Mass Poultry Culling and Usutu Virus Emergence An Open Letter Identifying Unmodelled Environmental Drivers in de Wit et al. (2026)

This open letter responds to de Wit et al. (2026), "Silent reservoir species are shaping the emergence of Usutu virus" (Nature Ecology & Evolution, doi: 10.1038/s41559-025-02973-4). It identifies a critical anthropogenic variable omitted from the published transmission model: the environmental consequences of mass poultry culling and disposal in the same geographic corridors where Usutu virus (USUV) emergence was subsequently observed. Using geotemporal-spatial overlay mapping (https://outbreak.mosaeq.com), the letter documents geographic co-location between HPAI cull zones, carcass disposal routes, and wild bird USUV mortality across three independent outbreak events: the Netherlands (2016), Italy/Po Valley (2017–2018), and eastern Germany (2018). It proposes that the exceptional Culex pipiens abundance in 2016—which the de Wit et al. model identifies as the principal outbreak driver but attributes to climate—was instead driven by anthropogenic eutrophication from mass organic disposal of culled poultry into standing water and shallow groundwater. The letter further identifies the absence of standard agricultural runoff indicators (ammonia, nitrate, biological oxygen demand, hydrogen sulfide) from all published epidemiological models for these outbreaks, and raises the potential role of environmental immunosuppression in inflating estimated USUV pathogenicity. It does not dispute mosquito-borne flavivirus transmission but argues that the model omits a critical upstream driver of vector abundance. The letter concludes with policy recommendations including a moratorium on mass CO2 culling in affected corridors, retrospective environmental characterisation of disposal sites, and the inclusion of environmental disease pathway expertise on EFSA advisory panels evaluating avian influenza control strategies.