Genetic and molecular characterization of a novel reassortant H3N2 influenza virus from a sick pig in Eastern China in 2019

Swine influenza viruses (SIVs) cause clinical respiratory symptoms associated with high mortality rates among pigs. Because pigs can be a "mixing vessel" for influenza viruses, the SIV might pose a serious threat to animal and human health. In this study, an H3N2 SIV [A/swine/Zhejiang/19/2019(H3N2) (ZJ-SW19)] was isolated from a sick pig in Eastern China in 2019, and its molecular genetics were characterized. Phylogenetic analysis demonstrated the hemagglutinin (HA) and neuraminidase (NA) segments of ZJ-SW19 are highly homologous with those of H3N2 SIVs, belonging to human-like lineages; in contrast, the remaining six SIV segments (PB2, PB1, PA, NP, M, and NS) demonstrate the highest similarity with H1N1 SIVs isolated in East Asia during 2014-2020. The in vitro analysis of the virus's growth kinetics revealed that ZJ-SW19 can replicate efficiently in various mammalian and avian cell lines (including MDCK, A549, and DF-1). The receptor-binding analysis results indicated that ZJ-SW19 can bind to human-like receptors (α-2,6-linked sialic acid) and avian-like receptors (α-2,3-linked sialic acid). Moreover, ZJ-SW19 demonstrated significant differences compared with avian- and human-origin H3N2 influenza viruses in the antigenic analysis. Finally, in the pathogenicity test, ZJ-SW19 effectively replicated in the mouse lungs with moderate virulence. Therefore, continuous circulation of novel reassortant H3N2 SIVs indicates the need for long-term, close surveillance of influenza viruses in pig herds.