Inactivation of the thymidine kinase gene of a gI deletion mutant of pseudorabies virus generates a safe but still highly immunogenic vaccine strain

Department of Virology, Central Veterinary Institute (CDI), Houtribweg 39, 8221 RA Lelystad, The Netherlands In an earlier report, we described the construction of the genetically engineered pseudorabies virus strain 2.4N3A which does not express glycoprotein gI. Although this strain showed a strongly reduced virulence in 10-week-old seronegative pigs, it could still cause severe disease or death in 3-day-old piglets. To attenuate the strain further, we constructed mutants with a deletion in the viral thymidine kinase gene. One mutant strain, designated 783, has a deletion of 19 base pairs and was shown to be highly immunogenic and safe for vaccination of pigs against pseudorabies virus. Received 12 December 1989; accepted 22 March 1990.