Researchers analysed the genetic diversity of the virus in Karelia and found traces of multiple long-distance transfers. Extensive mixing of virus variants had occurred, suggesting that other organisms — not only ticks — could be involved in the spread of the disease.

Tick-borne encephalitis is a serious viral disease that affects the central nervous system. Negative consequences may be long-lasting or even permanent in 10–20% of the infected persons, and the disease poses a serious threat to public health in Europe and Asia, as the number of cases has been growing within the last decades. In order to shed light on the changes in the virus, scientists from Sechenov University and their colleagues have analysed the genetic features of the virus isolated from ticks in Karelia, a region in the northwest of Russia which borders Finland, in 2008–2018. The study has been published in the journal Microorganisms.

Tick-borne encephalitis virus (TBEV) is an arbovirus which is found throughout Europe and Asia. Ixodes persulcatus, or the taiga tick, is the carrier of the virus and the main vector of the Siberian TBEV subtype. In the last two decades, this tick species has become more widespread in Karelia, raising a question whether this process has also led to changes in the TBEV subtypes in the area. The answer may have serious implications for public health.

The team of researchers collected ticks in Karelia and analysed ten viral sequences from these samples. The 1054 nucleotide fragment of the E-gene was used for the comparison with other existing samples. This phylogenetic analysis was expected to show a clear geographic pattern of virus migration, but instead the viruses proved to be quite different in neighbouring locations, suggesting multiple long-distance transfers.

For example, four virus variants from Kurgan Oblast were particularly close to the variants from Vologda Oblast, because in their E-gene, 1053 out of 1054 nucleotides were identical. At the same time, viruses found in the same (or neighbouring) location can be quite different. The most diverse pair — a sample from Gomselga (Karelia) and another one from Kokkola (Finland) — differed by 5.6%. Another pair of variants, both from Gomselga, differed by 4.5%. The authors speculate that there may have been several independent re-introductions of the TBEV in Karelia, along with movements between other areas. A possible explanation of this genetic mixing could be the involvement of flying animals, capable of carrying ticks, in the spreading of the TBEV.

‘The Baltic subgroup of the Siberian TBEV subtype is found in the vast territory stretching from Finland in the west to Chelyabinsk Oblast in the east. We found that its genetic diversity — within only one village in Karelia — was comparable to the diversity of all representatives of this group’, said Andrei Deviatkin, Senior Research Fellow at the Institute of Molecular Medicine (Sechenov University) and the first author of the paper. ‘In addition, the diversity of the viruses was commensurate in every region. In other words, active mixing of the viruses took place, and is possibly continuing on a large scale. We believe that the spread of TBEV may be caused by flying animals’.

The study was carried out by Sechenov University (Institute of Molecular Medicine; Institute for Translational Medicine and Biotechnology; Martsinovsky Institute of Medical Parasitology, Tropical, and Vector Borne Diseases), Chumakov Institute of Poliomyelitis and Viral Encephalitides (Moscow), Institute of Biology of the Karelian Research Centre of the Russian Academy of Sciences (Petrozavodsk), and Lomonosov Moscow State University.

Read more:

Deviatkin AA, et al. Baltic Group Tick-Borne Encephalitis Virus Phylogeography: Systemic Inconsistency Pattern between Genetic and Geographic Distances. Microorganisms (2020).