Vaccine properties of pseudorabies virus strain 783 are not affected by a deletion of 71 base pairs in the promoter/enhancer region of the viral immediate early gene

Strain 783 of pseudorabies virus (PRV) is a genetically engineered vaccine which contains three deletions. The purpose of this study was to examine the effect of one of the deletions, which until now has not been characterized. The deletion occurs within the inverted repeats. Seventy-one base pairs (bp) were deleted, including one of the repeat sequence elements related to the TAATGARATTC boxes detected within the promoter and enhancer region of the immediate early (IE) genes of herpes simplex virus. The deletion affected neither the transcription of the IE gene nor viral growth in vitro. In our animal experiments, one group of pigs was inoculated with the original strain 783 and another with strain 783 which had had the repeat sequences restored. These two groups were then compared to determine the protective efficacy of the two vaccine strains against PRV infection. The deletion in the inverted repeats does not affect the vaccine properties of PRV strain 783: strain 783, with and without the 71 bp deletion in the repeats, protected pigs equally well.