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

## TL;DR

This study finds that higher bird diversity in Italy is linked to lower West Nile Virus activity in mosquitoes, suggesting conservation could help control outbreaks.

## Contribution

The study provides the first empirical evidence supporting the dilution effect hypothesis for West Nile Virus in Europe.

## Key findings

- Higher avian biodiversity is associated with reduced West Nile Virus activity in Culex mosquitoes.
- Non-passerine bird species richness is linked to lower transmission risk, while passerine species richness is associated with higher risk.
- Findings support the dilution effect hypothesis in a European WNV hotspot.

## Abstract

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 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 detection frequency. 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 transmission risk quantified by vector index (VI) at 68 geolocated mosquito traps across the region.

We observed consistent negative associations between WNV detection frequency 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 VI while passerine species richness showed a positive association after adjusting for covariates and spatial random effects. These findings suggest that passerines may amplify WNV transmission, whereas 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 biodiversity conservation and integrated public health surveillance activities across Europe.

West Nile Virus (WNV) circulates primarily between Culex mosquitoes and birds, yet the association between avian biodiversity and WNV transmission remains debated. Focusing on Emilia-Romagna, Italy, one of Europe’s WNV hotspots, we analysed 11 years (2013–2023) of entomological surveillance data alongside two avian datasets: bird census data from the Farmland Bird Index project and abundance estimates from the citizen-science project eBird. These datasets allowed us to test the dilution effect hypothesis (DEH) at both broad and fine spatiotemporal resolutions. Our findings support the DEH for WNV, showing that higher avian biodiversity is associated with reduced WNV activity. Additionally, we found that non-passerine species richness is linked to lower WNV transmission risk, while passerine species richness is associated with higher risk. These results point towards the potential amplifying role of passerine species and the protective role of non-passerine species in WNV transmission dynamics, with implications for species conservation, public health practice, and future modelling studies.