A mutant virus strain that led to an atypical epidemic of gastroenteritis cases last summer in Hong Kong closely resembles the strain that caused a 2001 epidemic, the Centre for Health Protection said yesterday. It warned more new strains of the norovirus, a common cause of food poisoning, could emerge in coming years. Health authorities were puzzled by the off-season resurgence of the norovirus from May to July last year. It usually strikes from September to October and lasts until March. Last summer's epidemic also reached record proportions, involving 24 outbreaks in May, 42 in June and about 30 in July, mostly in homes for the elderly. At least 1,000 people fell ill with the disease. Genetic sequencing revealed yesterday showed the strain evolved from a 1995-96 strain that caused a local epidemic in 2001, the centre's head of public health laboratory services, Wilina Lim Wei-ling, said. Last year's variant was different from the strains that caused epidemics in 2002 and 2004 in the city, she said. All the variants belong to what is known as the Bristol cluster, which was responsible for causing a pandemic between 1995 and 2001. Dr Lim also said that last year's strain in Hong Kong was closely related to one of two dominant strains that caused the norovirus epidemic in Europe last year. 'With such an enormous capacity for genetic change, it is expected there will be more new variants emerging in the coming years,' she said. Symptoms of norovirus infection include nausea, vomiting, diarrhoea, abdominal pain, low-grade fever and malaise lasting two to four days. The infection could be transmitted in food or water contaminated with the virus, contact with vomit or faeces or any other objects contaminated by the virus. Laboratory tests of samples had turned up fewer cases of norovirus this year, data from the centre showed. Nine more cases of community-associated methicillin-resistant Staphylococcus aureus infections were recorded last month, the centre said. The victims - eight men and one woman, aged 12 to 79 - took to 25 the number recorded last month, the highest since surveillance began in January this year. There were 68 cases of the superbug infection in the first half of the year.