The nature of replication of tick-borne encephalitis virus strains isolated from residents of the Russian Far East with inapparent and clinical forms of infection.

We describe the biological properties and molecular characteristics of complete genomes of 33 tick-borne encephalitis virus (TBEV) strains that induced different forms of infection, from inapparent to severe focal ones resulting in fatal outcome. Hemagglutinating activity of Oshima-like strains was higher at pH 5.8, while activity of Sofjin- and Senhzang-like strains were higher at pH 6.2 and 6.8, respectively. We determined susceptibility of porcine kidney (PK) cell cultures to these TBEV strains by cytopathic effect (CPE), plaque formation, and size of plaques. The clinical TBEV strains had higher virus titers both in tissue culture infectious dose 50(TCID50) and in plaque-forming unit (PFU) titers and larger plaques than the inapparent strains. A comparison of virus multiplication kinetics by PFU in culture fluid with kinetics of ELISA antigen and hemagglutinin accumulation suggested a different mechanism of interaction between these virus strains and PK cells at the initial stage of cell infection.