Isolation of a variant of the felid herpesvirus 1-live attenuated vaccine strain F2 from unvaccinated cats

Introduction: Felid herpesvirus 1 (FeHV-1) is widespread worldwide and is sometimes associated with chronic diseases in cats; therefore, it is clinically important. Vaccination can be used to control FeHV-1 infection. We previously isolated a strain identical to the live attenuated vaccine (LAV) strain F2 from a feline eye with a dendritic ulcer, indicating that the F2 strain may be pathogenic. The origin of the F2 strain that we previously isolated is unknown, and we surmised that the F2 strain may exist in the field. To determine whether the LAV strain F2 exists in the field, we therefore performed a genomic analysis of FeHV-1 isolates from cats. Methods: FeHV-1 isolates from cats were genotyped using single nucleotide variants (SNVs) within two marker genes related to the F2 strain as indicators. Strains with markers were subjected to next-generation sequencing to determine their whole genome sequence (WGS). Results: Of the nine FeHV-1 isolates, none were found to have two SNVs related to the F2 strain, whereas two isolates were found to have one SNV related to the F2 strain. Because the cats from which these strains were isolated lived together, the isolates were considered to be of the same strain, and we determined the WGS of one strain (strain A-19-5). All coding sequences of strain A-19-5 were identical to those of one of the cloned strains of the Merial vaccine (likely a variant of the F2 strain within the Merial vaccine population). The cat from which the strain was isolated had never received vaccination. Conclusion: The results suggest that a strain identical to a variant of the F2 strain, as found in the FeHV-1-LAV, may exist in the field.