Incursion and spread of H5N1 highly pathogenic avian influenza viruses among wild birds in 2010-11 winter in Japan.

Many highly pathogenic avian influenza (HPAI) outbreaks occurred in Japan during the 2010-11 winter. H5N1 HPAI viruses were isolated from 63 wild birds including migrating and resident birds, and caused HPAI outbreaks in 24 chicken farms by the end of March. In the present study, all virus strains isolated from wild birds in western Japan together with the viruses in the preceding works were phylogenetically and epidemiologically analyzed. Furthermore, the virus distributions in the raptors that died of H5N1 HPAI virus infection were assessed. The virus isolates in Japan were classified into three groups by phylogenic analysis of their hemagglutinins, supporting the previous report (Sakoda et al., 2012). The viruses in each group were continuously isolated in respective limited areas, indicating that viruses were maintained in local bird populations throughout the outbreak periods. Some viruses were genetically closely related to the Korean isolates around the same periods, suggesting that migratory birds were suspected of contributing to transportation of the viruses across the sea. Viruses were recovered from systemic tissues including digestive organs of the deceased raptors, indicating that they were infected with HPAI viruses by their predatory behavior, eating infected birds or carrion in the environment.