ISOLATION AND PASSAGE IN ONE-DAY EGGS OF JAPANESE B ENCEPHALITIS (JBE) VIRUS RESULTING IN ATTENUATION OF ITS MOUSE PATHOGENICITY

Since Matumoto (1957) demonstrated that JBE virus could be readily grown in 1-day eggs, it had been a focus of our interest whether it was possible to isolate this virus by the 1-day egg inoculation, until an opportunity was given to test this possibility in 1957, when a brain material was supplied from an autopsy case of fatal JBE, and a strain was isolated which was identified as JBE virus.Subsequently serial passages of this strain were carried out by inoculation into 1-day eggs. A special purpose, thereby, was to obtain a variant possessing a low infectivity to nervous tissue. This was attained by applying limiting dilution passage in the serial 1-day egg passages. The decrease in infectivity to nervous tissue was evidenced by lowering of its infective titer for mice intracerebrally inoculated in contrast to consistently high infective titers for 1-day eggs.The present report describes in detail the changes in the mouse infectivity which ensued during the 1-day egg passages following the isolation of this strain and discusses the feasibility of the method employed to develop an attenuated live virus vaccine.