March 6 (Bloomberg) -- The bird flu virus that's infected people in a dozen countries can be traced to southern China, where it was discovered more than a decade ago, researchers found.

Variants of the H5N1 virus originated in Guangdong province where ``explosive growth'' in intensive poultry production and proximity to wild fowl promoted its diversification and spread, researchers at the University of California in Irvine found after studying genetic and geographic data.

``Guangdong appears the prime source of H5N1's diversity and diffusion,'' the UCI researchers, led by Robert Wallace, said in the study, published yesterday in the online early edition of the Proceedings of the National Academy of Sciences.

Eleven years after the lethal virus was found in a farmed goose in Guangdong, scientists are still struggling to understand how it spreads and whether it will spark the next flu pandemic. H5N1 has been found in birds in 56 countries across Asia, the Middle East, Africa and Europe.

Knowing where H5N1 strains develop can help scientists better limit the virus's spread by identifying outbreaks and culling infected birds faster. Vaccination programs can also be more effective with strains from regions that have repeatedly contributed to outbreaks, UCI said in a media release on its Web site.

``If you can control the virus at its source, you can control it more efficiently,'' Walter Fitch, professor of ecology and evolutionary biology at UCI and co-author of the study, said in the statement. ``With a road map of where the strain has migrated, you're more likely to isolate the strain that you should be using to make the vaccine.''

Virus Seeks Hosts

The researchers analyzed genetic sequences from 192 avian- flu samples collected across Asia and northern Europe to identify mutations that have occurred during the virus's evolution.

``The ability to develop the right mutation allows the virus to hop from one host type to the next,'' UCI's Wallace said in the statement. ``By spreading across a large area, the virus in essence can run multiple experiments in multiple locations, increasing the likelihood that it will mutate into a form that can be transmitted from human to human.''

Infections in people increase the opportunity for the virus to mutate into a form that spreads easily between humans.

At least 167 of the 277 people known to have been infected since late 2003 have died, the World Health Organization said on March 1. Millions could die if H5N1 becomes as contagious as seasonal flu.

`Regional Sink'

Birds from China have been associated with H5N1 outbreaks outside the country in the past.

In August, the Food and Agriculture Organization of the United Nations said an H5N1 variant found in southern China caused outbreaks in poultry in Thailand and Laos the previous month, and suggested the virus was re-introduced through trade.

Outbreaks in the Middle East and Europe last year were linked with a variant found at China's Qinghai Lake, where more than 6,000 wild birds died in April 2005.

This year, Laos reported its first two human cases, while neighboring Vietnam, Myanmar and Thailand had fresh outbreaks. Today, officials said the virus had infected 550 young chicken at last month in a province west of Hanoi in Vietnam.

While researchers at UCI found ``Guangdong is the source of multiple H5N1 strains spreading at both regional and international scales,'' the sub-region of Indochina ``appears to be a regional sink'' in which the virus circulates within its own ``endemic disease ecosystem,'' the authors wrote.

To contact the reporter on this story: Jason Gale in Singapore at j.gale@bloomberg.net

Last Updated: March 6, 2007 03:32 EST