A mutant of herpes simplex virus possessing an attenuated pathogenicity for baby mice obtained by egg passage of a newly isolated strain.

It has been attempted in this laboratory for the past years to isolate and passage herpes simplex virus by the chorioallantoic inoculation of embryonated eggs and pursue the changes in pathogenicities for various host animals occurring during the course of the passage. Among the pathogenicities examined were included those for baby mice intracerebrally or intraperitoneally inoculated, since Kilbourne and Horsfall (1951) stated that one-day old mice were equally susceptible to herpes virus inoculated by these two routes. A strain designated SK was studied most extensively. A fact came to light thereby that the SK strain after passaged through eggs about 50 times was hardly lethal to baby mice when given by the intraperitoneal route, whereas the classical HF strain experiencing more than 450 egg passages still possessed a high infectivity to baby mice even by the peripheral inoculation. This paradoxical result stimulated a quantitative analysis of the infectivities to baby mice of SK strain in comparison with HF strain, and it was eventually disclosed that a mutant had appeared incidentally in an early egg passage of SK strain, which subsequently had replaced the parent type virus after several more passages. Details of those experiments are reported in the present communication.