Comparative Neuropathogenesis of Equine Herpesvirus 9 and its Mutant Clone (SP21) Inoculated Intranasally in a Hamster Model

Summary The neuropathogenesis of equine herpesvirus 9 (EHV-9), a neurotropic herpesvirus, and its mutant clone (SP21) was studied experimentally in a hamster model. EHV-9-infected hamsters showed clinical signs of infection at 3 days post infection (dpi), while infection with SP21 resulted in clinical signs at 4 dpi. Clinical signs were more severe in the EHV-9-infected group than in the SP21-infected group. There was a significant difference in the time of anterograde transmission of EHV-9 and SP21 inside the brain. Viraemia was detected in the EHV-9-infected group at 4–5 dpi, while no viraemia was detected in the SP21-infected group. The serum concentration of tumour necrosis factor-α was significantly higher in EHV-9-infected animals than in those infected by SP21 group at 4–5 dpi, but there was no difference in the serum concentration of interferon-γ. The spatiotemporal profiles of viral replication and virus-associated histopathology were remarkably similar, were high in the olfactory bulb and cerebral hemispheres, and decreased progressively towards the medulla oblongata. The mean group scores of the histopathological changes for the entire brain were significantly higher in the EHV-9 group than in the SP21 group at all time points, starting from 3 dpi. These results suggest that the gene products of the open reading frame (ORF)19 and ORF14 play essential roles in the neuropathogenesis of EHV-9, as the two point-mutations detected in SP21 significantly altered the neuropathogenesis of the virus.