Onset of protective immunity in chicks after vaccination with a recombinant herpesvirus of turkeys vaccine expressing Newcastle disease virus fusion and hemagglutinin-neuraminidase antigens.
The onset of protective immunity from lethal Newcastle disease virus (NDV) challenge of chicks was determined after vaccination with a recombinant herpesvirus of turkeys (HVT) expressing the fusion and haemagglutinin-neuraminidase proteins of NDV. One-day-old SPF chicks free from maternal antibodies to NDV were vaccinated with 130 to 3300 plaque forming units of HVT (depending on the trial) and then challenged at 4, 7, 10, and 14 days after vaccination (DPV) with a neurotropic velogenic strain of NDV (GB Texas). The recombinant vaccine induced 0%, 35-75%, 85%, and 94-100% protection when the vaccinated birds were challenged at 4, 7, 10, and 14 DPV, respectively. In all trials, challenge caused 100% mortality in unvaccinated control chicks. Newcastle disease virus was reisolated from the lung, liver, spleen, and brain of birds dying in all trials regardless of vaccine dosage or time of challenge, except when challenge occurred at 14 DPV.