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WASHINGTON - Flu viruses can live for decades in frozen lakes and may be picked up by migrating birds thus infecting animals and people further afield, researchers say.

A Journal of Virology article says such frozen viruses could potentially become the source of new epidemics that sicken and kill generations long after they were last seen.

"We've found viral RNA in the ice in Siberia and it's along the major flight paths of migrating waterfowl," said Dr Scott Rogers, of Bowling Green State University, in Ohio.

"The lakes are along the migratory flight paths of birds flying into Asia, North America, Europe and Africa."

Migrating birds are blamed, in part, for the spread of H5N1 avian flu, which has killed or forced the culling of more than 200 million birds globally.

Since January, H5N1 has spread out of Asia, across Europe and into Africa. Now more than 50 countries have battled the virus, which has infected 258 people and killed 153 since 2003.

Experts fear it could mutate into a form that easily infects people and causes a pandemic such as the three of last century, the worst being in 1918-1919 that killed millions.

It was caused by a virus called H1N1, a descendant of which still circulates and causes illness today.

But the original form was only recently studied after being recovered from the frozen body of an Alaskan victim.

Were that strain of H1N1 to circulate today, Dr Rogers said it could cause another pandemic because no one alive now had immunity to it.

The original H1N1 appears to have passed fairly directly from birds to people.

Dr Rogers and colleagues at the Russian Academy of Sciences sampled three lakes in northeast Siberia in 2001-2002 and found an H1 strain that circulated from 1933 to 1938 and again in the 1960s in the lake that had attracted the most geese.

"These certain strains come back from time to time," he said.

"The data suggest the influenza A virus deposited as the birds begin their autumn migration can be preserved in lake ice. As birds return in the spring, the ice melts, releasing the viruses."