Abstract
Migratory waterfowl are widely acknowledged as the primary source of avian influenza (AI) virus introduction into poultry. However, recent studies have challenged the assumption of direct interactions between waterfowl and poultry, revealing a more complex interface (Shriner&Root, 2020; Verhagen et al., 2021). While waterfowl are crucial for maintaining and amplifying AI viruses in nature, other species may function as the actual bridges for introducing AI into the poultry sector (Caron et al., 2015). However, the nature of this wild-domestic interface remains elusive, leaving gaps in understanding which species play the bridging role.
An ornithocoenosis study was conducted in northern Italy between January and December 2019, in the area with the highest occurrence of Highly Pathogenic AI (HPAI) outbreaks during the 2017-2018 H5N8 epidemic. The study focused on ten poultry farms: five fattening turkey, four laying hen, and one duck premises. Three census transects were established within one kilometre from each farm, and visited biweekly, while two camera-traps per holding were installed adjacent to the sheds. A Bayesian Adaptive Regression Trees (BART) approach was adopted to define a series of Species Distribution Models (SDM) to predict the favourability of species detected by camera-trapping. These results were then input as geographical raster layers into further BART-SDMs to produce risk maps for AI and determine which wild bird species significantly contributed to the occurrence of the 2017-2018 H5N8 HPAI outbreaks in northern Italy.
The transect survey revealed a marked diversity despite the environmental and climatic homogeneity, with approximately 150 species detected. However, only 41 of these were also observed by camera-trapping in proximity to the farms, suggesting a fragmented community. The likelihood of observing an AI outbreak resulted positively associated with the presence of Ardea alba, Bubulcus ibis, Columba livia and Phasianus colchicus, and negatively with the Falco peregrinus, Phoenicurus phoenicurus and Picus viridis. In particular, the contribution of pheasants to the probability of observing AI outbreaks was likely due to the release of birds for repopulation or hunting, which might create bridges between wild and domestic populations. The tendency of released game birds to seek out anthropized areas, such as poultry farms, in search of food, further corroborates the hypothesis.
The study shed light on the intricate interface between wild birds and poultry in the dynamics of AI spread. Our results stressed the need to account for the role of both aquatic and non-aquatic bird species in the introduction and/or inter-farm spread of AIVs, emphasising the potential risks associated with wetland proximity and the impact of human activities, such as releasing game birds, on the risk of disease spread (Shriner&Root, 2020). These findings may be used to inform targeted surveillance activities and prevention strategies to mitigate the AI threat in domestic birds.
References
Caron, A., Cappelle, J., Cumming, G. S., de Garine-Wichatitsky, M., & Gaidet, N. (2015). Bridge hosts, a missing link for disease ecology in multi-host systems. Veterinary Research, 46(1), 83. https://doi.org/10.1186/s13567-015-0217-9
Shriner, S. A., & Root, J. J. (2020). A Review of Avian Influenza A Virus Associations in Synanthropic Birds. Viruses, 12(11), 1209. https://doi.org/10.3390/v12111209
Verhagen, J. H., Fouchier, R. A. M., & Lewis, N. (2021). Highly Pathogenic Avian Influenza Viruses at the Wild–Domestic Bird Interface in Europe: Future Directions for Research and Surveillance. Viruses, 13(2), 212. https://doi.org/10.3390/v13020212