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.