Abstract
Since the national vaccination program was implemented with the H5/H7
bivalent vaccine in poultry in September 2017, the prevalence of H7N9
avian influenza viruses (AIVs) has
been controlled effectively in China, and low pathogenic H7N9 viruses
have disappeared nationwide. However, highly pathogenic H7N9 viruses
still exist, causing sporadic outbreaks especially in some regions of
northern China. During our routine
surveillance in poultry in 2020, we isolated two strains of
H7N9 subtype AIV from breeder layer
farms in northern China. We found that these
two
chicken-origin H7N9 isolates were
both highly pathogenic (HP) based on
the sequence of the HA gene. Deduced amino acid sequences of the HA gene
revealed that both strains had a four-amino-acid (KRTA) insertion at
position 339-342 and an I335V mutation in the cleavage site to make the
motif PEVPKRKRTARāGLF. Remarkably,
both strains gained the F102V and
N157D mutations (H3 numbering) in their HA genes, which have never been
reported before. Solid-phase direct
binding assay showed that these two isolates both had
dual-receptor binding
characteristics, while thermal and
acid stability assays indicated
that they were relatively stable in high-temperature or acidic
conditions. In addition, the animal experiments demonstrated that both
strains were highly pathogenic to
chickens but low pathogenic to mice. These results suggested that the
evolution of H7N9 subtype AIV is still continuing, and they pose a
potential threat to poultry and public health. Thus, attentions should
be paid to the importance of continual surveillance of the H7N9 AIVs.
Keywords: H7N9 subtype avian influenza virus; highly
pathogenic; dual-receptor characteristics; Thermal and acid stability