Deviatkin et al.
Tick-Borne Encephalitis Virus: An Emerging Ancient Zoonosis? 
Viruses 2020, 12, 247, doi: 10.3390/v12020247

TBE is prevalent in the so-called „TBE belt “, which extends from the British Isles to Japan. Based on phylogenetic analyses, the TBE wirus has been divided into five subtypes: the European, the Siberian, the Far Eastern, the Himalayan, and the Baikalian subtypes. The geographic distribution of subtypes mostly corresponds to the nominal regions. However, there are some exceptions, e.g. the European subtype is also found in South Korea and the Irkutsk region, and the Far Eastern subtype also circulates in the Baltic countries and in Moldavia. Regarding the evolutionary history of the TBE virus, the most common hypothesis says that the virus has emerged from a common ancestor with the Omsk hemorrhagic fever in Central Eurasia about 3000 years ago.

Using GenBank, the authors compared the RNA of TBE viruses in different years in the territory of several countries. Knowing the „distance “, the proportion of divergent nucleotides between samples and the date of their receipt, the authors could estimate the time of divergence of subtypes. For example, the European subtype demonstrated explosive growth of population size in two periods, one from 1711 to 1834, and a second growth period started in 1956. An increase of virus population was suggested independently for all subtypes and was most pronounced in the European subtype, which is also most thoroughly intermixed geographically.

Most of the virus spreading events occurred in the last three or four centuries and the transfer of virus was observed for thousands of kilometers, not to mention that the viruses have completely mixed across Europe over the last 100 to 200 years as a result of transportation by wild (e.g. birds) and domestic animals. Thus, TBE is a highly dynamic disease, even as an emerging disease in many regions.

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