Influenza virus reservoirs and intermediate hosts: dogs, horses, and new possibilities for influenza virus exposure of humans

Abstract

Influenza A virus (IAV) infections in hosts outside the main aquatic bird reservoirs occur periodically. Although most such cross-species transmission events result in limited onward transmission in the new host, sustained influenza outbreaks have occurred in poultry and in a number of mammalian species, including humans, pigs, horses, seals, and mink. Recently, two distinct strains of IAV have emerged in domestic dogs, with each circulating widely for several years. Here, we briefly outline what is known about the role of intermediate hosts in influenza emergence, summarize our knowledge of the new canine influenza viruses (CIVs) and how they provide key new information on the process of host adaptation, and assess the risk these viruses pose to human populations.

FIG 1

Schematic showing the known emergence events of influenza A viruses in mammals that led to extended outbreaks, including epidemics and pandemics. The diagram indicates the known mammalian hosts (humans, swine, seals, mink, horses, and dogs), as well the approximate periods during which those were known to sustain natural transmission in that host. The viruses are all assumed to have emerged from avian reservoirs to infect the initial mammalian hosts. The three known transfers between different mammalian hosts that went on to establish epidemics in the new hosts are indicated by red lines, and the first mammalian host therefore acted as the intermediate reservoir host for the transferred virus. The direction of transfer of the 1918 H1N1 virus is not known for certain, but the virus is now thought to have transferred from humans to swine, rather than the other way around. The transfers of new AIV gene segments into the human virus lineage are shown by the vertical arrows, and the segments are listed; these are assumed to involve a direct transfer of the gene segments from the avian ancestor in humans. There have been complex mixtures and transfers of genome segments among some of the viruses in swine, but most of those are not shown because they have not involved transfer between different hosts.