Association of avian biodiversity and West Nile virus circulation in Culex mosquitoes in Emilia-Romagna, Italy

West Nile Virus (WNV) is a zoonotic arbovirus maintained in a transmission cycle between Culex mosquitoes and birds, occasionally spilling over into humans. The impact of avian biodiversity on WNV circulation remains debated, with studies reporting both negative and positive correlations (dilution and amplification effects respectively) across different settings. In Europe, this relationship remains largely unexplored, particularly in regions with high WNV transmission, such as Emilia-Romagna in Northern Italy. We explored the association between avian biodiversity and WNV circulation in Culex mosquitoes in Emilia-Romagna using 11 consecutive years (2013–2023) of entomological surveillance data paired with two avian data sources. We calculated avian biodiversity indices (Shannon’s, Simpson’s, and Chao2) from observation records from the Farmland Bird Index project and applied linear regression models to assess their relationship with WNV circulation levels. Moreover, we used Bayesian spatiotemporal regression models and gridded weekly avian abundance estimates from the eBird project, to analyse the associations between avian species richness indices and WNV-positivity rates at 68 geolocated mosquito traps across the region. We observed consistent negative associations between WNV circulation in the Culex population and avian biodiversity indices, supporting the dilution effect hypothesis (DEH). We found that non-passerine species richness was negatively associated with WNV mosquito circulation, whereas passerine species richness exhibited a positive association after adjusting for environmental factors and spatial random effects. These findings suggest that passerines may amplify WNV transmission, while the presence of non-passerine species is associated with reductions in WNV circulation. This study provides the first empirical evidence supporting the DEH for WNV in Europe. These findings have important implications for avian surveillance and future WNV modelling studies, and can inform regional, national and European policies on biodiversity conservation and public health planning.