Molecular evolution of emerging Banna virus

Banna virus (BAV) is an emerging pathogen that causes human viral encephalitis and has been isolated from types of blood-sucking insects and mammals in Asia. However, there are no reported systematic studies that describe the origin and evolution of BAV. Here, a phylogenetic analysis of BAVs isolated from a variety of potential vectors and vertebrate hosts worldwide revealed that BAVs emerged in the beginning of the 20th century and do not exhibit a species barrier. The mean substitution rate of BAVs was 2.467×10^-2substitution/site/year (95% HPD, 1.093×10^-3 to 5.628×10^-2). The lineage is mainly composed of BAVs from high-latitude regions, which are the most recently emerged viruses with significantly higher substitution rates compared with the lineage comprised of the isolates from middle or low-latitude regions. The genetic differences between BAV strains are positively correlated with the geographic distribution. Strains from the same latitude regions are almost 100% identical, whereas the differences between strains from long distance regions with different latitudes could be >60%. Our results demonstrate that BAV is an emerging virus at a stage that involves rapid evolution and has great potential for introduction into non-endemic areas. Thus, enhanced surveillance of BAV is highly recommended worldwide.