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.
"We find that viruses come out of east and southeast Asia as a region each year and it is not any one particular country that is the continual source of influenza viruses. So it is not as simple as saying out of China, because out of China is not the whole story," Russell said.
In tropical regions, flu tends to break out in the rainy season. "In east and southeast Asia there is a there a lot of variability in the timing of the rainy season and the timing of the epidemic," Russell said.
"Bangkok and Kuala Lumpur are only 700 miles apart but they have their flu epidemics at completely different times of year." This means flu epidemics can be occurring almost year-round in Asia, he said.
Then the viruses die out every year in the Americas, Europe, Australia and the rest of Oceania, making these areas "evolutionary graveyards," Russell said.
Even if travelers carry the flu viruses back from the Americas to Asia, for example, people living in Asia are already immune to those particular variants.
The World Health Organization estimates that annual influenza epidemics infect between 5 percent and 15 percent of the world population each year, cause 3 million to 5 million cases of severe illness, and between 250,000 and 500,000 deaths,
About 300 million people get the flu vaccine each year. Without it, said Smith, a person can expect to catch the flu about once every 10 years.
Smith said the findings are important for the experts who formulate the new flu vaccine each year. It typically includes a cocktail of three strains, and the scientists try to predict which strains will cause the most trouble each year.
"If we are trying to predict what will happen a year from now we should be paying attention to what is happening in east and southeast Asia," he said.
The researchers said their study does not have any bearing on what might happen in a pandemic of a new source of flu, such as the H5N1 virus now circulating mostly among birds in Asia, Europe and Africa.