Diverged evolution of recent equine-2 influenza (H3N8) viruses in the Western Hemisphere

We reported previously that equine-2 influenza A virus (H3N8) had evolved i nto two genetically and antigenically distinct "Eurasian" and "American" li neages. Phylogenetic analysis, using the HAI gene of more recent American i solates, indicated a further divergence of these viruses into three evoluti on lineages: A South American lineage, a Kentucky lineage, and a Florida li neage. These multiple evolution pathways were not due to geographic barrier s, as viruses from different lineages co-circulated. For the Kentucky linea ge, the evolution rate was estimated to be 0.89 amino acid substitutions pe r year, which agreed with the previously estimated rate of 0.8. For the Sou th American lineage, the evolution rate was estimated to be only 0.27 amino acid substitutions per year. This low evolution rate was probably due to a unique alternating Ser138 to Ala138 substitutions at antigenic site A. For the Kentucky lineage, there was a preference for sequential nonsynonymous substitutions at antigenic site B, which was also a "hot spot" for amino ac id substitutions. Convalescent sera had minimal crossreactivity to viruses of different lineages, indicating antigenic distinctions among these viruse s. In contrast to human H3N2 viruses, our results suggested that the evolut ion of equine-2 influenza virus resembled the multiple evolution pathways o f influenza B virus.