First detection of Usutu virus in wild birds in Denmark, 2024

In 2024, the first known infections by Usutu virus (USUV) in Denmark were detected in wild birds, primarily affecting the common blackbird (Turdus merula). USUV is a mosquito-borne virus that has been circulating in Europe for the last three decades. A total of 149 dead birds, including 85 blackbirds, were investigated for the presence of USUV. Using an USUV-specific RT-qPCR assay, viral RNA was detected in brain tissue from 56 of the 85 blackbirds. In addition, two great spotted woodpeckers (Dendrocopos major) and one red kite (Milvus milvus) were found positive, however with lower levels of viral RNA. Pathological examinations performed on the affected blackbirds were largely consistent with those observed during previous outbreaks of USUV in European blackbirds. Cloacal and tracheal swabs from USUV - positive blackbirds were also tested for USUV RNA and results corresponded qualitatively with those found in brain tissue. Additionally, liver and kidney samples from infected blackbirds revealed comparable levels of viral RNA to their corresponding brain tissue samples. Sequencing and phylogenetic analysis revealed that three distinct lineages (Europe 2, Europe 3 and Africa 3) were present in Denmark in 2024. These findings emphasize the importance of surveillance of emerging vector-borne viruses in Europe.