Lack of virulence of the murine fibroblast adapted strain, Kentucky A (KyA), of equine herpesvirus type 1 (EHV-1) in young horses.

The virulence of the cell culture adapted KyA strain of equine herpesvirus type 1 (EHV-1), which lacks at least six genes by deletions in its genome, was assessed by intranasal inoculation of six young horses that were serologically negative for EHV-1. No horses showed clinical signs, and a neutralizing antibody response against EHV-1 was detected in two horses which had antibodies against EHV-4 prior to the inoculation. A challenge experiment using a highly virulent strain of EHV-1 conducted 4 weeks later against 4 of the 6 horses inoculated intranasally with the KyA strain and 2 control horses revealed that (i) the KyA inoculated horses were protected from manifestation of clinical signs detected in both control horses, with the exception of pyrexia, (ii) duration of virus isolation from the KyA inoculated horses after the challenge was remarkably shortened as compared to that from control horses; (iii) thus, animals inoculated with the KyA and challenged with pathogenic EHV-1 showed a reduction in the time of virus shedding and viremia; (iv) two horses which exhibited no antibody responses after the KyA inoculation showed antibody responses after the challenge significantly higher than those of control horses. The results reveal that the KyA strain has no virulence but still possesses immunogenicity for horses, suggesting that some of the genes deleted from the KyA strain might have importance in the expression of EHV-1 virulence.