Third, we identified a new set of significant risk factors that help to refine the current understanding of the HPAI H5N1 epidemic in Thailand. Previous studies have suggested that broiler and layer ducks do not constitute a risk factor for HPAI risk in Thailand [8, 9] . In contrast, we found a significantly increased risk of HPAI, both in chicken and duck flocks, in areas with a high density of broiler and layer ducks. During the period studied, only a quarter of the total number of broiler and layer ducks were raised in closed facilities with high biosecurity systems [20] . Since it has been proven that farm duck breeds can shed the H5N1 virus with minimal clinical signs [11] , our results suggest that farm ducks may also have played the role of silent carriers during the second wave of the epidemic, contributing to the spread of the disease. In addition, an increased risk in duck flocks was shown for subdistricts with a high density of broiler and layer chickens (> 500 chicken/km 2 ). In Thailand, broiler and layer chicken production range from large-scale industrial farms to small, family-run operations [24] . The latter refer to small or medium-scale businesses with links to several middlemen or companies for the transformation and transportation of both farm inputs and outputs (feed, wastes, poultry products. . .) [23] . During the second wave of the epidemic, it is possible that biosecurity rules were not applied fully throughout these complex poultry production chains, thus resulting in the spread of H5N1 in subdistricts with a high density of broiler and layer chickens.