INTRODUCTION

Avian influenza viruses (AIVs) can be separated into two broad categories: highly pathogenic avian influenza viruses (HPAIV) and low pathogenic avian influenza viruses (LPAIV), based on their pathogenicity in chickens and molecular characterization of the hemagglutinin (HA) protein. H9N2 AIVs, the subject of this study, have become endemic in poultry across much of Asia, the Middle East, and West and North Africa[1,2]. In the mid-1990s, H9N2 AIV was first reported in chickens in Guangdong province of China, and subsequently became epidemic nationwide[3]. At present, the poultry-adapted H9N2 AIVs are not only a major threat to poultry health, but also human health, as some of the H9N2 lineage AIVs are zoonotic[1,4,5].
To prevent H9N2 AIV infection in chickens, over 20 different commercial vaccines are used in China, and these vaccines are frequently updated[3]. Unfortunately, H9N2 AIVs continue to persist in chickens, even in vaccinated chicken flocks, which is possibly due to antigenic drift[6-8]. Owing to the potential pressure of antigenic drift in poultry-adapted H9N2 AIVs, a large number of variant viruses evade the host’s neutralizing antibodies.
Not only does antigenic drift lead to the emergence of variant viruses, but also the exchange of gene segments among different virus particles via gene reassortment generates a large number of novel viral descendants[9]. H9N2 AIVs are often found co-circulating in poultry with other AIV subtypes, such as H5 and H7 HPAIVs[10]. Like other AIVs, H9N2 is prone to gene reassortment that can affect host specificity, virulence and pathogenicity. Live poultry are host to co-circulating subtypes of AIVs, while live poultry trading may influence the transmission of AIVs[11]. Live poultry markets (LPMs), which are a key link in live poultry trading, require close monitoring. H9N2 AIVs may act as donors of genes to other AIVs, such as H5N1, H5N6, H7N9 and H10N8, which are responsible for high mortality rates in humans[1,12-14]. Based on this information, it was crucial to investigate the molecular characteristics and genetic reassortment of H9N2 AIVs isolated from LPMs to assess whether these viruses possess the ability to infect and transmit to a new host.