Avian Influenza: How Can We Predict Future Outbreaks?

In Europe, outbreaks of highly pathogenic avian influenza among wild birds and farmed poultry are, unfortunately, no longer rare. An epidemiological, ecological, and evolutionary overview of H5 viruses allows researchers to better guide their future research and the surveillance of the influenza viruses responsible for these outbreaks.   Over the past fifteen years, outbreaks of highly pathogenic avian influenza (HPAI) have become increasingly frequent in Europe. In particular, outbreaks caused by H5 viruses derived from the A/Goose/Guangdong/1/1996 lineage—which emerged in Southeast Asia in 1996—are rampant. Between 2005 and 2020, at least ten outbreaks of H5 HPAI were identified in Europe, resulting in massive mortality among wild birds and domestic birds. Initially, and until 2009, H5N1 viruses of clade 2.2 were particularly prevalent, whereas starting in 2014, H5 viruses of clade 2.3.4.4 have dominated, with numerous genetic reassortments giving rise to the subtypes H5N1, H5N2, H5N3, H5N4, H5N5, H5N6, and H5N8. The majority of these outbreaks coincide with fall avian migration (southwest/west) and large concentrations of waterfowl during the wintering season. In fact, wild birds belonging to the orders Anseriformes (primarily ducks, geese, and swans) and Charadriiformes (gulls, terns, and shorebirds) serve as the natural reservoir for avian influenza viruses. From these birds, influenza viruses can be transmitted directly or indirectly to domestic birds, as well as to other wild or domestic animals and to humans. But the reverse is also true. Viral evolution and epidemiology result from interactions between viruses, their hosts, and their environment. The wild-domestic interface is particularly conducive to the shift of influenza from low pathogenic to highly pathogenic forms. Recurring incursions of the highly pathogenic H5 virus raise questions about the mechanisms behind this new emergence, particularly regarding its origin and the routes of its introduction into Europe and onto poultry farms. Significant progress in characterizing the evolution and epidemiology of the H5 HPAI virus (particularly in terms of identifying host species, infection periods, habitats, and geographic areas associated with an increased risk of introduction) have been made possible through collaborations between virologists, ornithologists, ecologists, pathologists, and mathematicians. Together, they have developed faster and more specific diagnostic tests based on genetic sequencing technologies. These advances have revealed a high degree of viral genetic diversity during the various outbreaks that have occurred since 2005. This variability, combined with differing susceptibility to viral strains, facilitates extensive reassortment of genomic segments and the emergence of new variants. Not to mention the massive growth of the global poultry industry, which has only increased viral circulation and, consequently, the impact of these outbreaks. Outbreaks of H5 HPAI must therefore be closely monitored in the coming years, as soon as they appear in Europe. Researchers recommend integrating passive surveillance of wild birds and farmed birds, in addition to active surveillance of wild birds, which aims to detect the virus in target species at priority sites along migratory routes. Today, the integration of avian ecology into the detection and surveillance parameters for new H5 viruses is inevitable. Long-term surveillance programs will provide a better understanding of the ecology, prevalence, and diversity of viruses, enabling more accurate predictions of viral outbreaks over time and space. Ultimately, this is the fastest way to address the global “One Health” challenge posed by avian influenza. We need to better understand the ecology and evolution of avian influenza, while effectively controlling the disease in poultry to support safe and sustainable food production, which in turn would reduce both the threat to wildlife and the long-term zoonotic risk to humans.