Circulation of West Nile virus and Usutu virus in birds in Germany, 2021 and 2022.

Background: Usutu virus (USUV) and West Nile virus (WNV) are zoonotic arthropod-borne orthoflaviviruses. The enzootic transmission cycles of both include Culex mosquitoes as vectors and birds as amplifying hosts. For more than 10 years, these viruses have been monitored in birds in Germany by a multidisciplinary network. While USUV is present nationwide, WNV used to be restricted to the central-east.
Methods: In 2021 and 2022, over 2300 live bird blood samples and organs from over 3000 deceased birds were subjected to molecular and serological analysis regarding the presence of WNV and USUV. The samples were collected at sites all over Germany.
Results: Circulation of both viruses increased in 2022. For USUV, the nationwide presence of lineages Africa 3 and Europe 3 reported in previous years was confirmed. Lineage Europe 2, formerly restricted to the German east, was able to expand westward. Nonetheless, USUV neutralizing antibody (nAb) detection rates remained low (< 9%). Years 2021 and 2022 were characterized by stable enzootic circulation of WNV lineage 2, dominated by one previously identified subcluster (95% of generated sequences). In 2022, >20% of birds in the endemic region in eastern Germany carried nAb against WNV. Serological data also indicate expanding WNV circulation west and south of the known hotspots in Germany.
Conclusions: USUV circulates enzootically nationwide. Emergence of WNV at several new locations in Germany with a potential increase in human infections may be imminent. In this context, wild bird monitoring serves as a capable early warning system in a One Health setting.