Equine Herpesvirus Type 1 ORF76 Encoding US9 as a Neurovirulence Factor in the Mouse Infection Model

Equine herpesvirus type 1 (EHV-1) causes rhinopneumonitis, abortion and neurological outbreaks (equine herpesvirus myeloencephalopathy, EHM) in horses. EHV-1 also causes lethal encephalitis in laboratory small animals such as mice and hamsters experimentally. EHV-1 ORF76 is a homologue of HSV-1 US9, which is a herpesvirus kinase. Starting with an EHV-1 bacterial artificial chromosome clone of neuropathogenic strain Ab4p (pAb4p BAC), we constructed an ORF76 deletion mutant (Ab4p∆ORF76) by replacing ORF76 with the rpsLneo gene. Deletion of ORF76 had no influence on replication, cell-to-cell spread in cultured cells or replication in primary neuronal cells. In western blots of EHV-1-infected cell lysates, an EHV-1 Us9-specific polyclonal antibody detected multiple bands ranging from 35 to 42-kDa. In a CBA/N1 mouse infection model following intranasal inoculation, the parent and Ab4p∆ORF76 revertant caused the same histopathology in the brain and olfactory bulbs. The parent, Ab4p∆ORF76 and revertant mutant replicated similarly in the olfactory mucosa, although Ab4p∆ORF76 was not transported to the olfactory bulbs and was unable to infect the CNS. These results indicated that the ORF76 (US9) plays an essential role in the anterograde spread EHV-1.