Rate of evolution and molecular epidemiology of tick-borne encephalitis virus in Europe, including two isolations from the same focus 44 years apart.

Tick-borne encephalitis virus (TBEV) is a member of the family Flaviviridae. It is transmitted by Ixodes spp. ticks in a cycle involving rodents and small mammals. TBEV has three subtypes: European, Siberian and Far Eastern. The virus causes thousands of cases of meningoencephalitis in Europe annually, with an increasing trend. The increase may be attributed to a complex network of elements, including climatic, environmental and socio-economic factors. In an attempt to understand the evolutionary history and dispersal of TBEV, to existing genetic data we add two novel complete ORF sequences of TBEV strains from northern Europe and the completion of the genome of four others. Moreover, we provide a unique measure for the natural rate of evolution of TBEV by studying two isolations from the same forest on an island in Åland archipelago 44 years apart. For all isolates, we analysed the phylogeny, rate of evolution and probable time of radiation of the different TBEV strains. The results show that the two lineages of TBEV in different Ixodes species have evolved independently for approximately 3300 years. Notably, rapid radiation of TBEV-Eur occurred approximately 300 years ago, without the large-scale geographical clustering observed previously for the Siberian subtype. The measurements from the natural rate of evolution correlated with the estimates done by phylogenetic programs, demonstrating their robustness.