Adaptation of the “New Guinea B” Strain of Dengue Virus to Suckling and to Adult Swiss Mice

SummaryThe New Guinea "B" strain of dengue virus has been propagated in mouse brain. Initially, the virus was carried through eight serial intracerebral passages in mice of the dba strain. It produced weakness or paralysis in about 50 per cent of these animals. A sample derived from the fourth passage was transferred to Swiss baby mice after it had been lyophilized for five years. After two passages, the virus was consistently infectious and lethal for 1 to 7 day old baby mice. The lethal titer of infected baby mouse brain was of the order of 10-7 to 10-8. Upon continued serial transfer in baby mice, the virus gradually acquired an increasing capacity for producing paralysis and death in adult mice as well. After a total of 26 passages, the transformation of the virus into a form consistently pathogenic for adult Swiss mice was complete. The titer in such animals then was similar to that in baby mice. Adult mice infected with samples derived from earlier baby mouse passages also supported viral multiplication, but the virus arising in them was qualitatively similar to the virus inoculated in that it was likewise of low-grade pathogenicity for adult mice but invariably lethal for baby mice. The nature of the change in the character of the mouse-adapted virus has been discussed. It is believed that the virus arising in early baby mouse passages consists of two types of particles, one pathogenic for baby mice only, the other for mice of either age, and that the latter gradually replace the former upon continued adaptation to the mouse brain.Neutralization tests carried out with the mouse-adapted New Guinea "B" strain indicate that it is immunologically identical with the New Guinea "C" and "D" strains and distinct from the Hawaiian type. It is concluded that all known strains of dengue virus belong to one of two separate immunological types.