Effect of infectious bursal disease virus on protection against Marek's disease by turkey herpesvirus vaccine.

Chickens inoculated at hatch with pathogenic infectious bursal disease virus (IBDV) and the turkey herpesvirus (HVT) vaccine were poorly protected against challenge at 1 week with virulent Marek's disease virus (MDV). However, if vaccination with HVT preceded IBDV inoculation such as by embryonal vaccination with HVT and exposure to IBDV at hatch, or by vaccination at hatch and IBDV exposure 5 days or longer postvaccination, then the vaccinal efficacy of HVT was not influenced. Further, the inhibitory effect of pathogenic IBDV was transient: chickens that received IBDV and HVT at hatch were poorly protected against challenge with virulent MDV at 1 week but not against challenge at 2 weeks of age. Pathogenic IBDV routinely interfered in vaccine efficacy of HVT in chickens lacking maternal antibody to IBDV but not in chickens that had such antibodies at the time of exposure to IBDV. In certain experiments, pathogenic IBDV caused a high rate of mortality in newly hatched chicks. Chickens receiving IBDV and MDV had a lower incidence of Marek's disease than those receiving MDV alone.