Evidence of exposure to West Nile virus and Usutu virus in migratory birds in Sweden

Abstract

Objectives

The recent detection and expansion of West Nile virus (WNV) and Usutu virus (USUV) in the Netherlands, Germany, and Austria point to the likelihood of the viruses spreading to Northern Europe. Migratory birds and ornithophilic mosquitoes may spread these viruses to new areas. We sampled birds during the spring and autumn bird migration of 2021 in Southern Sweden to investigate the risk of the introduction of mosquito-borne zoonotic avian viruses like WNV and USUV.

Methods

We collected blood samples from 1775 birds comprising 59 species and determined the seroprevalence of WNV using a competitive enzyme-linked immunosorbent assay (ELISA). WNV and USUV belong to the Japanese encephalitis serocomplex, and antibodies against both viruses are detected in the WNV ELISA. Focus-forming assays or fluorescence-based neutralization assays were performed to verify ELISA results and to differentiate between antibodies against WNV and USUV.

Results

We found nine (0.51%) samples to be WNV-antibody-positive. Cross-neutralization experiments with WNV and USUV confirmed that seven (0.41%) had WNV-neutralizing antibodies and two (0.11%) had USUV-neutralizing antibodies. Interestingly, the two samples had neutralizing antibodies of both viruses. All samples but one with anti-flavivirus antibodies came from long-distance migrants wintering in sub-Saharan Africa. Antibodies were detected in samples taken during spring and autumn and only in adult birds.

Conclusion

The findings show that migratory birds in Sweden have been exposed to WNV and USUV.