Distribution of Far-Eastern tick-borne encephalitis virus subtype strains in the former Soviet Union

Abstract

European and Asian viruses within the tick-borne encephalitis flavivirus complex are known to show temporal, spatial and phylogenetic relationships that imply a clinal pattern of evolution. However, the isolation of recognized Far-Eastern tick-borne encephalitis virus (TBEV) strains in the European region of the former Soviet Union (SU), i.e. thousands of kilometres west of the region in which they are considered endemic, appears to contradict this concept. Here, we present a parsimonious explanation for this apparent anomaly based on analysis of the dates and regions in which these non-endemic strains were isolated, together with their phylogenetic relationships and the records of redistribution of animals under the All-Union programme for acclimatization of game animals within the former SU. Our evidence supports the concept that the anomalous distribution of Far-Eastern TBEV strains in Europe and Siberia arose primarily as the result of the large-scale westward redistribution of game animals for economic purposes.Tick-borne encephalitis virus (TBEV), belonging to the tickborne flavivirus group, genus Flavivirus, family Flaviviridae, is a prototype representative of the seroviruses group of the same name, which were discovered in 1937 in the Russian Far East. TBEV is the causative agent of tick-borne encephalitis in humans, usually after the bite of an infected tick. Two types of host are required for TBEV circulation in nature. The first is the tick as the reservoir and carrier of TBEV and the second the vertebrate animal whose blood is the nutrient source for ticks and also the way in which the virus is transmitted from infected to non-infected ticks by their feeding on the same animal (Labuda et al., 1993). Ixodes ricinus L. and Ixodes persulcatus Schulze are the two main TBEV vectors. Small mammals are the principal hosts for pre-imaginal ticks, whereas mature ticks feed on large mammals such as lagomorphs, predators, hoofed animals and birds (Pavlovsky, 1947;Filippova, 1985). All these animals are natural reservoirs of TBEV in the infection hot spots.According to phylogenetic analysis, there are three TBEV subtypes: Far-Eastern (FE-TBEV) and Siberian (S-TBEV), which are both transmitted by I. persulcatus, and European (Eu-TBEV), which is transmitted by I. ricinus. Each TBEV subtype has specific nucleotide substitutions in the protein E gene and their classification is based on this characteristic (Ecker et al., 1999). Strains belonging to the different TBEV subtypes have their own geographical distributions. Specifically, Eu-TBEV is endemic in Europe and includes strains collected in Austria, Switzerland, Germany, Sweden, Hungary, the Czech Republic, Slovenia, Finland, Latvia, Lithuania, Estonia, Byelorussia (Belarus) and the European part of Russia (Ecker et al., 1999;Lundkvist et al., 2001;Haglund et al., 2003), FE-TBEV is distributed mainly in the Russian Far East, the eastern part of North China and northern Japan. S-TBEV is commonest in eastern and western Siberia, in the Ural region, the European pa...

The Three Subtypes of Tick-Borne Encephalitis Virus Induce Encephalitis in a Natural Host, the Bank Vole (Myodes glareolus)

“…TBEV-FE is endemic in the area ranging from the Lake Baikal region to Northern Japan and the eastern part of Northern China. Some individual foci have been found also in the Baltics and Western Siberia [3], [43], [44]. The geographical distribution of M. glareolus covers only the endemic areas of TBEV-Eur and TBEV-Sib, while other Myodes species, M. rutilus and M. rufocanus inhabit the area endemic for TBEV-FE [45].…”

Molecular evolution of the tick-borne encephalitis and Powassan viruses

“…Their common ancestor existed approximately 130 years ago, most likely in the Far East. The hypothesis postulating that these strains spread due to the movement of wild and domestic ani mals is quite plausible [40]. However, most movement of wild animals occurred in the Soviet period, which disagrees with the divergence ages calculated above.…”

Prevalence of tick-borne encephalitis virus in Ixodes ricinus ticks in northern Europe with particular reference to Southern Sweden

“…However, it is present in populations of I. persulcatus in the Baltic area [10] and western Russia not far from the Finnish border. Its geographical range extends eastwards to China and Japan [9,11]. The Siberian subtype is found in Siberia, eastern Europe and western Russia [9,10,12], but also in Finland [13].…”