Post-outbreak surveillance reveals contrasting circulation dynamics of West Nile, Usutu and Bagaza viruses in southern Portugal, 2021 - 2023

In 2021-2023, a detailed investigation into mosquito-borne orthoflaviviruses in southern Portugal unveiled distinct circulation patterns for West Nile (WNV), Usutu (USUV), and Bagaza (BAGV) viruses. The research, conducted through active surveillance of birds and mosquitoes using serological and molecular methods, alongside viral genome sequencing and phylogenomic analysis, revealed varying prevalence rates of these viruses.

WNV, a major concern due to its sustained presence, was found in 1.5% of birds and Culex univittatus mosquitoes, with a seroprevalence of 28.5% in birds, indicating ongoing local circulation. USUV, detected in Alectoris rufa in 2021 and 2023, exhibited low seroprevalence (1.3%) during the outbreak years, suggesting intermittent circulation.

BAGV, meanwhile, was only found during the 2021 outbreak in Alectoris rufa and Emberiza calandra, with BAGV-specific antibodies persisting for up to two years post-outbreak, indicating potential continued exposure or low-level circulation.

Overall, the study highlights the need for enhanced avian and human surveillance in Portugal to address the sustained threat from WNV, while also underscoring the importance of long-term, integrated One Health surveillance to improve early warning and preparedness for emerging mosquito-borne diseases.