Pathogenicity and vaccine efficacy of a thymidine kinase-deficient mutant of feline herpesvirus type 1 in cats.

We constructed a recombinant feline herpesvirus type 1 (FHV-1) which was deleted in a defined region (450 bp) within the thymidine kinase (TK) gene (C7301dlTK) [Yokoyama et al. (1995) J Vet Med Sci 57: 709-714]. In this report, we carried out two experiments to assess the pathogenicity and vaccine efficacy of the recombinant C7301dlTK in cats. The first experiment showed that, following multiple inoculation of the recombinant C7301dlTK by intraocular, intranasal and oral routes, the virus was sufficiently attenuated in cats, although a high titer of the virus was recovered from target organs (eye, nose, and mouth). In the second experiment, two intramuscular vaccinations with the recombinant C7301dlTK protected cats to a significant degree against subsequent challenge with the parent FHV-1 strain C7301 at 4 weeks after the last vaccination. These results demonstrate that the recombinant C7301dlTK is effective as a live attenuated vaccine with a clear genetic marker.