Surveillance of Highly Pathogenic Avian Influenza in Wild Birds in France, 2016-22: What Did We Learn from Mortality...

Highly pathogenic avian influenza (HPAI) poses major conservation issues worldwide. In France, recurrent outbreaks of HPAI (H5) in wild birds have occurred since 2020, yet our understanding of the disease's dynamics have remained limited. By leveraging data from the national wildlife health surveillance network (SAGIR), we conducted a spatiotemporal analysis of the HPAI outbreaks in wild birds. Between 2016 and 2022, two different spatiotemporal patterns of the disease were observed in France: sporadic episodes of the virus in four episodes, forming either isolated cases or self-limited clusters at the maximum and epizootic circulation in 2022. During sporadic circulation episodes, observations were concentrated in well-defined spatiotemporal clusters with low prevalence. Those self-limited clusters, places where the density of positive events was substantially larger than in the rest of France, reflected three epidemiologic patterns: 1) recurrent clusters linked to migration and waterfowl habitats; 2) clusters involving synanthropic species in diverse areas and related to outbreaks in poultry farms; and 3) outbreaks in colonial bird species, observed once in 2020 and once in 2022, involving Red Knots (Calidris canutus) and Eurasian Griffon Vultures (Gyps fulvus), respectively. Beginning in summer 2022, the epizootic episode, characterized by a high prevalence along the northern French coasts, involved Northern Gannets (Morus bassanus) and Laridae. During this epizootic, cluster boundaries were far less well-defined. The ecology of the affected bird species and the characteristics of the circulating viral strains (often adapted to these species) may explain this new spatiotemporal dynamic compared with previously observed sporadic circulation, driven mainly through migration. Our study provides a better understanding of the dynamics of HPAI outbreaks in wild birds; nevertheless, knowledge gaps remain, and improved surveillance of HPAI in wild birds is still needed.