Investigations of the biological properties of tick-borne encephalitis viruses immediately after isolation from naturally infected Ixodes persulcatus from Western Siberia has revealed their different pathogenicity for monkeys and mice. Strains highly pathogenic for mice on extraneural inoculation and for rhesus monkeys on intracerebral inoculation have been isolated, but also strains with a low extraneural pathogenicity for mice and a complete non-pathogenicity or a mild pathogenicity for monkeys on intracerebral inoculation.

The viruses have been classified into three main groups by their virulence for mice and monkeys, and their ability to grow in tissue culture at 40° C. and the findings are summarized in Table 6. Group I included viruses highly virulent for mice by any route of inoculation, which caused fatal paralytic disease in monkeys, and which grew well in chick embryo culture at 40° C.

Group II strains also had a high virulence for mice, but in monkeys caused milder disease with a peculiar clinical course indicative of primary damage to the cerebellum. About half of the infected monkeys recovered completely or with mild sequelae (ataxia, pareses of the limbs). This group grew moderately well in chick embryo cultures at 40° C. Groups I and III differed essentially in their virulence for mice by subcutaneous inoculation, and virulence for monkeys decreased distinctly with transition from group I to group III. Group III strains showed a lower reproduction rate at 40° C. In chick embryo culture than group I. Groups I and II showed good correlation between extraneural pathogenicity for mice, pathogenicity for monkeys by IC inoculation, and growth rate in chick embryo cultures at 40° C.

Group I strains were isolated chiefly from foci with a high TBE morbidity and the strains of groups II and III in foci with sporadic cases. It is, however, highly probable that in any tick-borne encephalitis focus several variant strains of differing virulence exist.

Recent experience of vaccinating volunteers with naturally attenuated group III strains suggests that safe and effective live vaccines can be developed against tick-borne encephalitis. Vaccines against many other arbovirus diseases can probably also be derived from naturally occurring strains of virus isolated from arthropods in areas of low disease but high infection rate. This approach looks more promising than the usual approach of attempting to develop attenuated strains from highly virulent strains.