Evolution of H5N1 avian influenza viruses in Asia

Abstract

An outbreak of highly pathogenic avian influenza A (H5N1) has recently spread to poultry in 9 Asian countries. H5N1 infections have caused > or =52 human deaths in Vietnam, Thailand, and Cambodia from January 2004 to April 2005. Genomic analyses of H5N1 isolates from birds and humans showed 2 distinct clades with a nonoverlapping geographic distribution. All the viral genes were of avian influenza origin, which indicates absence of reassortment with human influenza viruses. All human H5N1 isolates tested belonged to a single clade and were resistant to the adamantane drugs but sensitive to neuraminidase inhibitors. Most H5N1 isolates from humans were antigenically homogeneous and distinct from avian viruses circulating before the end of 2003. Some 2005 isolates showed evidence of antigenic drift. An updated nonpathogenic H5N1 reference virus, lacking the polybasic cleavage site in the hemagglutinin gene, was produced by reverse genetics in anticipation of the possible need to vaccinate humans.

Figure 1

Phylogenetic relationships among H5 hemagglutinin (HA) genes from H5N1 avian influenza viruses and their geographic distribution. Viral isolates collected before and during the 2004–2005 outbreak in Asia and selected ancestors were included in the analysis (Table A1). HA clades 1, 1′, and 2, discussed in the text, are colored in blue, red, and green fonts, respectively. Virus names in boldface denote isolates from human infections. Phylogenetic trees were inferred from nucleotide sequences by the neighbor-joining method with A/chicken/Scotland/56 genes as outgroup (not shown, denoted by arrowhead). Bootstrap analysis values >90% are shown. A) HA gene tree phylogeny was based on the coding region of the segment. Presence of a motif for glycosylation in HA is indicated as A156T by an arrow at the root of clade 1 and a diamond for other clades (Table 1). Stars denote absence of 1 arginine residue at the polybasic cleavage site, which starts at position 325 of HA1. Isolates to which ferret antisera were made for antigenic analyses are boxed (Table 2). B) Geographic distribution of H5N1 in east Asia: blue denotes countries reporting infections with clade 1 H5N1 in humans and birds (solid) or in birds only (hatched). Green denotes countries reporting bird infections with clade 2 H5N1 viruses.

Figure 2

Phylogenetic relationships among N1 neuraminidase (NA) genes of H5N1 influenza viruses. The clade of the hemagglutinin of each of these viruses is indicated by font coloring as in Figure 1A. Brackets denote genes encoding NA protein with deletions in the stalk region; residues 49–68 for clades 1–2 and 57–75 in clade 3.