mammalian adaptation of B3.2 viruses in sub-Antarctic
Since its initial detection in the Antarctic region in late 2023, clade 2.3.4.4b H5N1 high pathogenicity avian influenza virus (HPAIV) has exerted significant impacts on diverse avian and mammalian populations within the sub-Antarctic and Antarctic region. In this study, we report continued detections of H5N1 HPAIV in both wild birds and marine mammals across this region during the austral summer of 2023/2024. Genetic characterisation revealed that all virus sequences belonged to the B3.2 genotype, while phylogenetic analysis indicates multiple independent incursions from continental South America into South Georgia and the Falkland Islands, with subsequent evidence of viral transmission between the islands. Importantly, from a zoonotic perspective, genotypic analysis revealed that B3.2 isolates from both avian and mammalian hosts harbour key markers of mammalian adaptation. Notably, polymerase complex mutations PB2 Q591K, PB2 D701N, and PA M86I were identified. Functional characterisation demonstrated that viruses bearing these mutations replicated efficiently in primary human airway epithelial cell cultures, reaching titres comparable to or exceeding those of H5N1 isolates from North American dairy cattle, and that these polymerase mutations alone contributed to significantly enhanced polymerase activity in human cell lines. Despite these adaptive changes in the polymerase complex, no mammalian-adaptive mutations were detected in the haemagglutinin or neuraminidase genes, with viruses exhibiting an avian-like receptor binding preference. Antigenic characterisation showed no substantial deviation from cross-reactivity with existing pre-pandemic candidate vaccine strains, and the viruses remained susceptible to currently licensed frontline antiviral therapeutics. Collectively, these findings highlight a complex and evolving viral ecology in the sub-Antarctic region involving both avian and marine mammal hosts, with significant implications for regional wildlife populations and zoonotic risk.