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LONDON (Reuters) - Outbreaks of H5N1 flu among birds in Europe came at the edges of cold fronts that caused wild birds to change migration patterns, scientists said on Thursday, suggesting cold snaps may signal future outbreaks.Dutch and American researchers found European outbreaks of avian influenza during the 2005-2006 winter were driven by collective movements of wild waterbirds to places where the fresh water they need to feed and survive had not frozen."This has important implications for surveillance, which should target areas where temperatures are close to freezing in winter, especially in poultry-dense regions close to areas where waterfowl aggregate," the researchers wrote in a study in the Public Library of Science journal PloS Pathogens.It is difficult for people to catch H5N1 bird flu, but when they do it can be deadly. Since 2003 it has infected 492 people and killed 291 of them, according to the World Health Organisation, and experts fear the H5N1 virus could mutate at any time into a form easily passed from one person to another.The virus emerged more than a decade ago in poultry in Southeast Asia. In 2005 it spread outside Asia infecting both poultry and wild birds in the Middle East, Europe and Africa.Most human cases have been in Asia but Egypt has had 108 cases and 33 deaths.Romanian officials reported an outbreak of bird flu last month on a poultry farm close to Ukraine in an area on an important migratory pathway for wild birds. Leslie Reperant of Princeton University in the United States and Thijs Kuiken of the Erasmus Medical Centre in the Netherlands said their findings offered a possible way to predict and control where and when bird flu might erupt again."Forecasts predicting near-freezing temperatures in Europe may act as an indication for concern," they wrote.They found that most H5N1 outbreaks occurred at sites where maximum temperatures were between 0 degrees Celsius and 2 degrees Celsius. This was usually on the edge of cold fronts where fresh water remained unfrozen. "Many wild waterbirds need unfrozen bodies of fresh water in winter to feed," they wrote. "To minimise the distance flown, they also try to stay as close as possible to the northern breeding grounds to which they will migrate during spring...The resulting congregation of different species of waterbirds along the freezing front likely created ideal conditions for the transmission of the H5N1 virus."The now waning pandemic of H1N1 swine influenza has driven the development of new drugs to fight flu, which kills 250,000 to 500,000 people globally in a normal year and far more in a severe pandemic.