Transmission of lethal H5N1 clade 2.3.4.4b avian influenza in ferrets

Abstract Highly pathogenic avian influenza viruses of the H5N1 subtype are highly lethal in many avian species. Since 2014, viruses of the genetically distinct clade 2.3.4.4 have been circulating in Eurasia, Africa and briefly in North America, killing millions of wild and domestic birds. In December, 2021 clade 2.3.4.4b H5N1 viruses were first isolated from poultry and wild birds in Canada. Further spread of H5N1 viruses has since led to infection in many terrestrial and aquatic mammalian species. Some of the H5N1 viruses isolated from mammals have acquired adaptations previously shown to increase viral replication in mammals as well as novel patterns of reassortment of internal gene segments with North American (NA) lineage influenza A viruses (IAVs). While there have been few human cases of infection with clade 2.3.4.4b viruses reported to date, the potential for spillover, particularly of viruses harboring mammalian adaptation signatures remains a critical concern. Here we show that multiple naturally circulating reassortant H5N1 viruses can replicate in primary human airway epithelial cells and cause lethal disease in multiple mammalian species. One isolate, A/Red Tailed Hawk/ON/FAV-0473-4/2022, efficiently transmitted by direct contact between ferrets, resulting in lethal outcomes. Historically, H5 subtype viruses do not transmit effectively between mammals, and recent studies of currently circulating H5N1 viruses showed mild infection outcome in ferrets without detectable transmission. While known specific mutations within the Eurasian lineage H5Nx viruses have allowed for increased viral fitness in mammals, reassortment of H5N1 viruses with NA lineage avian IAVs may be unpredictably (unexpectedly) contributing to further enhancements in virulence and transmissibility in mammals, with potential increased risk to humans. Our research has determined that certain, as yet uncharacterized, genetic signatures may be important determinants of mammalian adaptation and pathogenicity of these viruses. Ongoing surveillance of circulating HPAI A(H5N1) viruses across species, including humans, should be a top priority so as to promptly identify viruses that may have pandemic or outbreak potential in mammals.