Evolution of Influenza Viruses

Abstract

The rate of evolution in influenza A viruses is the fastest followed by influenza B, C viruses. The key factor in the evolution of influenza B and C viruses is prolonged co-circulation of antigenically and genetically distinct lineages. However, predominantly clonal selection, and to a very limited extent co-circulation of sublineages, is responsible for the evolution of influenza A viruses. Studies on phylogenetic analysis have identified several host-specific virus lineages for various viral proteins, except HA and NA genes. The evolution of influenza A viruses particularly is influenced by several factors such as origin and evolution of HA gene, receptor specificity, antigenic drift and shift, recombination, mixing vessels, host species jumping, etc. Phylogenetic analysis has helped to compare past and present influenza viruses as well as the determination of the common ancestor of the virus. Considerable genetic diversity, divergence and antigenic drift observed in the H5N1 virus during the last 16 years of its circulation in poultry have led to the development of unified nomenclature system in which these viruses were classified into various virus clades. This is required to understand the evolutionary mechanism of the development of pandemic H5N1 strains. The discovery of new subtypes, H17N10 and H18N11, from bats has increased the repertoire of known subtypes of influenza viruses, and the known range of mammals that can be infected by these viruses. Multiple reassortments were responsible for the generation of the novel H7N9 isolates that caused disease and death in humans in 2013.