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Researchers Chart Flu's Global Journey
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But why is East-Southeast Asia always the starting point? The researchers believe it's because of the unusual concentration of different climates there. The region has both tropical environments, where flu flourishes during the rainy season, and temperate zones. There are places that are relatively close -- Russell cited Bangkok and Kuala Lumpur, Malaysia, 700 miles apart -- that have totally different flu seasons.
This effectively allows new strains to be passed around the region like a baton in a relay race, even though in each climate zone the virus completely dies out once a year.
The reason new variants don't cause epidemics if they are carried back to East Asia from elsewhere is because people already have immunity to them. They're old news. At least that's the theory.
The Science paper complemented one published this week in the competing journal Nature.
Edward C. Holmes, a biologist at Pennsylvania State University, and his collaborators studied about 1,300 influenza virus samples gathered from New York state in the Northern Hemisphere and New Zealand in the Southern Hemisphere over 12 years. They examined the genetic fingerprints of all eight gene segments of the flu virus, not just hemagglutinin.
They found that one region's year-to-year virus strains were not directly descended from each other but instead appeared to come from a "reservoir" population somewhere, Holmes said. He added that the results were "completely compatible" with the Cambridge group's findings.
The Penn State group also found that new, successful flu variants weren't just ones that had different hemagglutinin proteins to which people were not likely to be immune. Many also showed evidence of having traded genes with other flu viruses, with some of these "reassorted" strains more successful than those that evolved by small steps.
Neither of these papers address the origin of flu viruses that cause severe global pandemics such as the one that killed at least 50 million people in 1918 and 1919. Those arise from the mixing of human flu viruses and ones carried by birds or pigs -- a much rarer event than the predictable evolution of human viruses year to year.
