Influenza from a One Health Perspective: Infection by a Highly Versatile Virus

Abstract

Influenza A viruses (IAVs) are among the most versatile viruses, in terms of host range, pathogenesis, route of transmission, transmissibility efficiency, and evolutionary dynamics. Several IAVs are recognized pathogens of a wide range of avian and mammalian species. On the one hand, AIVs that evolved in wild water birds, their natural reservoirs, cause mild or asymptomatic infections of their intestinal tract, resulting in annual epidemic cycles fueled by IAV fecal-oral transmission in watershed habitats. On the other hand, they may cause disease manifestations in avian and mammalian species, including humans, upon sporadic cross-species transmission—including spill-back transmission into wild birds—and during self-limiting outbreaks or large-scale airborne epidemics. In these spillover host species, clinical signs and symptoms range from inapparent to severe and often fatal respiratory and extra-respiratory conditions. A feature of IAVs in animal spillover hosts including humans is their ability to adapt to these new species to eventually be maintained independently of new introductions from their natural host reservoirs. In humans, IAVs of animal origin can be the precursors of pandemic influenza viruses. These pandemic viruses eventually evolve into seasonal influenza viruses that cause recurring epidemics of seasonal influenza. The unprecedented diversification and spread of highly pathogenic avian influenza viruses of the H5N1 and other H5Nx subtypes in wild birds in and across Asia, North America, Europe, and Africa, with spillover into wild mammals in Europe, over the past few years mark a worrying paradigm change in influenza epidemiology. Through their versatile nature, IAVs are a striking example of the flexible and ever-evolving nature of zoonotic threats and of the richness of avenues zoonotic pathogens can take to burden animal and public health.