Food Safety Risks Posed by Parasites in Oysters for Raw Consumption


Live oysters and other shellfish are common food products served in buffets. But before savoring the seafood, it is helpful to have an idea of the growth and characteristics of oysters so as to enjoy them safely.


  Oysters are an aquatic species of filter-feeding shellfish found in seawater or brackish waters. Environmental contaminants can accumulate in their bodies, like chemical contaminants derived from chemical substances and heavy metals present in industrial waste water, pesticide and veterinary drug residues from farming and animal-breeding practices, as well as naturally-occurring microbial contaminants like bacteria and viruses. Owing to these potential contamination sources, the environment and water quality of oyster farming are subject to specific requirements and control. Oysters intended for raw consumption can only be cultivated or caught from designated waters.


  Marine worms are common parasites found in oysters. They usually live in the benthic sediments of oyster growing areas. Owing to the presence of substrate that is essential to marine worms on oyster shells, marine worms are often found in the external crevices of oyster shells. The Cliona sp. is a species of shell-boring sponges that bore holes on the oyster shells causing their surfaces to become uneven and covered with boreholes. The marine worms live in these boreholes they created. It was once viewed that the presence of marine worms in oysters is an indicator of freshness and shows that the oysters are freshly removed from the sea. Though marine worms inhabit the outer surfaces of oyster shells, a worm on the shell surface may fall or be pushed by the oyster knife onto the oyster flesh during the shucking process. Marine worms pose no harm to oysters, but studies indicated that immersing oysters in saturated brine for 15 seconds is effective in removing the marine worms in them.


  So far, there are no studies indicating that the marine worms found living on the outer surfaces of oyster shells have impact on human health. However, the risk of viral and microbial contamination cannot be ignored. Therefore, it is advisable to cook all aquatic products thoroughly before consumption. Please refer to the following suggestions before purchase and consumption of oysters and shellfish.


Advice to the public:

  • Patronise hygienic and reputable food establishments. Do not buy live oysters from unknown sources or those which are unusual in appearance;
  • Upon purchase of shellfish, check whether their shells are tightly closed. Do not buy those with damaged or opened shells;
  • Wash, clean and scrub the shells of shellfish thoroughly. It takes at least three to five minutes for them to be thoroughly cooked by high-temperature cooking. Live oysters have to be cooked until the flesh shrinks noticeably and the mantle becomes wavy before consumption;
  •  High risk individuals, like young children, the elderly, pregnant women and those with sensitive bowels or weak immunity, should refrain from eating raw or undercooked oysters;
  • Spicy or acidic seasonings, such as Japanese wasabi sauce, chilli sauce, lemon juice, vinegar and alcohol, cannot kill pathogenic bacteria and viruses.


Advice to the trade:

  • Purchase live oysters from hygienic and reliable suppliers. Before placing any order, be aware of information such as their country of origin and make sure they come with health certificates issued by competent authorities of their country of origin indicating they are suitable for human consumption. Check whether their packaging condition is consistent with what is stated in their health certificates. Keep the purchase orders, invoices and sales records;
  • Upon receipt of delivery of live oysters, check whether they are kept at under 5oC, their shells are tightly closed and without damage. Do not accept or sell them when doubtful about their safety and hygiene condition;
  • Use potable water to rinse and clean the oyster flesh. Place it on ice upon serving to keep it at low temperature;
  • Avoid handling live oysters and cooked food at the same time to prevent cross-contamination;
  • Stay informed about food alerts and announcements issued by the Food Safety Department of the Municipal Affairs Bureau (IAM). Upon receiving an alert from the Centre to stop the sale of the suspected contaminated live oysters or recall them, the concerned entity has to immediately stop the sale or supply of the products to consumers and cooperate with the Centre to carry out product recall.