Epidemiological and Virological investigation of high pathogenicity avian influenza outbreaks in Danish poultry during 2020/2021 and 2021/2022

Incursions and persistence of high pathogenicity avian influenza viruses (HPAIVs) in Europe have resulted in devastating epidemiological seasons in 2020/2021 and 2021/2022 compared to previous seasons. HPAIVs were confirmed in 27 outbreaks in poultry and other captive birds, and in 553 wild birds throughout Denmark. Our data revealed that the viruses detected in Denmark were highly similar to contemporary HPAIVs in other European countries in both seasons. The HPAIVs detected in Denmark belonged to clade 2.3.4.4b and were of different subtypes (H5N1, H5N3, H5N5, and H5N8). Several viruses identified in 2021/2022 were found to be highly similar to those found in the previous season, whereas others were identified as novel viruses. The genetic data combined with epidemiological information on the outbreaks in Denmark were used to explore the most likely route of introduction into each infected poultry production site. In a few cases, the data supported that the outbreaks were caused by between-farm HPAIV transmission. However, the majority of outbreaks in poultry are believed to have been a result of direct or indirect contact with HPAIV-infected wild birds. With a logistic regression model, it was determined that the only significant risk factor for experiencing an outbreak was the poultry production type. Turkey and mallard farms had a significantly higher risk of experiencing an outbreak in comparison to chicken farms. These findings prompt reconsideration of AIV preventative measures limiting HPAIV outbreaks, not only at the wild bird-poultry interface but also due to secondary transmissions between poultry farms.