The global H5N1 influenza panzootic in mammals

Influenza A viruses have caused more documented global pandemics in human history than any other pathogen^1,2. High pathogenicity avian influenza viruses belonging to the H5N1 subtype are a leading pandemic risk. Two decades after H5N1 ‘bird flu’ became established in poultry in Southeast Asia, its descendants have resurged³, setting off a H5N1 panzootic in wild birds that is fuelled by: (1) rapid intercontinental spread, reaching South America and Antarctica for the first time^4,5; (2) fast evolution via genomic reassortment⁶; and (3) frequent spillover into terrestrial^7,8 and marine mammals⁹. The virus has sustained mammal-to-mammal transmission in multiple settings, including European fur farms^10,11, South American marine mammals^{12, 13, 14–15} and US dairy cattle^{16, 17, 18–19}, raising questions about whether humans are next. Historically, swine are considered optimal intermediary hosts that help avian influenza viruses adapt to mammals before jumping to humans²⁰. However, the altered ecology of H5N1 has opened the door to new evolutionary pathways. Dairy cattle, farmed mink or South American sea lions may have the potential to serve as new mammalian gateways for transmission of avian influenza viruses to humans. In this Perspective, we explore the molecular and ecological factors driving the sudden expansion in H5N1 host range and assess the likelihood of different zoonotic pathways leading to an H5N1 pandemic.