West Nile virus in Portugal

Background: West Nile virus (WNV) is a mosquito-borne pathogen with a globally expanding geographical range. Many European countries regularly report outbreaks affecting both human and equine populations, with an increase in epidemic activity in the past decade. Portugal, on the far west of Europe, has not yet experienced large outbreaks of WNV, despite its suitable Mediterranean climate. Objectives: To provide a comprehensive, data-driven perspective on the past epidemiology, surveillance and transmission suitability of WNV in Portugal. Methods: Literature and database reviews were performed, covering the period between 1966 and 2020 on existing evidence for WNV circulation. Climatic data was used to estimate the theoretical transmission potential of WNV, and to analyse long-term climate trends between 1981 and 2019. Results: Existing data show that WNV-capable mosquito species are spread across the country, while WNV molecular and serological evidence is vastly concentrated in the south. Our analyses suggest that two different climate types, characteristic of the north and south of Portugal, contribute to quantitative differences in WNV theoretical transmission potential. This supports the north-south divide observed from previous molecular and serological evidence. Due to long-term changes in local climate, suitability for WNV transmission is shown to have been slowly increasing in the country. Discussion: Data and theoretical analyses support the notion that the south of Portugal is more suitable for WNV transmission, while not rejecting the possibility of transmission in the north. Mosquito monitoring has been effective, but surveillance of key sentinel species remains passive, creating important knowledge gaps for epidemic control and public health. To date, evidence supports WNV circulation, but it remains unclear whether it is endemic or sporadically transmitted. This study supports a timely change towards WNV active surveillance, to avoid and mitigate potential public health emergencies as those recently observed in other European countries.