Figure 1. How avian influenza viruses adapt to mammals.

(A) One way that avian viruses can adapt to mammals is through virus reassortment, as shown here. The avian virus (red) reassorts with another influenza virus that is already adapted to mammals (green), yielding a new mammalian-adapted virus. For this to happen, a single individual must be infected by both viruses simultaneously. (B) The other way that avian viruses can adapt to mammals is by direct infection and subsequent adaptation, as is illustrated here. As an avian virus (red) starts to spread between individuals in a mammalian population, it can acquire mutations that make it better at infecting mammals (an evolutionary intermediate is shown in blue). Eventually it may become well-adapted to infecting mammals (green). (C) Chen at al. isolated H3N2 canine influenza viruses from samples collected between 2012 and 2019 (middle columns), and tested them for a range of phenotypic properties that are associated with influenza viruses being able to infect mammals (rows). As controls, they included avian H3N2 (left column) and human H3N2 (right column) viruses. They found that, over time, the canine H3N2 virus gained multiple properties that make it compatible with mammals (green squares), with several of the key adaptations occurring around or after 2016/2017. Yellow squares indicate intermediate phenotypes. A blank square indicates that the test was not performed.