Between-species contagion drives HPAI transmission and mass mortality in seabirds.

The unprecedented avian panzootic caused by High-Pathogenicity Avian Influenza (HPAI) caused widespread mortality in seabird populations, but the exact scales and patterns of transmission, basic epidemiological parameters and mortality outcomes are unknown. Mass mortalities combined with potentially detrimental sub-lethal effects of exposure will have currently unpredictable consequences for population viability of affected wild bird species, many of which are already subjected to acute anthropogenic pressures. Here, we focus on one of the most severely affected seabird species, the Northern gannet (Morus bassanus). Using a metapopulation epidemiological framework, fitted to counts of dead gannets in or near gannet breeding colonies across the NorthEast Atlantic, we estimate key epidemiological parameters, quantify spatial patterns of mortality and isolate external transmission routes. Our epidemiological parameter estimates suggest that the 2022 strains of HPAI were highly virulent and transmissible over wide geographical ranges, within the gannet network and beyond. Within gannet colonies, we found evidence that transmission rates may have declined as mortality reduced local breeding density. Inter-colony spread in the network declined with distance, and epidemiological connectivity had an estimated half-distance of 5 km. Large-scale outbreak initiation was best explained by external infection sources, most likely, other seabird species, with an estimated half-distance of 66 km. Our modelling suggests that these multi-scale, multispecies processes resulted in 28% (95% CI: 21%-34%) HPAI-mortality across the Northeast Atlantic gannet breeding metapopulation, in addition to normal levels of annual mortality (5%-10%). Our study demonstrates that if analysed holistically, carcass data can provide epidemiological insights but also act as an early warning system for continent-wide patterns of spread. For pandemic preparedness, it is essential that data-integrative approaches such as ours are placed in the mainstream toolbox of planetary health monitoring. By disentangling intra- and inter-specific transmission, our approach highlights the need to integrate colonial species into global avian influenza surveillance and response.