WASHINGTON, March 18 (Reuters) - Southern China may have been the source for much of the spread of the H5N1 avian flu virus, researchers suggested on Tuesday.

A genetic analysis of the virus shows that strains that showed up in Vietnam, Thailand and Malaysia in 2002 and 2003 closely resemble a strain from poultry markets in China's Yunnan Province, the flu experts found.

Two viruses found in poultry in China's Hunan province in 2002 and 2003 were most closely related to viruses from Indonesia, they reported in the Journal of Virology.

"These results suggest a direct transmission link for H5N1 viruses between Yunnan and Vietnam and also between Hunan and Indonesia during 2002 and 2003," wrote the researchers, who included Guan Yi of the University of Hong Kong and Robert Webster of St. Jude Children's Research Hospital in Memphis, Tennessee.

"Poultry trade may be responsible for virus introduction to Vietnam, while the transmission route from Hunan to Indonesia remains unclear," they wrote.

The H5N1 bird flu virus was first seen in a goose in southern China's Guangdong province in 1996. A 1997 outbreak in Hong Kong killed six people.

The virus reemerged in 2003 when two members of a Hong Kong family who had recently visited Fujian province became ill and one died.

Since 2003, H5N1 bird flu has been found in more than 60 countries and territories. It has killed 236 people out of 373 infected in 14 countries -- Myanmar, Turkey, Djibouti, Azerbaijan, Egypt, Pakistan, Iraq, Indonesia, Thailand, Vietnam, China, Nigeria, Laos and Cambodia.

Bird flu now almost exclusively infects birds. But it can occasionally pass to a person.

Experts say the danger is that the virus may evolve into a form that people can easily catch and pass to one another, in which case the transmission rate would soar, causing a pandemic in which millions of people could die.

Yi and Webster's team wanted to find out if H5N1 viruses all descend from one type. "Due to the lack of influenza surveillance prior to these outbreaks, the genetic diversity and the transmission pathways of H5N1 viruses from this period remain undefined," they wrote.

In 2007, a team at the University of California Irvine reported that Guangdong appeared to be the source of renewed waves of the H5N1 strain.

Chinese officials denied the report at the time. (Reporting by Maggie Fox; Editing by Will Dunham and Eric Walsh)

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