Genetic characterisation of avian influenza A(H7N9) shows that the H7N9 virus that infects human beings resulted from the recombination of genes between several parent viruses noted in poultry and wild birds in Asia. It is most closely related to sequences found in samples from ducks in Zhejiang province in 2011. Evidence so far suggests that the new H7N9 virus might have evolved from at least four origins. It is hypothesized that the gene that codes for HA has its origin in ducks and the gene that codes for NA has its origin with ducks and probably also wild birds. Six internal genes originated with at least two H9N2 chicken viruses. The HA genes were circulating in the East Asian flyway in both wild birds and ducks, while the NA genes were introduced from European lineages and transferred to ducks in China by wild birds through migration along the East Asian flyway.
