Preparedness for a possible influenza pandemic caused by highly pathogenic avian influenza A subtype H5N1 has become a global priority. The spread of the virus to Europe and continued human infection in Southeast Asia have heightened pandemic concern. It remains unknown from where the pandemic strain may emerge; current attention is directed at Vietnam, Thailand, and, more recently, Indonesia and China. Here, we report that genetically and antigenically distinct sublineages of H5N1 virus have become established in poultry in different geographical regions of Southeast Asia, indicating the long-term endemicity of the virus, and the isolation of H5N1 virus from apparently healthy migratory birds in southern China. Our data show that H5N1 influenza virus, has continued to spread from its established source in southern China to other regions through transport of poultry and bird migration. The identification of regionally distinct sublineages contributes to the understanding of the mechanism for the perpetuation and spread of H5N1, providing information that is directly relevant to control of the source of infection in poultry. It points to the necessity of surveillance that is geographically broader than previously supposed and that includes H5N1 viruses of greater genetic and antigenic diversity. genetics ͉ human ͉ influenza A ͉ virus evolution ͉ avian

Highly pathogenic avian influenza (HPAI) H5N1 viruses are now endemic in many Asian countries, resulting in repeated outbreaks in poultry and increased cases of human infection. The immediate precursor of these HPAI viruses is believed to be A/goose/Guangdong/1/96 (Gs/GD)-like H5N1 HPAI viruses first detected in Guangdong, China, in 1996. From 2000 onwards, many novel reassortant H5N1 influenza viruses or genotypes have emerged in southern China. However, precursors of the Gs/GD-like viruses and their subsequent reassortants have not been fully determined. Here we characterize low-pathogenic avian influenza (LPAI) H5 subtype viruses isolated from poultry and migratory birds in southern China and Europe from the 1970s to the 2000s. Phylogenetic analyses revealed that Gs/GD-like virus was likely derived from an LPAI H5 virus in migratory birds. However, its variants arose from multiple reassortments between Gs/GD-like virus and viruses from migratory birds or with those Eurasian viruses isolated in the 1970s. It is of note that unlike HPAI H5N1 viruses, those recent LPAI H5 viruses have not become established in aquatic or terrestrial poultry. Phylogenetic analyses revealed the dynamic nature of the influenza virus gene pool in Eurasia with repeated transmissions between the eastern and western extremities of the continent. The data also show reassortment between influenza viruses from domestic and migratory birds in this region that has contributed to the expanded diversity of the influenza virus gene pool among poultry in Eurasia.

Antigenic profiles of post-2002 H5N1 viruses representing major genetic clades and various geographic sources were investigated using a panel of 17 monoclonal antibodies raised from five H5N1 strains. Four antigenic groups from seven clades of H5N1 virus were distinguished and characterized based on their cross-reactivity to the monoclonal antibodies in hemagglutination inhibition and cell-based neutralization assays. Genetic polymorphisms associated with the variation of antigenicity of H5N1 strains were identified and further verified in antigenic analysis with recombinant H5N1 viruses carrying specific mutations in the hemagglutinin protein. Modification of some of these genetic variations produced marked improvement to the immunogenicity and cross-reactivity of H5N1 strains in assays utilizing monoclonal antibodies and ferret antisera raised against clade 1 and 2 H5N1 viruses, suggesting that these sites represent antigenically significant amino acids. These results provide a comprehensive antigenic profile for H5N1 virus strains circulating in recent years and will facilitate the recognition of emerging antigenic variants of H5N1 virus and aid in the selection of vaccine strains.