Characterization of practices influencing avian influenza virus transmission risk in live bird markets in Northern Vietnam.

Live bird markets (LBMs) play a central role in the transmission and maintenance of avian influenza viruses (AIVs) in Vietnam, yet the relevance of trading practices for discriminating LBMs according to their AIV transmission risk remains poorly understood. We characterised LBM sellers in northern Vietnam according to trading practices likely to influence AIV introduction into LBMs and subsequent transmission, and assessed whether the resulting typology could stratify LBMs according to AIV prevalence. A cross-sectional study was conducted in 27 LBMs across four provinces of northern Vietnam. LBM-level observations and structured interviews with LBM managers and poultry sellers were used to describe practices. Factor analysis for mixed data and hierarchical clustering identified LBM- and seller-level typologies. Differences in avian influenza H9N2 prevalence in chickens across LBM typologies were evaluated using zero-inflated Bayesian models. Distinct LBM types (periodic small-scale, daily medium-scale, and daily large-scale) and seller types (small-scale high-surplus sellers, medium-scale farmer-supplied sellers, and large-scale low-surplus wholesalers) were identified. Small-scale, high-surplus sellers were considered at higher risk of promoting AIV circulation due to frequent sourcing from traders, high proportions of unsold birds, and prolonged poultry stay. LBMs hosting such sellers showed the highest H9N2 prevalence, whereas LBMs dominated by large-scale, high-turnover sellers had the lowest prevalence. Seller composition provided a stronger discrimination of H9N2 prevalence than market-level characteristics. These findings highlight the importance of poultry trading practices in shaping AIV circulation within LBMs and suggest that accounting for seller composition of LBMs may improve the effectiveness of surveillance and control interventions aimed at reducing zoonotic risk.