There is no need to worry about a mass gastroenteritis outbreak, despite an unusually persistent series of norovirus infections this year, researchers said yesterday. There was no evidence that the virus, usually most common in winter, had mutated into a form that could handle higher temperatures, they said. The researchers, from the Chinese University, were speaking a day after the government's Centre for Health Protection expressed concern about a prolonged norovirus season persisting into summer. Leung Wai-keung, from the university's department of medicine and therapeutics, said the warmer weather now being experienced should stop the bug's spread. The researchers tested stool samples from 651 patients with acute sporadic diarrhoea from December 2004 to November 2005 and found that the virus finds it difficult to survive in temperatures above 25 degrees Celsius. Professor Leung said the 35 outbreaks at hospitals and homes for the elderly in the past few weeks were related to cooler weather brought about by heavy rain. There have been 74 outbreaks this year affecting 99 people. The director of the Centre for Emerging Infectious Diseases, Joseph Sung Jao-yiu, quashed suggestions that the virus was spread through the air. 'If it was airborne, a lot more people would have been affected,' he said. The researchers said it was hoped a faster testing procedure could be developed that could be carried out in general clinics and laboratories. The director said there is only one laboratory in the city that can test for the highly infectious virus.