Emergence of Novel Highly Pathogenic Avian Influenza Virus A(H5N1) Clade 2.3.4.4b in Commercial Poultry, Bangladesh.

The high-pathogenicity avian influenza (HPAI) viruses represent a main threat to animal and human health, poultry productivity, and ecosystems. The newly evolved A(H5N1) clade 2.3.4.4b has become a worldwide concern for its expanding multihost ecology, including poultry and a wide range of mammalian species. Avian influenza virus (AIV) was isolated from an outbreak in commercial quail farms associated with severe respiratory signs and high mortality. Molecular characterization of the detected virus was performed through gene sequencing via whole-genome sequencing technology and phylogenetic analysis. Genetic and phylogenetic analyses revealed that the hemagglutinin (HA) gene clustered closely with a recent HPAI A(H5N1) clade 2.3.4.4b strain from live bird markets (LBMs) in Bangladesh, with a most recent common ancestor dated to July 2023. The NA gene and four internal genes (PB1, NP, MP, NS) were derived from contemporary A(H5N1) viruses of ducks in LBMs, while the PB2 and PA segments exhibited similarity to low pathogenicity avian influenza (LPAI) viruses, suggesting reassortment events within avian hosts. The presence of a multibasic cleavage site (PLREKRRKR↓GLF) with avian-specific receptor-binding residues (Q224 and G226) and five conserved glycosylation motifs, consistent with high-pathogenicity phenotypes and strong avian receptor affinity. The PB2 gene harbored a rare I147T substitution; however, key mammalian adaptation markers such as E627K and D701N were absent. The reassortant A(H5N1) clade 2.3.4.4b virus with high pathogenicity and genetic links to LBM-origin strains is now circulating in the commercial poultry population in Bangladesh, increasing the risk of viral adaptation, the potential for cross-species transmission, and threatening the poultry sector and public health.