Clusteron structure of tick-borne encephalitis virus populations.

Tick-borne encephalitis is a natural focal transmissible zooanthroponosis. The causative agent of the disease is a tick-borne encephalitis virus (TBEV) belonging to the genus Flavivirus of the family Flaviviridae and is widespread in Eurasia. Current TBEV classification based on molecular genetic data comprises three phylogenetically separate subtypes: Far Eastern, European and Siberian (TBEV-Sib). Further differentiation of TBEV isn't developed, making it difficult to investigate the origins, distribution and evolution of the virus. In the present study we determined the nucleotide sequence of the gene E fragment for 282 TBEV-Sib isolates from Ixodes persulcatus ticks or their pools from various natural foci in Russia. Analysis of these sequences and sequences obtained from the GenBank database (more than 600), made it possible to cluster TBEV-Sib strains by identical amino acid sequences of a glycoprotein E fragment. In total, 18 groups were identified (from 3 to 285 strains in the group). It was shown that TBEV strains belonging to the same group are phylogenetically related and have a territorial attachment showing either a local or a corridor type distribution. These groups were named as clusterons showed to be the smallest unit of TBEV classification. The grouping of TBEV strains allows characterization of endemic areas both in quantitative and qualitative composition of the clusterons. The approach could be successfully used to record and monitor the TBEV populations.