[Gradual variability of the parasitic system of tick-borne encephalitis].

It has been shown that the temperature conditions of hibernation in the major vectors of tick-borne encephalitis viruses (TBEV) the ticks Ixodes ricinus and Ixodes persulcatus occupy a special place among a lot of factors that determine the spatial distribution and the specific features of functioning of the parasitic system of tick-borne encephalitis. These conditions determine the borders of distribution of the major vectors of TBEV, their sympatric zone, and the physiological features of individual races of the ticks. The conditions of hibernation in the vectors also affect the properties of circulating RBEV strains. The reasons for variability in TBEV virulence in different landscape and climatic zones are discussed.