Bird flu outbreaks 'on edge of cold fronts'

Outbreaks of bird flu in Europe occurred along the edge of cold weather fronts as wetland birds gathered on stretches of water which remained unfrozen, researchers said.

Most outbreaks of the potentially deadly H5N1 avian flu virus among wild birds in the winter of 2005/06 occurred at sites where the temperature was just above freezing - around 0C to 2C, a study published in the journal PLoS Pathogens found.

These sites were usually on the edge of cold fronts where freshwater remained unfrozen, providing a place for the birds to feed, researchers from Princeton University said.

Waterbirds spend the winter in areas where waters remain unfrozen because it allows them to forage, and icy spells can cause massive movements of birds.

Many of them choose to congregate on the nearest unfrozen sites to their breeding grounds, rather than migrate long distances south, to conserve energy.

Even birds which do migrate can start their spring journey to their breeding grounds as early as February and can be held up by cold weather, which leads them to stop off on unfrozen waters.

This behaviour can lead to waterbirds congregating in large numbers in suitable habitats along the "freezing front" where water remains unfrozen - helping avian flu spread between species and cause outbreaks among wild birds.

The scientists said such gatherings of wild birds close to areas with poultry farms made it likely the disease would spill over into domestic birds - and increase the chance of transmission to humans. The scientists said forecasts of near-freezing temperatures should prompt close surveillance along cold fronts - particularly in areas with lots of poultry farms - to help detect the disease early.

The researchers analysed 52 outbreaks of H5N1 in wild birds in 15 European countries and the temperatures at those sites in the days leading up to the emergence of the highly pathogenic virus.

"Initial outbreaks of highly pathogenic avian influenza virus H5N1 infection in wild birds occurred significantly more often at locations where maximum surface air temperatures were between 0C and 2C on the day of the outbreaks and the two preceding days," they wrote.

Their research suggested the outbreaks "appear to closely follow the surge and ebb of the cold front".

Because of improved surveillance of avian flu in EU states since 2002 it is unlikely outbreaks in areas with other weather conditions went unnoticed and it did not appear that surveillance efforts were greater in more densely populated or richer areas, they said.

So the presence of outbreaks on the edge of the cold front was likely to be a "real and important phenomenon" which should be used to focus surveillance to tackle future outbreaks.