HONG KONG - China may have unwittingly introduced swine flu viruses when it imported pigs from Europe and North America for breeding over the past few decades, researchers said.

Three virus families are endemic in pigs in southern China and one of them - the Eurasian avian-like H1N1 flu virus from Europe - is viewed as most threatening because humans have no antibodies against it, said the researchers, who published their findings in Nature magazine on Thursday.

The researchers in Hong Kong, Singapore and Chinese mainland reached their findings after monitoring swine flu viruses in pigs in Hong Kong over a 12-year period.

"We found that since 2001, the Eurasian (flu) viruses and North American viruses had entered pig populations in southern China and replaced the earlier viruses," said Vijaykrishna Dhanasekaran, assistant professor at the Duke-NUS Graduate Medical School's Program of Emerging Infectious Diseases in Singapore.   

"The import of breeding pigs has increased in southern China over the 20 years, this was done to improve the breeds," he said by telephone from Singapore.

From 1998 to 2010, Dhanasekaran and colleagues collected more than than 650 flu virus samples from pigs that ended up in a Hong Kong abattoir and found they all belonged to three lineages.

The most dominant was the Eurasian avian-like H1N1 virus. First detected in pigs in Belgium in 1979, it quickly became the most common flu virus in pigs in Europe.

The other 2 lineages are the North American H1N2 swine flu virus which has been circulating in pigs in North America since the 1990s and the H1N1 swine flu virus which has been circulating worldwide, including in China, for more than 80 years.