Assessing the impact of outdoor farming, farm size, and farm density on highly pathogenic avian influenza epidemics: A modelling study in the Netherlands.

Poultry farming in the Netherlands is moving toward more extensive systems to promote animal welfare, a shift that has led to the rising number of organic and free-range farms, which provide poultry with outdoor access, lower maximum stocking densities, and limited flock sizes. However, the effect of these changes on the risk of infectious diseases for poultry remains unclear. In this study, we assessed the effects of these changes on the risk of highly pathogenic avian influenza (HPAI) introduction from the environment into poultry farms and HPAI spread between farms. We focused on the changes in farm production system in broiler and layer farms, which are the majority of the poultry population in the Netherlands. The increasing number of outdoor farms increased the introduction risk of HPAI and HPAI epidemic size (number of affected farms), with outdoor layer farms contributing more substantially to the risk than outdoor broiler farms due to the higher susceptibility of layer farms. Spatial clustering of outdoor farms sharply increased the epidemic size. Farm size reduction and decreasing the number of farms reduced the epidemic sizes. Although outdoor farming practices can promote better animal welfare, they potentially increase risk of HPAI introduction and spread. However, this risk can be mitigated by preventing clusters of outdoor farms, reducing farm size, or decreasing the overall number of farms.