Effect of reticuloendotheliosis virus and infectious bursal disease virus on MAREK'S disease herpesvirus latency

Birds infected with reticuloendotheliosis virus (REV) were exposed to Marek's disease virus (MDV) to determine if the establishment of MDV latency was affected by REV-induced immunosuppression, while other chickens, already latently infected with MDV, were challenged with REV or infectious bursal disease virus (IBDV) to determine if the consequent immunosuppression caused a return to cytolytic infection. Immunosuppression was assessed by in vitro mitogen stimulation assays with spleen cells. Latently MDV-infected cells were free of viral internal antigen(s) (VIA) but could be identified by their ability to produce VIA after in vitro cultivation. The results were unexpected: chickens infected with either of these viruses had very low, and often undetectable, levels of MDV infection when compared with appropriate controls. REV infection interfered with early cytolytic MDV infection, and IBDV and REV both failed to activate latent MDV infection in the face of inferred (IBDV) or demonstrated (REV) immunosuppression by these viruses. Apparently, both viruses reduced the number of MDV infected cells since neither cytolytic nor latent infection could be demonstrated. This was based on an absence of cells with VIA either before or after cultivation and, in the case of REV infection, on failure to detect MDV-DNA using a dot-blot hybridisation technique.