Experimental characteristics of viraemia caused by two strains of tick-borne encephalitis virus in small rodents.

Two strains of tick-borne encephalitis (TBE) virus differing in virulence for subcutaneously (s.x.) inoculated adult white mice elicited different levels of viraemia in small rodents. Strain Skalica of a lowered virulence caused no detectable viraemia in adult Clethrionomys glareolus and only negligible viraemia in adult Apodemus sylvaticus, Microtus arvalis and Pitymys subterraneus species. The virulent strain 204 gave rise to higher levels of viraemia, sufficient for infection of TBE virus vectors-ixodid ticks, in most individuals of the rodent species tested. Viraemia surpassing the threshold values of infectivity for tick vectors was also found in some juvenile and adult Clethrionomys rufocanus, C. rutilus and Micromys minutus. The level of viraemia depended on rodent species and age, and exhibited individual variability.