Influenza A viruses: an ecology review

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.

Keywords: ecology; host range; influenza; influenza A virus; review; species.

Fig. 1

Illustration of the host range of influenza A virus with the natural reservoir of influenza A virus, accidental hosts, and the subtypes that have been identified in the different groups. Illustration by Rebecca Rönnmark and Eric Gisaeus.

Fig. 2

Illustration of localized low pathogenic avian influenza (LPAI) infection versus systemic HPAI infection. Adapted by permission from Macmillan Publishers Ltd: [Nature Reviews Microbiology] ‘Influenza: lessons from past pandemics, warnings from current incidents’ copyright 2005.