Alternate circulation of recent equine-2 influenza viruses (H3N8) from two distinct lineages in the United States.

Phylogenetic and antigenic analyses indicate that recent circulating equine-2 influenza viruses in the United States have been alternating between two genetic and antigenic distinct lineages since 1996. The evolution rates for these two lineages, the Kentucky and the Florida lineage, are very similar. For the earlier isolates in the Kentucky lineage, there are multiple and sequential nonsynonymous substitutions at antigenic sites B and D. However, there are no changes at any of these antigenic sites for KY98 and OK00. In the Florida lineage, except for NY99 with one amino acid substitution at antigenic site B, viruses in this lineage do not have nonsynonymous substitutions at any of the antigenic sites. The lack of amino acid substitutions at these antigenic sites suggests a mechanism other than immune selection is responsible for the maintenance of these viral lineages. Serological analysis indicates that these two lineages are antigenic distinct, and the pattern of reactivity of horse sera towards these two lineages alternates in consecutive years, parallel to the "switching" of virus lineage seen in the phylogenetic tree. This alternate circulation may play a role in the maintenance of these two lineages of equine-2 influenza virus.