Figure 3.

Reassortment and emergence of influenza viruses. Within the wild bird reservoir, reassortment is most recognizable when viruses contain gene segments with a mosaic of geographic lineages (see Fig. 2). These intercontinental reassortants are most often of the H13/H16 subtype and found in gull species (Wille et al. 2011). Reassortment in birds can cross host species barriers, such as the H7N9 viruses in China that involve viruses isolated in wild birds, domestic ducks, and poultry. According to the model proposed by Wang et al. (2014), an H7Nx virus from a domestic duck reassorted with an HxN9 from a wild bird. This H7N9 virus, once introduced to poultry, reassorted with an H9N2. More recent literature suggests this virus has segments from more than one H9N2 lineage, suggesting additional reassortment events. Following the emergence of this virus in poultry, it continued to reassort, continually producing new genotypes. Cross-species reassortment may also involve mammals. North American triple reassortant viruses in swine populations emerged following a number of reassortment events; segments have been traced back to classical swine lineages, and human seasonal viruses as well as North American avian viruses.