EVOLUTION OF H5 SUBTYPE AVIAN INFLUENZA-A VIRUSES IN NORTH-AMERICA

The phylogenetic relationships of the hemagglutinin (HA) and non-struc tural (NS) genes from avian influenza (AI) H5 subtype viruses of North American origin are presented. Analysis of the HA genes of several pr eviously uncharacterized isolates from waterfowl and turkeys provided clear evidence of significant sequence variation and existence of mult iple virus clades or sub-lineages, maintained in migratory waterfowl. Phylogenetic analysis of NS gene sequences further demonstrated multip le sub-lineages and also demonstrated re-assortment of two NS alleles in wild duck populations. Based on currently available HA, gene sequen ces, at least four clades exist with waterfowl isolates included in th ree of the four groups. The most genetically unstable of these sub-lin eages is composed of recent poultry isolates from the outbreak of AI i n Central Mexico. This group of viruses, which replicated unabated in chickens for at least 16 months, exhibited an increased rate of mutati on in both the HA and NS gene. Comparison of the HA, sequence data for all available North American H5 subtype viruses demonstrated minimal variation both in and around the amino acids predicted to be involved in the HA receptor binding site. The sequences also revealed that migr atory waterfowl, live poultry market chicken, and turkey isolates unif ormly lack a glycosylation site at amino acid 236 in the HA protein wh ich is present in commercial chicken isolates. (C) 1997 Elsevier Scien ce B.V.