Modelling spillover risk of highly pathogenic avian influenza from wild birds to poultry in Denmark.

Spillover risks of contagious diseases affecting both wildlife and farm animals are of growing concern. Since late 2020, several waves of highly pathogenic avian influenza virus (HPAIV) clade 2.3.4.4b have been observed in poultry farms in Denmark, impacting the poultry industry. Concurrently, many wild birds have been found infected and dead of this disease. As the epidemic developed, 49 farm outbreaks and over 2000 infected wild birds were detected in Denmark from late 2020-2024. Thus, it is important to investigate the interactions between wild birds and poultry farms and identify the spillover risks of HPAIV from free-living wild birds to domestic poultry. Using a previously developed mechanistic model for HPAIV in Danish wild birds and statistical tests (including correlation tests and logistic regressions), we quantified this spillover risk in Denmark in space and time. The mechanistic model identified a constant hot spot of HPAIV spillover risk in southern Zealand which corresponds to the hot spot previously identified using reported poultry outbreaks. The model estimated farm-specific risks with a precision level of 78 % for the simulated period. The population changes of the barnacle goose (Branta leucopsis) were found to be significantly associated with poultry outbreaks. A 3-week time lag across Denmark between wild bird HPAIV detections and subsequent poultry outbreaks was discovered, together with regional differences of this time lag. Our study showed the relationships between wild bird populations/detections and poultry outbreaks and the simulated weekly risk maps can help decision-making for prevention and surveillance.