People have been warned against eating raw oysters after nearly 400 people in Hong Kong went down with the potentially fatal Norwalk virus infection in the first half of the year. Food authorities said 11 samples of oysters were found to be contaminated with Norwalk-like viruses in tests on 500 over the same period, the first time the virus has been detected in the mollusc sold in Hong Kong. Dr Gloria Tam, assistant director (food surveillance and control) of the Food and Environmental Hygiene Department, said: 'Pregnant women, the young, elderly and chronically sick may come down with symptoms ranging from a simple temperature, nausea, vomiting, diarrhoea, and abdominal pain. Very severe cases can lead to fatalities. 'I'm appealing to these vulnerable groups to refrain from eating raw oysters,' she said yesterday as the department released findings of microbiological and chemical testing on 27,200 food items from January to June. Thirty-four outbreaks of the Norwalk virus were reported involving 360 people, according to Department of Health figures. Of these, 29 were due to eating oysters, a spokeswoman said. All have recovered. The virus is commonly found in sewage-contaminated waters. The only known hosts are infected humans. It can be spread by the faecal-oral route via contaminated food and water, by person-to-person contact, by contact with a contaminated object and through the air. The virus was first identified in 1972 after an outbreak of gastro-intestinal illness in Norwalk, Ohio, in the United States. Later, other viruses with similar features were described as Norwalk-like viruses. Dr Tam said international authorities believed the Norwalk viruses could become the most dangerous food-borne agent in the next decade. 'It's a new kid in town. It is an emerging food-borne pathogen which has only been studied in detail in the past decade,' she said. 'There have been well-published results in international literature that this virus could cause massive food-poisoning outbreaks, particularly in people who have consumed improperly cooked seafood, notably the bivalves,' she said. In December 1999, 136 students of the Diocesan Preparatory School in Kowloon Tong were struck with the Norwalk virus in two separate outbreaks. Health officials at that time ruled out contamination of food or water. Microbiological tests on 9,400 of the 27,200 samples in the current batch also found 34 to be contaminated with other bacteria or viruses. Of these 34, three samples of lo mei, a Chinese sausage, and siu mei, or roast meat, were contaminated with listeria bacteria. In tests on the other 17,800 samples, 109 failed standards. For the first time, tests showed non-leafy vegetables, such as chives, tomatoes and sugar peas, to be contaminated with pesticides, Dr Tam said. 'Besides washing and soaking leafy vegetables in water for at least an hour, people should also do the same for non-leafy ones,' she said.