Comparison of the Genomes of Attenuated Equine Herpesvirus-1 Strains with Their Parent Virulent Strain

To elucidate the virulence factors of equine herpesvirus-1 (EHV-1), we compared the genomic architecture of the HH1 strain, a Japanese isolate of EHV-1, with that of its four descendant strains obtained by serial passage in bovine kidney (BK) cells, i.e., BK77, BK161, BK271, and BK343. In preliminary studies high-passaged BK271 and BK343 lost virulence in colts, and in the present study digestion with restriction endonuclease BamHI showed that the DNAs of BK271 and BK343 differed markedly from the DNA profiles of HH1, BK77, and BK161. Employing all the molecularly cloned BamHI fragments of the HH1 strain as probes, Southern blot analysis revealed seven variable fragments. All BamHI fragments of BK343 were subsequently cloned into plasmid pUC18 and on mapping to the HH1 DNA maps, two additional variable fragments were disclosed. Finally, the precise variable regions identified in these nine fragments were fine mapped on the viral prototype genome, and among the open reading frames (ORFs) located in the variable fragments, three (ORFs 1, 24, and 71) were found to have changed via serial passage in the BK cells. Thus these three ORFs are thought to play an important role on the pathogenicity of EHV-1.