HPAI spread driven by poultry farm density and proximity
The article published in Communications Biology (Guinat et al., 2025) examines the dynamics of highly pathogenic avian influenza (HPAI) H5N8 spread in southwestern France during 2020–2021, using phylogeographic and phylodynamic tools to analyze viral genetic sequences from numerous outbreaks. The study focuses on a region with exceptionally high poultry farm density, particularly duck farms, which acted as the main driver of viral dissemination. The analysis showed that a single introduction of the virus into the area resulted in extensive spread, with an average velocity of 10.5 km per week and a peak of 27.8 km per week in December 2020. Farm infectiousness was estimated at about nine days, highlighting a critical silent period of viral shedding before detection. The effective reproduction number (Re) between farms peaked at 3.8, significantly higher than in previous epidemic waves. Duck farm density and farm proximity were identified as the principal determinants of transmission, whereas greater inter-farm distances effectively reduced the risk. The study further demonstrated that standard 3- and 10-km surveillance zones alone were insufficient to halt spread during periods of high transmission, but their effectiveness improved substantially when combined with extensive preventive depopulation.
