The complexity of protecting poultry from HPAIV: Findings from a case-control study in Sweden.

High pathogenicity avian influenza (HPAI) has caused severe disruptions to poultry production worldwide and in Sweden in the last decade. During 2020 and 2021, 18 outbreaks were confirmed in poultry due to viral introduction from wild birds. The aim of the study was to investigate farm-level risk factors for HPAI in Swedish poultry, to guide prevention measures. A case-control study based on 15 cases from 2020 and 2021 and 33 controls, representing four poultry categories and three counties in southern Sweden was conducted. Data were collected through interviews with farmers/ farmworkers and on-farm observations using a questionnaire and Biocheck.UGent™. Predictor variables were selected based on a hypothesised causal diagram and included in a case-control model using stability selection based on three regularisation models. In addition, a cross-sectional analysis was done based on poultry registry data, and six years of outbreak data. The case-control results showed an association between HPAI outbreaks and presence of waterfowl near the poultry farms. A higher number of birds per farm was positively associated with outbreaks in both parts of the study. Poultry category was associated with HPAI status where turkeys had higher risk and broilers lower risk of outbreaks in the cross-sectional study. The results didn't show a significant association between biosecurity score or other biosecurity covariates, but trends were identified that may deserve further research.