Poultry farm density and proximity drive highly pathogenic avian influenza spread

The continuous spread of highly pathogenic avian influenza H5 viruses poses significant challenges, particularly in regions with high poultry farm densities where conventional control measures are less effective. Using phylogeographic and phylodynamic tools, we analysed virus spread in southwestern France in 2020 and 2021, a region with recurrent outbreaks. Following a single introduction, the virus spread regionally, mostly affecting duck farms, with an average velocity of 10.5 km/week and a peak at 27.8 km/week in mid-December. The effective reproduction number between farms (R_e) exceeded 1 in December, peaking at 3.8 in mid-December. Transmission declined after late December, coinciding with extensive preventive culling, suggesting that standard 3- and 10-km zones were more effective when combined with timely, large-scale interventions. Farm infectiousness was estimated around 9 days, highlighting a critical window of undetected viral shedding and transmission risk. Duck farm density and poultry farm proximity were key drivers of virus spread, likely due to the higher virus susceptibility and transmission efficiency of ducks compared to chickens. We identified density and proximity thresholds required to maintain effective control (R_e < 1). These findings offer actionable guidance to support regional biosecurity and to improve the robustness of the poultry sector to mitigate future outbreaks.