Changes in the prevalence of H5N1 influenza viruses in Hong Kong

Abstract

Background: The highly pathogenic Asian H5N1 avian influenza virus first emerged in 1997 and by 2006 had spread in 3 major transmission waves to more than 60 countries across Eurasia and Africa. During each wave transmission, a different H5N1 clade or genotype was disseminated, which widened the geographic range affected by the viruses and led to some viruses becoming enzootic in affected regions. More recently, clade 2.3.2.1 H5N1 viruses have been transmitted to many Eurasian countries and are forming different subgroups. Introduction of live poultry market management measures in Hong Kong has meant that H5N1 viruses are mainly isolated from migratory wild birds. The presence of clade 2.3.2.1 viruses in wild birds in Hong Kong could be part of the widespread dissemination of these viruses. Materials and Methods: Avian influenza H5N1 viruses were isolated from dead wild birds or poultry carcasses found in Hong Kong and identified using real-time PCR and hemagglutinin inhibition tests at the laboratory of the Agriculture, Fisheries and Conservation Department of the Hong Kong SAR Government. Three human viruses were isolated from specimens taken from RT-PCR–confirmed H5N1 human infections in Hong Kong and Shenzhen, Guangdong, China. Genetic and antigenic analyses of these viruses were performed as described previously. A phylogenetic tree of each gene segment was generated using a maximum likelihood model, and the robustness of the tree topologies was evaluated with the approximate likelihood log ratio test, implemented in PhyML (version 3.0). Results: We characterized 71 H5N1 viruses isolated from poultry, wild birds, and humans in Hong Kong and Shenzhen from June 2008 to June 2013. One third of the viruses belonged to clade 2.3.4; the others belonged to clade 2.3.2.1 viruses that had diverged into 2 phylogenetic subgroups: Barn-swallow-like and 6841-like. The last virus belonging to clade 2.3.4 was isolated in January 2011; clade 2.3.2.1 viruses were last isolated in January 2013. H5N1 viruses showed a distinct preference for the winter season in Hong Kong. Viruses from clades 2.3.4 and 2.3.2.1 were antigenically distinct. However, the internal genes of the clade 2.3.2.1 viruses were mainly derived from clade 2.3.4 H5N1 viruses that were present in southern China from 2006 to 2008. All clade 2.3.2.1 viruses had a T156A substitution (most also had an N154D substitution) in the HA, which removed the glycosylation site at position 154. Loss of this site was implicated in the recent attainment of airborne transmission by H5N1 viruses in ferrets. S223R in HA, which may confer an increasing binding affinity for α-2, 6-NeuAcGal receptors, was found in 13 of the 2.3.2.1 viruses. Conclusions: Clade 2.3.2.1 viruses appear to have replaced clade 2.3.4 viruses in Hong Kong as part of the apparently new transmission wave of clade 2.3.2.1 viruses throughout Eurasia. Wild bird 2.3.2.1 viruses from Hong Kong belonged to 2 of the 4 subgroups in which this clade appears to be evolving. Possible antigenic and dissemination differences among these subgroups need to be evaluated.