Surveillance of West Nile and Usutu virus in birds in Luxembourg for animal and public health

Although the burden of mosquito-borne diseases has traditionally been high in (sub-)tropical regions, it is expected to also increase in Europe in the future. West Nile (WNV) and Usutu (USUV) viruses are already enzootic in certain European regions, with widening geographic repartition. The two viruses share similar epidemiological characteristics such as bird-to-bird transmission through mosquito (mainly Culex pipiens) bites. Both viruses may also be transmitted to mammals, including humans, leading to a wide range of symptoms. Therefore, WNV and USUV surveillance in birds – shown to be cost-effective and timely to provide early warning signal for increased risk of transmission to human - was initiated in Luxembourg in 2018. The first USUV case was detected in 2020 in the capital [1]. Starting in 2024, the surveillance activities were intensified in the framework of the EU project “OneHealth4Surveillance” by screening systematically a wider range of bird species. While WNV has not yet been detected in Luxembourg, USUV was detected again in several birds during the summer and fall of 2024 and in the summer of 2025. Phylogenetic analyses based on whole genomes suggest that the 2024-2025 strains do not originate from a new virus introduction but that USUV was rather maintained in the region since 2020. This finding, together with the overlapping host and vector range of WNV and USUV, suggests that local conditions would likely also allow the maintenance of WNV in the area once introduced, with wider consequences for public and animal health.