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.