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Recombination and Reassortment in H5N1 Wild Bird Flu in Henan
December 1, 2005
Four complete sequences from tree sparrows in Henan, China (A/Tree sparrow/Henan/1/2004(H5N1), A/Tree sparrow/Henan/2/2004(H5N1), A/Tree sparrow/Henan/3/2004(H5N1), A/Tree sparrow/Henan/4/2004(H5N1)) have been deposited at GenBank. These 2004 isolates are reassortants with H5and N1 on the outside and sequences related to H9N2 on the inside, similar to the 1997 isolates linked to the Hong Kong outbreak. Thus, although 1.5 million birds were culled in Hong Kong in 1997, related sequences were circulating in wild birds in China.
The four sequences share polymorphisms with the 1997 isolates as well as 2000-2001 H5N1 isolates from Hong Kong. These "older" sequences however are present in 2004 tree sparrow isolates. One of the 2004 isolates also has the 20 amino acids in NA that are deleted in the Z genotype.
These isolates also still have PB2 E627 and although they are pathogenic to chickens, they are not pathogenic in ducks or mice. However, these sequences may be endemic in China, offering genetic variations used in the evolution of H5.
Indeed there are also polymorphisms found in LPAI H's recently deposited. These H5N3 and H5N2 serotypes are from ducks and swans in Mongolia and Japan. They also share polymorphisms with H5 isolates from Primorie and northern Europe, which identify an additional northern migratory pathway stretching from Sweden to Japan.
The presence of HPAI H5N1 in tree sparrows also raises the possibility that the dominant wild bird serotype in Asia, LPAI H9N2, is being replace by HPAI H5N1. Moreover, this indigenous HPAI H5N1 is assuming several configurations via reassortment, and acquiring novel polymorphisms via recombination. These genetic changes may be contributing to the lack of effectiveness of vaccines in China and the mismatches may be driving genetic diversity.
These new sequences from China also highlight the need for wider surveys of wild bird populations, including water fowl and well as local terrestial birds. Screening and publishing of these sequences will aid in the development of effective vaccines to better control the accelerating evolution of H5 in Asia and throughout the world.