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 (Shannons, Simpsons, 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. Author summaryWest Nile Virus (WNV) is a zoonotic, vector-borne disease maintained in a mosquito-bird-mosquito transmission cycle. The role of avian biodiversity in WNV transmission remains debated, with studies reporting both dilution effects (negative associations) and amplification effects (positive associations) across different settings. Here, we examined the impact of avian biodiversity on WNV circulation in Culex mosquitoes in Emilia-Romagna, Italy, one of Europes WNV hotspots. We analysed 11 years (2013-2023) of entomological surveillance data alongside two ornithological datasets, respectively collected by the Farmland Bird Index project and the citizen science project eBird. These different datasets allowed us to test the dilution effect hypothesis (DEH) at broad and fine spatiotemporal resolutions. Our findings provide support for the DEH for WNV, revealing that higher bird community diversity is associated with reduced WNV circulation. Additionally, we found that non-passerine species richness is linked to lower levels of WNV circulation in the vector, whereas passerine species richness is positively associated with WNV transmission from entomological surveillance. These results point towards the potential amplifying role of passerine species and the protective role of non-passerine species in WNV transmission dynamics, which has important implications for species conservation, public health policy and future modelling studies.