An international team of scientists said on Sunday they had sequenced the genome of the duck as part of an investigation into a natural source for avian influenza.

Ducks and other fowl are carriers of bird flu virus, which can mix among pigs and humans in close proximity, mutating into more dangerous forms.

But, intriguingly, these birds are often immune to the virus they carry, as appears to be the case with the newly emerged H7N9 strain in China.

Publishing in the journal Nature Genetics, researchers said they had unravelled the DNA code of a 10-week-old female Beijing duck (Anas platyrhynchos) and identified immune-system genes that defend the bird against flu infection.

Many of these genes are not found in chicken and the zebra finch, whose genomes have previously been transcribed, the scientists found.

In addition, the duck's chromosomes show that it has pairs of some of these genes, rather than a single gene -- and this could explain why it appears to have a better shield against flu than chickens and turkeys.

Overall, ducks have an "optimised immune system" against flu, says the study.

For reasons that are unclear, though, the system can be sidestepped by the highly pathogenic form of H5N1 bird flu but not by the weaker type, it says.

H5N1 bird flu first erupted as a health scare among poultry in live markets in Hong Kong in 1997.

It is not very contagious among humans but is extremely dangerous if infection does occur. Out of more than 600 recorded cases of H5N1 among humans, almost 60 percent have been fatal.