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.