Image: Map of flu spread
NASA/University of Cambridge
Seasonal influenza strains constantly evolve in overlapping epidemics in Asia and sweep the rest of the globe, dying out in South America along the pathways shown here.
updated 4/16/2008 1:08:56 PM ET 2008-04-16T17:08:56

Asia is the world’s cradle for new flu strains. South America is where each year’s epidemic goes to die.

In between, the virus catches a ride to North America and Europe almost simultaneously, report international researchers who have finally pinpointed exactly how the most common type of influenza sweeps the globe each year.

It’s work that promises to help health authorities better prepare each winter’s flu vaccine — and already monitoring is being beefed up in parts of East and Southeast Asia in hopes of more accurately predicting which strains will jump continents in coming years.

The good news: Outside of Asia, flu strains don’t seem to get stronger as they migrate from continent to continent.

“Once the viruses leave that region, they’re really on a pathway to an evolutionary graveyard,” said Derek Smith of Britain’s University of Cambridge, who helped lead the team that analyzed 13,000 flu samples collected around the world over the last five years.

The World Health Organization blames flu for 250,000 to a half-million deaths worldwide each year. The virus evolves so quickly that slightly different strains circulate each winter, and specialists have long suspected that China was the world’s main incubator.

The new work, being published in Friday’s edition of the journal Science, shows flu’s annual evolution is far trickier.

'Like relay runners passing a baton'
Every year, the WHO’s Global Influenza Surveillance Network painstakingly collects nose or throat swabs of flu patients in over 80 countries, to identify what strains are circulating. The researchers culled samples of the most common flu subtype, a version of harsh Type A influenza called H3N2, collected since 2002. They tracked small changes in a protein on that virus’ coat that are enough to let the flu evade the immune system and sicken people — in other words, new H3N2 strains.

The first surprise: Flu may be a winter problem in most of the world, but H3N2 virus is constantly circulating in some part of East and Southeast Asia. In tropical countries, it prefers the rainy season; in more temperate zones, it thrives in chillier weather.

But there was enough overlap in these densely populated, closely neighboring countries that “we actually see ... viruses passing from epidemic to epidemic like relay runners passing a baton,” said study lead author Colin Russell of the University of Cambridge.

Like following a trail of footprints, the researchers then used a tool called “antigenic cartography” to map how and when those newly emerged strains then moved to other continents.

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The second surprise: Like clockwork, each year new strains reached Australia, North America and Europe six to nine months after emerging in Asia.

Several months after that, they reached South America — and died out.

Why that progression? Travel and trade, says Russell: There is far less direct air travel between Asia and South America than Asia and North America, for example. By the time the virus hitched a ride to South America, it seemed destined to die out because the rest of the planet already had been exposed.

Africa might be a last stopover, too, Russell cautioned. There simply is too little tracking of influenza in Africa to be able to tell.

What countries in East and Southeast Asia are the key incubators of each year’s flu trouble? It’s too soon to know. But already, the U.S. Centers for Disease Control and Prevention and the WHO are working to increase monitoring throughout the region. Most countries have some flu tracking, but Myanmar, Cambodia and Laos in particular need more, said CDC’s Dr. Michael Shaw.

“We have been increasing surveillance in that area as fast as we can,” Shaw said. Now, “at least we know the part of the world to look in and the probable time of the year to look.”

The work dovetails with a major study of influenza’s genes published Wednesday in the journal Nature, which examined how the virus ebbs and flows in the Northern and Southern Hemispheres. That research also pointed to single, tropical world source of new flu strains that spreads to temperate regions in the winter before dying out by summer.

“The geography’s important in the context of vaccination,” stressed Edward Holmes of Pennsylvania State University, who led the Nature genetic study. In picking the yearly vaccine’s recipe, “sometimes we get that wrong. Part of it is not looking in the right population. We need to look in Southeast Asia every season.”

The flu mapping research was funded by the U.S. National Institutes of Health.

© 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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