After 37 passages in L TK-celJs, the TNL strain of pseudorabies virus was further attenuated either alternatively in L TK-and ESK cell lines or straightly in ESK cell under the presence of BUdR of a concentration of 1,000 mcg/ml. activity and stabiblity of the kinase of the mutant in vitro were studied. Results indicated that the TK-mutant of TNL-L TK-ESK could not replicate in HAT medium. It was not virulent to day-old chicks and its virulence to rabbits was greatly reduced. A little reversion of the virulence was found when it was serially passaged 10 times in RK-13 cell without BUdR.
The TK-mutant was safe to pregnant sows. The control sow lost appetite and showed deep depression and febrile reaction for 4 days after challenge. On the other hand, the vaccinated sow had temperature rise for one day only and did not show any clinical signs.