Flu comes fresh from Asia each year, study finds

By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters) - Flu viruses evolve freshly somewhere in east or southeast Asia every year, spreading around the world over the next nine months before dying out, researchers reported on Wednesday.

Genetic analysis by two teams of international researchers show that there are just a few initial sources of annual, seasonal influenza epidemics. The viruses spread around the world from these before dying.

Then every year, new strains emerge to infect people, according to the studies published in the journals Nature and Science.

One team led by Edward Holmes of Pennsylvania State University could not pinpoint the source but said that both H3N2 and H1N1 strains of influenza appear to arise every year from a "reservoir," perhaps in the tropics.

A second team led by Colin Russell and Derek Smith of the University of Cambridge in Britain analyzed 13,000 samples of H3N2 flu taken since 2002 to demonstrate this source must be in east and southeast Asia, perhaps a different place every year.

"For over 60 years the global migration pattern of influenza viruses has been a mystery," Russell told reporters in a telephone briefing.

Many experts have long believed Asia, and specifically China, to be the source of most influenza viruses.

Others hypothesized that flu viruses migrated back and forth between the northern and southern hemispheres, or that they cooked year-round in the tropics, to pop out every once in a while to the rest of the world, Russell said.