Title: Ecogeographic drivers of the spatial spread of highly pathogenic avian influenza outbreaks in Europe and North America, 2016-2022Authors: Jonathon D. Gass, Jr.1,2, Nichola J. Hill3, Elena N. Naumova4, Felicia B. Nutter1, Jonathan A. Runstadler11 Department of Infectious Disease and Global Health, Cummings School of Veterinary Medicine, Tufts University, North Grafton, MA, USA2 Department of Public Health and Community Medicine, Tufts University School of Medicine, Boston, MA, USA3 Department of Biology, University of Massachusetts, Boston, Boston, MA, USA4 Friedman School of Nutrition Science and Policy, Tufts University, Boston, MA, USACorresponding author email: [email protected] H5Nx highly pathogenic avian influenza (HPAI) viruses of clade 2.3.4.4. have caused outbreaks in Europe since 2016, and most recently these viruses were introduced to North America via avian migratory transport in December 2021. We sought to analyze the spatiotemporal extent of these viruses within the two continent outbreak system, and to characterize predictors of virus spread between geographic regions through a Bayesian phylodynamic generalized linear model (phylodynamic-GLM). Findings from this study reveal for the first time the geographic extent and directionality of the H5Nx HPAI virus outbreak system in Europe and North America during 2016-2022. Data 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 via wild migratory birds, likely via stopover locations in the North Atlantic. Once in the US, H5Nx HPAI viruses spread at a greater rate between US-based regions along migratory flyways, and no evidence points to spread back to any European region. Our GLM demonstrated that geographic proximity is a predictor of virus spread between regions, which implies that inter-continental transport across the Atlantic is relatively rare and may require northward virus movement that coincides with spring migration of susceptible avian species to regions in the North Atlantic, possibly Iceland and Greenland. Finally, mean change in precipitation at destinations of viral transitions along phylogenetic branches was predicted to reduce H5Nx HPAI virus spread, which may reflect the effect of climate change on declines in host species abundance or changes in migratory patterns as a result of ecological alterations. Our data provide new knowledge about the spread and directionality of H5Nx HPAI virus dispersal in Europe and North America during an actively evolving 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.Keywords: Influenza A virus, North Atlantic, wild birds, phylodynamic-GLM, outbreak, spatial spread

Authors: Jonathon D. Gass Jr. 1,†, Robert J. Dusek2, Jeffrey S. Hall2, Gunnar Thor Hallgrimsson3, Halldór Pálmar Halldórsson4, Solvi Runar Vignisson4,5, Sunna Bjork Ragnarsdottir6, 7, Jón Einar Jónsson8, Scott Krauss9, Sook-San Wong9,ŧ, Xiu-Feng Wan10, 11, 12, 13, Sadia Akter10, 11, 12, 13,*, Srinand Sreevatsan14, Nídia S. Trovão15, Felicia B. Nutter1, Jonathan A. Runstadler1, Nichola J. Hill1,16 1 Department of Infectious Disease and Global Health, Cummings School of Veterinary Medicine, Tufts University2 U.S. Geological Survey, National Wildlife Health Center3 Institute of Biology, University of Iceland 4 University of Iceland’s Research Centre in Suðurnes 5 Suðurnes Science and Learning Center6 Icelandic Institute of Natural History 7 The Southwest Iceland Nature Research Centre8 University of Iceland's Research Center at Snæfellsnes9 Department of Infectious Diseases, St. Jude Children’s Research Hospital 10 Center for Influenza and Emerging Infectious Diseases (CIEID), University of Missouri, Columbia11 Department of Molecular Microbiology and Immunology, University of Missouri, Columbia12 Bond Life Sciences Center, University of Missouri, Columbia13 Department of Electronic Engineering and Computer Science, University of Missouri, Columbia14 College of Veterinary Medicine, Michigan State University15 Division of International Epidemiology and Population Studies, Fogarty International Center, National Institutes of Health16 Department of Biology, University of Massachusetts, Boston† Corresponding author Corresponding author email: [email protected] ŧ Present address: State-Key Laboratory of Respiratory Diseases, Guangzhou Medical University and School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong * Present address: Department of Molecular Microbiology, Washington University School of Medicine in St. LouisAbstractInfluenza A viruses (IAV) circulate endemically among many wild aquatic bird populations that seasonally migrate between wintering grounds in southern latitudes to breeding ranges along the perimeter of the circumpolar arctic. Arctic and subarctic zones are hypothesized to serve as ecologic drivers of the intercontinental movement and reassortment of IAVs due to high densities of disparate populations of long distance migratory and native bird species present during breeding seasons. Iceland is a staging ground that connects the East Atlantic and North Atlantic American flyways, providing a unique study system for characterizing viral flow between eastern and western hemispheres. Using Bayesian phylodynamic analyses, we sought to evaluate the viral connectivity of Iceland to proximal regions and how inter-species transmission and reassortment dynamics in this region influence the geographic spread of low and highly pathogenic IAVs. Findings demonstrate that IAV movement in the arctic and subarctic reflects wild bird migration around the perimeter of the circumpolar north, favoring short-distance flights between proximal regions rather than long distance flights over the polar interior. Iceland connects virus movement between mainland Europe and North America, consistent with the westward migration of wild birds from mainland Europe to Northeastern Canada and Greenland. Though virus diffusion rates were similar among avian taxonomic groups in Iceland, gulls play an outsized role as sinks of IAVs from other avian hosts prior to onward migration. These data identify patterns of virus movement in northern latitudes and inform future surveillance strategies related to seasonal and emergent IAVs with potential public health concern. Keywords: Influenza A virus, Iceland, wild birds, phylodynamics, transmission, subarctic