The ecology of influenza A viruses

Influenza A viruses (IAVs) are important pathogens of humans and animals. They have caused four pandemics between 1918 and 2009 and several outbreaks and epizootics in wild and domestic animals. The figure shows the complex ecology of influenza viruses as they circulate in the form of species-specific strains among pigs, bats, harbor seals, horses, dogs and humans (represented clockwise from top left). Wild waterfowl are pictured at the center of the figure as they are considered the main reservoir of the viruses in nature. Arrows represent "cross-species jumps" that are thought to have led to the establishment of the various IAV lineages. For example, two influenza strains circulate among dogs, one originated from horses and the other from birds. In turn, equine influenza viruses are thought to have originated from birds, and an avian-origin IAV lineage causes fatal infections in harbor seals. Pigs not only have their own influenza strains, but also are susceptible to infection by human and avian viruses. In fact, the 2009 influenza pandemic was the result of a cross-species jump of a swine influenza virus that itself possessed genes of avian, human and swine origin. Direct transmission of avian viruses to people frequently results in fatal infections, but fortunately so far avian viruses do not transmit from human to human. Finally, IAVs have also been identified in bats but their actual origins are unknown (represented with a question mark). Viral emergence can be devastating: more than 50 million people died during the 1918 influenza pandemic. Therefore, understanding how viruses jump the species barrier is essential to prepare against future pandemics.