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A pseudorabies virus mutant lacking thymidine kinase activity (TK-) was isolated and characterized. The mutant replicated as well in cell culture as the parental TK+ strain, was not temperature sensitive at 38.5 degrees C, and did not revert to TK+. Two pseudorabies virus field isolates and three commercial modified live virus vaccine strains were compared for TK activity and virulence for the mouse; all strains expressed TK: Km values for thymidine of the viral TKs ranged from 2.9 to 3.9 microns; the commercial modified live virus vaccine strains were reduced in virulence for the mouse two to ten-fold. The TK- mutant was avirulent for the mouse. Restriction enzyme analysis of the pseudorabies virus DNA from the strains under study revealed that two of the modified live virus vaccine strains are closely related and that all three modified live virus vaccine strains lack the largest PstI fragment characteristic of the other strains included in the study.

Characterization of virulent and attenuated strains of pseudorabies virus for thymidine kinase activity, virulence and restriction patterns