Risk factors for the incursion of highly pathogenic avian influenza virus into poultry and other captive bird holdings in Denmark from 2020 to 2023: A case-control study

Highly pathogenic avian influenza (HPAI) is a major concern in terms of animal and human health. Between October 2020 and September 2023, there were 36 HPAI outbreaks detected in poultry and other captive birds in Denmark. However, it is often not possible to determine the exact route of introduction. We conducted a case-control study to compare the odds of exposure to a range of potential risk factors for HPAI virus incursion into Danish poultry or other captive bird holdings with HPAI outbreaks (cases) and with no HPAI outbreaks (controls) during the HPAI epidemiological seasons 2020/2021, 2021/2022 and 2022/2023. The owners of 38 % of the eligible case holdings and 45 % of the eligible control holdings declined to participate, suggesting that HPAI virus infection is a sensitive subject for some owners. The study population included 18 cases and 34 controls. We collected data primarily through questionnaire-based interviews and estimated odds ratios (OR) within a Bayesian framework using a Beta conjugate prior to define the odds directly, with Monte Carlo integration from these posterior distributions of odds to estimate the relevant OR with 95 % credible intervals (CI) and Bayesian p-values. The results indicated that the odds of observing wild waterfowl or gulls on the roof or around farm buildings compared to observing none within 500 m of the holding was higher for case holdings (OR: 18.7, 95 % CI: 3.1–148, p: 0.022). This information can be used for future risk-based monitoring, biosecurity management and placement of captive bird holdings.