Researchers Chart Flu's Global Journey
Thursday, April 17, 2008
New strains of seasonal influenza virus all arise in East or Southeast Asia and take a largely predictable route around the world before dying out for good in South America, the global glue-trap for the pathogen.
That is the conclusion reported yesterday by a large team of researchers who analyzed the genetic ancestry of about 13,000 virus samples collected from six continents over a five-year period to answer long-standing questions about the flu's life cycle.
The findings help explain the biological mechanisms that underlie two long-held observations about flu: New strains tend to appear first somewhere near China, and Australia's flu season is a preview of what will happen in North America six months later. They also help explain why one winter's flu is always at least a little bit different from the previous winter's, even though the virus disappears over the summer.
The researchers hope the insights may help virologists better predict what strains of virus to include in annual flu shots, which must be manufactured anew each year. Although usually accurate, those predictions occasionally miss, as they did this year when the North American vaccine provided little protection against the strain causing most illness.
"If we want to know what will be happening in a year, we really need to pay attention to what is happening in East and Southeast Asia," said Derek J. Smith, a biologist at the University of Cambridge in England.
Previously, scientists were uncertain how and where influenza virus evolves between epidemics, which occur in winter in temperate climates.
One theory was that flu virus survives largely unnoticed each summer, with the first cases each winter caused by leftover -- and slightly changed -- strains. Another theory held that the virus dies out completely in temperate climates each summer but flourishes year-round in tropical zones, which are the nurseries for new variants. A third theory was that all new strains come from China, largely because of its population density.
The researchers analyzed virus samples collected from 2002 to 2007 and compared the molecular fingerprints of the gene for hemagglutinin, the protein that gives flu strains their distinct identities.
They found that each year's dominant strain in Europe, the Americas or other regions was traceable to ancestors first seen in a large region of East and Southeast Asia (an area bigger than China alone). It was never directly descended from a strain seen in temperate zones the previous year. In fact, the evidence strongly suggested that flu virus goes extinct each summer in temperate climates -- there is none left to smolder and evolve.
What the researchers now believe happens is that the world is reseeded each year by new, slightly different variants. "The strains are coming out [of Asia] fully formed," Colin A. Russell, one of the researchers, said in a telephone news conference.
The route the new strains then take seems to reflect both proximity to East Asia and the amount of travel between regions. The first stops are Australia and the Pacific islands known as Oceania, which the virus reaches about three months after it arises in Asia. Three to six months later it crops up in Western Asia, Europe and then North America. The last stop is South America.
"It is a surprise to everyone that South America is the end of this seeding hierarchy," Russell said.