Background: In 2021, the H5N1 clade 2.3.4.4b Avian Influenza Viruses (AIVs) emerged on the American continent. At the same time, a further global spread took place. Infections have been reported in avian species as well as in over 50 mammalian species in 26 countries, and often result in severe disease with notable neurological pathology. Outbreaks in dairy cattle in the United States in 2024 illustrate viral transmission at a non-traditional interface and cross-species transmission. This development raises significant global concern regarding the virus´s potential for wider spread. Given that H5N1 infections in birds reached record-high levels in Germany by late 2022, it is important to investigate whether Influenza A Virus (IAV) infections were also occurring in mammals sharing habitats with wild birds.

Methods and results: Selected wild and domestic mammal populations were monitored over a two-year period (from January 2022 to December 2023), which coincided with a major infection period in wild birds in Bavaria. Genomes of Highly Pathogenic Avian IAV H5N1 (clade 2.3.4.4b) were detected in red foxes but not in samples from ruminants such as red deer or domestic cattle. Analyses of viral whole genome sequences revealed several mutations associated with mammalian adaptation.

Conclusion: Our results indicate a high frequency of spillover events to red foxes at a time when there was a peak of H5N1 infections in wild birds in Bavaria. Phylogenetic analyses show no specifically close genetic relationship between viruses detected in mammalian predators within a geographic area. While direct fox-to-fox transmission has not yet been reported, the H5N1 clade 2.3.4.4b AIVs´ ability to spread through non-traditional interfaces and to cross species barriers underlines the importance of continuous IAV surveillance in mammals and possibly including previously unknown host species.