Pathogenicity of the skalica strain (from the tick-borne encephalitis complex) for white mice

Outbred white mice of different age were inoculated by subcutaneous (s. c.) route with the Skalica strain of tick-borne encephalitis virus (TBE). In the CNS of 3-days-old mice diffuse necrotizing encephalitis, abundant cytoplasmic fluorescence of viral antigen in nearly each neuron and high levels of virus (8.8 log LD50/mg tissue) were found. In 10-days-old mice, the extent of encephalitis and that of immunofluorescence in neurons were less widespread; the peak titre of the virus did not exceed 5.5 log LD50/mg brain tissue. In the CNS of 21-days-old mice the infectivity titre was either very low (1.5 log50/mg on day 3 post infection) or the virus was not detected at all. A few neurons revealed positive fluorescence of viral antigen in the basal ganglia in 1 out of 2 mice examined by day 3 post infection (p. i.). No virus was isolated from the CNS of 2-months-old mice observed for 53 days. In the CNS of 3 out of 10 juvenile mice examined histologically within 8 days post infection, minimal inflammatory changes were seen; foci of neurons showing positive immunofluorescence were not found. The failure to recover infectious virus from cultured brain tissue fragments coming from these mice confirmed the negative outcome of direct virus isolations. It is concluded that the Skalica strain was not pathogenic for juvenile mice when administered by s. c. route.