Ecogeographic drivers of the spatial spread of highly pathogenic avian influenza outbreaks in Europe and North America, 2016–2022

H5Nx highly pathogenic avian influenza (HPAI) viruses of clade 2.3.4.4 have caused outbreaks in Europe among wild and domestic birds since 2016 and were introduced to North America via wild migratory birds in December 2021. We examined the spatiotemporal extent of HPAI viruses across continents and characterize ecological and environmental predictors of virus spread between geographic regions through constructing a Bayesian phylodynamic generalized linear model (phylodynamic-GLM). Findings demonstrate localized epidemics of H5Nx throughout Europe in the first several years of the epizootic, followed by a singular branching point where H5N1 viruses were introduced to North America, likely via stopover locations throughout the North Atlantic. Once in the US, H5Nx viruses spread at a greater rate between US-based regions and no evidence demonstrated spread back to any European region. We establish that geographic proximity is a predictor of virus spread between regions, which implies that inter-continental transport across the Atlantic Ocean is relatively rare. Increase in mean ambient temperature over time was predictive of reduced H5Nx virus spread, which may reflect the effect of climate change on declines in host species abundance, decreased persistence of the virus in the environment, or changes in migratory patterns due to ecological alterations. Our data provide new knowledge about the spread and directionality of H5Nx virus dispersal in Europe and North America during an actively evolving inter-continental outbreak, including predictors of virus movement between regions, which will contribute to surveillance and mitigation strategies as the outbreak unfolds, and in future instances of uncontained avian spread of HPAI viruses.