Isolation of the feline herpesvirus-1 modified live vaccine strain F2 from one of four cats with dendritic ulcers

Objectives To investigate the pathogenicity of feline herpesvirus-1 (FHV-1) to the cornea, FHV-1 strains isolated from feline eyes with dendritic ulcers were subjected to genomic analysis to determine whether FHV-1 vaccine strains are involved in the formation of dendritic ulcers. Methods All open reading frame (ORF) sequences of the three F2 strains (Virbac, Intervet and Merial) and the FHV-1 clinical isolates from cats registered in GenBank were compared to detect nucleotide variants unique to the F2 strains, with those nucleotides then being used for simple genotyping of the F2 strains. In all isolates from feline eyes with dendritic ulcers, the regions including nucleotide variants of the F2 strain were amplified with PCR and sequenced. Isolates with nucleotide variants of the F2 strain were then subjected to next-generation sequencing to determine their full genome sequences, which were compared with all ORF sequences of the three F2 strains. Results Analysis of ORF sequences for simplified genotyping of F2 strains detected a single nucleotide variant in ORF28 and in ORF44. These were considered to be nucleotide variants unique to the F2 strain. Among the four FHV-1 isolates from eyes of four cats with dendritic ulcers, nucleotide variants of the F2 strain were detected in 1/4 strains (the NS strain). Next-generation sequencing of the NS strain was performed, and all ORF sequences of the NS strain were compared with the those of the three F2 strains. All ORF sequences of the NS strain were completely identical to those of two F2 strains (Virbac and Intervet) and some clones of the Merial vaccine strain. Conclusions and relevance The F2 strain was isolated from an eye with a dendritic ulcer, indicating that the strain has the potential to replicate in the corneal epithelium and form lesions.