Evidence of an emerging triple-reassortant H3N3 avian influenza virus in China

The H3 subtype of avian influenza virus (AIV) stands out as one of the most prevalent subtypes, posing a significant threat to public health, and a novel triple-reassortant H3N3 AIV, designated A/chicken/China/16/2023 (H3N3), was isolated from a chicken in northern China. The complete genome of the isolate was determined using next-generation sequencing, and the AIV-like particles were confirmed via transmission electron microscopy. Phylogenetic analyses revealed that eight genes of the H3N3 isolate clustered within the Eurasian lineage of AIVs, indicating the novel H3N3 isolate has various constellations and was generated by complex reassortment events involving H3N8, H9N2, and H10N3 subtype influenza viruses. Strikingly, the HA gene of the H3N3 isolate exhibited the closest evolutionary relationships to a human-derived H3N8 influenza virus, posing a potential threat to public health. Additionally, the presence of Q226 and T228 in the HA protein suggests the H3N3 virus preferentially binds to α-2,3-linked sialic acid receptors. While the HA cleavage site motif (PEKQTR/GIF) and the absence of E627K and D701N mutations in PB2 protein classify the virus as a characteristic low pathogenicity AIV. However, several mutations in internal genes raise concerns about potential increases in viral resistance, virulence, and transmission in mammalian hosts. Overall, this study provides valuable insights into the molecular and genetic characterization of the emerging triple-reassortant H3N3 AIVs, and continued surveillance of domestic poultry is essential for monitoring the H3N3 subtype evolution and potential spread.