Continuing evolution of H9N2 avian influenza A viruses in poultry in southern China

Abstract

Our systematic influenza surveillance in southern China revealed that two lineages of H9N2 influenza viruses, represented by Chicken/Beijing/1/94 and Quail/Hong Kong/G1/97, became endemic in the poultry in southern China since 1990’s. These established H9N2 lineages continually evolved to generate many different reassortants (or genotypes) and caused sporadic human infection cases. As co-circulating with H5N1 influenza viruses, the increasing genetic diversity and the capability to cause sporadic human infection make the H9N2 viruses become one of the major candidates with pandemic potential. Even though highly pathogenic H5N1 influenza viruses were seldom detected at the live-poultry markets of Hong Kong since 2002, H9N2 viruses were still commonly isolated in our surveillance program. The accumulated H9N2 isolates provided an opportunity to get insights into the continual evolution of this subtype virus in the region. In present study, we have systematically analyzed the H9N2 influenza viruses isolated from 2005 to 2010. Antigenic and phylogenetic analyses of 60 representative H9N2 viruses showed that the Ck/Bei-like H9N2 virus lineage continued endemic in the terrestrial poultry during the survey period in southern China. Genotyping analyses revealed four prevalent genotypes or reassortant variants in the field. Fifty-three of the viruses analyzed belonged to genotype B14 and B15, which were also the major reassortant variants prevailing in southern China from 2000 to 2005. The remaining seven viruses belonged to novel genotypes that have not been identified before. Our findings suggested that the Ck/Bei-like lineage continually maintained high genetic diversity in this region. The epidemiological findings showed that the isolation rate of H9N2 virus at the marketing poultry in Hong Kong was dramatically dropped down since 2009, which was different from what have observed in other provinces in southern China, but was closely correlated with the hygiene measures implemented in live-poultry markets in Hong Kong, e.g. not keeping live chicken overnight. These findings suggest the proper market policy would directly impact the prevalence of influenza virus in the field.