Habitats of oysters and their potential food safety risks

　　Oysters mainly live in brackish habitats at the estuary waters. They cannot swim or move around on their own, being dependent on planktons as their source of food. The oysters available for sale in the market are usually of two types, ones used for “preparing sashimi” and those “required high-temperature cooking before consumption”. In general, live oysters used for “preparing sashimi” are cultivated in clean waters with water quality control and those “required cooking” are raised in turbid waters.

        Regardless of the type of cultivation, the body of live oysters may have accumulated heavy metals, natural toxins and pathogenic microorganisms if their habitats and water quality are being polluted. Upon consumption of contaminated oysters, it enhances the potential risk of contracting diseases leading to health hazards.

Live oysters and heavy metals

　　The common types of chemical pollutants detected in live oysters are mainly heavy metals, namely mercury, lead, arsenic and cadmium, and organic compounds that are harmful to human body. Since these pollutants cannot be destroyed through high-temperature cooking, they accumulate in human body and cannot be decomposed. Long term intake of these pollutants damages the nervous system and renal functions and poses potential health risks.

Live oysters and natural toxins

　　Live oysters mainly feed on planktons, including marine plants under the genus of gymnodinium breve which contain natural toxins. These marine toxins, which accumulate in the viscera of shellfish, cause poisoning of four major types: paralytic shellfish poisoning (PSP), neurotoxic shellfish poisoning (NSP), diarrhetic shellfish poisoning (DSP) and amnesic shellfish poisoning (ASP). Upon consumption of shellfish containing marine toxins, symptoms of food poisoning may develop, like nausea, vomiting and diarrhea. In case they contain paralytic toxins or neurotoxins, symptoms of numbness sensation of mouth and lips, dizziness and muscular incoordination may appear. In severe cases of paralytic shellfish poisoning, symptoms of muscle paralysis, dyspnea and cardiovascular collapse could lead to death, particularly with highest fatality rate in the case of children.

Live oysters and pathogenic microorganisms

　　Consumption of live oysters is susceptible to pathogenic microbial infection. Parasites, pathogenic bacteria and viruses are the main types of microbes hazardous to residents’ health.

• Parasites: Helminths and protozoa are the common parasites found in live marine products which can cause intestinal discomfort and inhabit the intestines to breed their next generation. They can also cause their host to develop malnutrition and in severe cases, destroy the visceral tissues to cause irreparable health problems.
• Pathogenic bacteria: Consumption of live oysters is susceptible to infection of Vibrio spp., which includes Vibrio cholerae O1 and O139, vibrio parahaemolyticus and vibrio vulnificus. The usual symptom is gastroenteritis-related while in the case of severe vibrio vulnificus infection, the victims could suffer from life-threatening septicemia.
• Viruses: Namely norovirus (NoVs), enteric virus, aichi virus (AiV), sapoviruses (SaVs), rotavirus (RV) and hepatitis A and E virus. No matter norovirus or enteric virus, majority of victims suffer from gastroenteritis-related symptoms like fever, vomiting and diarrhea, and even dehydration in severe cases. For victims of hepatitis A or E viral infection, they suffer from jaundice that causes liver damage.

        Consumption of live oysters containing pathogenic microbes poses direct health hazards. Yet, aforesaid foodborne infection can be prevented by high-temperature cooking, which will kill the parasitic bacteria and viruses inside the marine products.

Think twice before consumption of live oysters to reduce health hazards

　　Food-poisoning cases caused by consumption of live oysters happened worldwide. As we are unable to judge whether any marine product contains health-damaging substances, like heavy metals, natural toxins in shellfish and harmful microorganisms, with our naked eyes, it is important to be aware of food safety risks behind consumption of live oysters, regardless in Macao or elsewhere abroad.

Advice to the public

1. Patronize reputable food establishments with good hygiene condition; do not buy live oysters from unknown sources or with abnormal appearances;
2. High-risk individuals, like young children, elderly, pregnant women and those with poor immunity, should avoid consumption of raw or undercooked oysters. For those with poor immunity and low recovery capacity, they are susceptible to suffer from other life-threatening complications upon bacterial, viral and marine toxic invasion;
3. Handle cooked and raw food separately to avoid cross-contamination; be cautious in handling shellfish to prevent skin injury or bleeding wound inflicted by the sharp edge of their shells and fragmented parts;
4. Rinse, scrub and clean the shells of shellfish thoroughly; remove their viscera and discard ones whose shells are damaged. They must be thoroughly cooked under high-temperature for at least 3 to 5 minutes until meat shrinks and mantle curls up before consumption;
5. Maintain a balanced diet and avoid excessive consumption of shellfish at one time;
6. Pathogenic bacteria and viruses cannot be killed by spicy and sour seasonings, such as Japanese wasabi sauce, chili sauce, lemon juice, vinegar and spirits, etc.;
7. Consider one’s immune capacity and be aware of pathogenic and chemical risk factors before consumption of raw oysters. All in all, bear in mind the Chinese proverb “The less you eat, the more tasteful it is; the more you eat, the more problematic it causes your stomach”.

Advice to the trade

1. Purchase shellfish from reliable suppliers; be aware of information such as the place of origin of the food ingredients before ordering them and make sure that they are suitable for human consumption; keep purchase invoices and sales records;
2. Upon receipt of products, check whether live oysters are kept under temperature requirement specified by their producer and if they look normal. In case of having doubts about the hygiene and safety of the products, do not buy or sell them;
3. Handle live oysters properly to avoid cross-contamination with cooked food;
4. It is necessary to wear protective gloves to avoid hand injury upon opening the shells of live oysters. Never leave any shell debris over the meat of oyster;
5. Pay attention to the food alerts and announcements issued by the Food Safety Department of the IAM. Upon receiving a stop-sale or recall alert from the Department, the concerned entity has to immediately remove the suspected contaminated live oysters from sale or supply to consumers and cooperate with the Department by recalling them.