AN EQUINE HERPESVIRUS TYPE-1 RECOMBINANT WITH A DELETION IN THE GE AND GI GENES IS AVIRULENT IN YOUNG HORSES

The cell culture-adapted KyA strain of equine herpesvirus type 1 (EHV- 1) has been found to be attenuated in young horses (Matsumura at al., 1996, Vet Microbiol, 48, 353-365), The KyA strain lacks at least six g enes in its genome, including those encoding glycoproteins gE and gl. To elucidate whether EHV-1 glycoproteins gE and gl play a role in vira l virulence, we have constructed an EHV-1 recombinant that has the gen es encoding both gE and gl deleted from its genome and its revertant. Growth properties of the deletion mutant virus in vitro were compared with those of the parent and the revertant viruses. Plaque size of the mutant virus in fetal horse kidney (FHK) cells was significantly smal ler than those of the parent and the revertant viruses. In one-step gr owth experiments. however, the yields of infectious virus from FHK cel ls infected with the deletion mutant, the parent, or the revertant Vir us were approximately the same. The results suggested that gE and/or g l of EHV-1 promoted cell-to-cell spread of the virus, but that these g lycoproteins were not involved in the process of virus maturation and release or in virus attachment and penetration. Subsequently, the viru lence of mutant and revertant viruses was examined in young horses. No clinical signs were observed in six horses, including three colostrum -deprived foals inoculated intranasally with the deletion mutant virus , whereas three colostrum-deprived foals inoculated intranasally with the revertant virus manifested clinical signs typical for EHV-1 respir atory infection (i.e., pyrexia, nasal discharge, and swelling of subma ndibular lymph nodes). The results obtained from in vivo studies revea led that the EHV-1 mutant defective in both gE and gl genes was avirul ent in young horses, suggesting that gE and/or gl of the EHV-I have an important role in EHV-1 virulence. However, the EHV-1 mutant defectiv e in both gE and gl genes induced only a partial protectivity in inocu lated feats from manifestation of respiratory symptoms after challenge infection. (C) 1998 Academic Press.