Development of an attenuated strain for Japanese encephalitis live virus vaccine for porcine use.

To develop Japanese encephalitis live virus vaccine for the prevention of stillbirth in swine, an attempt was made to produce an attenuated virus. Primary bovine kidney cell culture was used to carry out serial passages of a field strain at 30 degrees C for a long time. During these passages cloning was repeated with the character of reproductive capacity at given temperature (rct) and plaque sizes as markers. As a result, an attenuated S- strain was produced successfully. It was rct/37- and rct/40- and formed small-sized plaques. In this strain a marked attenuation was seen in peripheral infectivity to suckling mice and in intracerebral infectivity to adult mice. When inoculated with this strain, newborn piglets manifested no abnormal clinical signs or pathological changes of the brain tissue at all. A minute amount of virus was rarely recovered from some lymph nodes and blood. Little virus was recovered from various organs. No viremia was detected from any piglet more than one month of age inoculated with recovered virus. No infection of the placenta or fetus was recognized in any pregnant sow inoculated with recovered virus. Inoculated sows gave birth to normal young. Furthermore, when mosquitoes of Culex tritaeniorhynchus summorosus were allowed to suck infected blood by the membrane feeding technique, the virus recovered from them showed an infectivity more reduced than the field strain. From these results, it was presumed that the S- strain might be safe enough to be used as live virus vaccine for swine.