters.

Scientists found oysters are a high-risk product for import, particularly for certain groups, given their filter feeding, which allows the bioaccumulation of hazards and likelihood for raw consumption. Still, measures are available to reduce risks in many cases. However, risk mitigation depends on the hazard.

During growing area selection and management, measures applied early in the supply chain may reduce the risk in the latter phases. Other options are depuration and short-term relay of oysters from Class B waters.

The review found factors such as vulnerable population changes, emergence of hazards, climate change, globalization of the seafood market, and changing human behaviors will impact the risk.

The main hazards in the supply chain were Vibrio parahaemolyticus; marine biotoxins (such as amnesic, paralytic, and lipophilic toxins); hepatitis A virus (HAV), norovirus and Salmonella; heavy metals like cadmium mercury and lead; and diseases caused by other Vibrio species.

The severity of the microbiological hazards associated with oysters was considered low for the general population. However, it could be higher for some hazards, and susceptible individuals are likely to suffer more severe illness for a number of the potential hazards.

Some viruses transmitted by the fecal-oral route can persist for months and are more resistant than bacteria to standard control measures such as refrigeration, freezing, drying, heat, or UV radiation. As shellfish are often consumed raw, contamination with the infectious stages of parasites represents a public health concern, found the review.

Impact of controls on hazard

Control measures at different supply chain stages will vary with hazard type. One example was marine biotoxins, which cannot be removed by purification or controlled post-harvest except for removing contaminated products from the market. The report summarizes regulations on oyster control in the EU, United States, Canada, China, and Australia.

Interventions during harvest are suspension of activity, transfer of live animals to cleaner sites, or altering onward processing requirements. Steps during processing include purification through re-immersion in clean water. During the consumption phase, labeling and traceability, education of workers on cold chain breaches and contamination by staff with gastrointestinal symptoms, and advice on avoiding raw products by vulnerable groups can help reduce the risk.

International guidance on risk management is available and is based on mitigating risks via monitoring and includes steps at the growing and farming phase, harvesting, processing, transport, and retail.

Where market access requests are made, measures in the country of origin should be investigated to estimate the safety of products. If initial investigations do not provide clarity or indicate a concern, it is recommended that a country audit or full import risk assessment be considered, according to the risk profile.