Antigenic and biological characterization of H5 avian influenza viruses
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Avian influenza (AI) undergoes antigenic drift, enabling it to evade the host immune response, allowing higher replication and enhanced transmission. Ongoing surveillance and biological characterization is necessary to monitor currently circulating viruses, in part to allow the optimal use of vaccines to protect vaccinated birds from disease and to reduce the amount of virus that is shed. Current methods of characterization include sequence analysis, antigenic characterization, and vaccine efficacy studies, but a simpler and more reliable method for characterization is needed. Nineteen highly pathogenic H5N1 AI isolates from poultry in northern Vietnam in 2005 were characterized, and commercial AI vaccines were evaluated in their abilities to prevent disease and to reduce viral shedding. The H5N1 viruses have drifted, not only from other Asian H5N1s, but also amongst themselves; the Vietnamese viruses were in two genetic and antigenic subgroups, clades 2.3.2 and 2.3.4. These H5N1 viruses were exceptionally virulent in both chickens and Pekin ducks. When two representative strains were tested in a vaccine efficacy study, current vaccines protected against disease and reduced viral shedding. However, the vaccine produced from an older virus provided marginal protection and should be replaced. In analysis of another H5 influenza lineage, the Mexican H5N2, previous studies had characterized antigenic drift in the face of vaccination, and had identified potential antigenic sites important for virus neutralization. Using DNA vaccines and reverse genetics, point mutations in the hemagglutinin (HA) protein were introduced to determine the influence that particular amino acids have on the antigenicity of this H5 lineage. Based on data obtained from cross-HI tests, virus neutralizations in embryonating chicken eggs, and an in vivo study comparing levels of virus shed between groups of chickens, differences were detected which corresponded to overall sequence similarity of the HAs used. However, it appears that none of these differences could be attributed to the point mutations alone. These studies emphasize the importance of continuous monitoring of currently circulating HP AI H5N1 viruses. Until an alternative, simplified method is developed, the best way to assess vaccine efficacy and seed strain selection is by direct, in vivo testing.