Comprehensive assessment of the genetics and virulence of tick-borne encephalitis virus strains isolated from patients with inapparent and clinical forms of the infection in the Russian Far East.

We analyzed the genetics and virulence of 35 strains of TBEV isolated from patients with different forms of the infection living in the southern Far East region of Russia. The results of moleculargenetics studies of the TBEV strains showed that most of the strains that cause inapparent infections form a single cluster (I) with the Oshima 5-10 strain from Japan on the phylogenetic tree. A comparison of the amino acid sequences of the viral polyproteins of the studied strains identified 17 amino acid residues distributed unevenly across the polyprotein that distinctly differed between the clusters of inapparent and virulent strains. We detected additional substitutions in the NS1 and NS5 proteins. These substitutions might influence the pathogenic potential of the strains. Using a model of inbred mice of different ages, we examined the virulence of these strains and showed the different pathogenic potentials of strains belonging to different clusters.