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.
Keywords: Dendritic ulcer; F2 strain; feline herpesvirus-1; next-generation sequencing.
Since feline herpesvirus-1 (FHV-1) is a major cause of corneal ulcers, we investigated the pathogenicity of FHV-1 to the cornea via genomic analysis of FHV-1 isolated from feline eyes with dendritic ulcers. Previous genomic analyses of FHV-1 have not identified any nucleotide variants related to pathogenicity, but the FHV-1 modified live vaccine (MLV) F2 strain has unique nucleotide variants that may be distinguishable from field strains. FHV-1 MLVs are often administered, and so identification of feline clinical isolates as vaccine strains would be important information. Four FHV-1 isolates from the eyes of four cats with dendritic ulcers were subjected to genomic analysis. Genomic analysis of nucleotide variants unique to the F2 strain in these isolates resulted in one isolate (the NS strain) being tentatively classified as the F2 strain. The NS strain was further subjected to next-generation sequencing to determine its full genome sequence, which was compared with that of the F2 strain. All open reading frame sequences of the NS strain were completely identical to those of two F2 strains (Virbac and Intervet) and some of clones of the Merial vaccine strain, and the NS strain was identified as the F2 strain. The patient from which the NS strain was isolated had been vaccinated with the Merial vaccine 17 days before the virus was isolated. However, the NS strain was one of the strains included in the Merial vaccine, and so whether the strain originated from the Merial vaccine was unclear. Three of the four patients in this study had a history of corticosteroid treatment, and it was inferred that administration of corticosteroids may have been involved in the formation of dendritic ulcers. These results suggest that the F2 strain has the potential to replicate in the corneal epithelium and form lesions, and, in a cat with no previous history of allergy, corticosteroid administration is not recommended in the period immediately after MLV vaccination.