Avian Influenza H5N1 Virus

Avian Influenza H5N1 Virus

Abstract

Pandemic influenza viruses can emerge through continuous evolution and the acquisition of specific mutations or through reassortment. This study assessed the pandemic potential of H5N1 viruses isolated from poultry outbreaks occurring from July 2006 to September 2008 in the Lao People's Democratic Republic (PDR). We analyzed 29 viruses isolated from chickens and ducks and two from fatal human cases in 2007. Prior to 2008, all H5N1 isolates in Lao PDR were from clade 2.3.4; however, clade 2.3.2 was introduced in September 2008. Of greatest concern was the circulation of three isolates that showed reduced sensitivity to the neuraminidase (NA) inhibitor oseltamivir in an enzyme inhibition assay, each with different NA mutations - V116A, I222L and K150N, and a previously unreported S246N mutation. In addition, six isolates had an S31N mutation in the M2 protein, which conferred resistance to amantadine not previously reported in clade 2.3.4 viruses. Two H5N1 reassortants were isolated whose polymerase genes, PB1 and PB2, were homologous to those of Eurasian viruses giving rise to a novel H5N1 genotype, genotype P. All H5N1 viruses retained avian-like receptor specificity, but four had altered affinities for {alpha}2,3-linked sialic acid. This study shows that, in a genetically similar population of H5N1 viruses in Lao PDR, mutants emerged with natural resistance to antivirals and altered affinities for {alpha}2,3-linked sialic acids, together with reassortants with polymerase genes homologous to Eurasian viruses. These changes may contribute to the emergence of a pandemic influenza strain and are critical in devising surveillance strategies.

Introduction

Since the introduction of highly pathogenic H5N1 influenza viruses in 1997, H5N1 has become endemic in the southern People's Republic of China and has spread to over 60 countries in Europe, Asia and Africa. H5N1 outbreaks have resulted in the loss of millions of poultry and in sporadic transmission to humans (Li et al., 2004; Wang et al., 2008), and have raised concerns about its pandemic potential (Chen et al., 2004; Guan et al., 2004; Peiris et al., 2007). H5N1 viruses have evolved locally from this unprecedented spread, resulting in genetically and antigenically divergent H5N1 sublineages (Chen et al., 2004) that typically originate from one isolate emerging via antigenic drift (Guan et al., 2004) or reassortment (Guan et al., 1999; Li et al., 2004) within a specific region. Evolution of the H5 haemagglutinin (HA) glycoprotein has altered the antigenic properties of H5N1 viruses and produced antigenically distinct strains among genetically similar strains (Guan et al., 2004; Horimoto et al., 2004), which are identified by ten different clade designations (Donis et al., 2008).

Despite extensive surveillance programmes worldwide, short-term regional evolution of H5N1 viruses within domestic bird populations is not well understood. In this study, we showed that, within a genetically similar population of H5N1 influenza viruses in Lao People's Democratic Republic (PDR), different phenotypes have emerged with reduced susceptibility to adamantanes and NA inhibitors (NAIs) and altered receptor affinities for {alpha}2,3-linked sialic acid. We also identified two novel reassortant H5N1 viruses whose PB1 and PB2 genes are derived from viruses ...