Abstract
In humans, influenza A viruses cause yearly outbreaks with high morbidity and excess fatality rates as a direct effect. Placed in its ecological niche, however - in dabbling ducks - avian influenza virus (AIV) induces quite a mild disease. It is when the virus crosses the species barrier that pathogenic traits are attributed to infection. When infecting phylogenetically more distant species (i.e. chicken and turkeys), the AIV can cause high morbidity and may in some cases change the virus into a highly pathogenic variant with nearly 100% fatality rate. Being a very adaptable virus, these spill-over events are frequent and numerous species are susceptible to influenza virus. When a subtype of AIV that has not previously infected humans crosses the species barrier, adapts to humans, and spreads easily, a pandemic event is imminent. There is no cure for influenza infection and vaccination is a cumbersome endeavor so, currently, the strategy when a pandemic strikes is damage control. The interest in AIV ecology has increased dramatically since the beginning of the millennium as a key factor for preventive work for future pandemics. This review gives a broad overview of influenza A virus ecology: in the natural host, accidental hosts, new endemic hosts, and humans.