Infectious bronchitis virus vaccine interferes with the replication of avian pneumovirus vaccine in domestic fowl

Experiments were performed in chickens to ascertain whether application of infectious bronchitis (IB) H120 vaccine had an effect on the replication of an attenuated avian pneumovirus (APV) strain, using as indicators virus detection, humoral antibody responses and clinical protection against in vivo APV challenge. A preliminary experiment demonstrated that pharyngeal swabs were as efficient for recovery of APV as were buccal cavity swabs, and that either site was superior to swabbing the nasal cavity. APV was detected to a similar extent by both a reverse transcriptase-polymerase chain reaction (RT-PCR) and virus isolation; therefore, RT-PCR was used in subsequent experiments. In chickens vaccinated with APV alone, APV was detected by RT-PCR in most birds for 1 week after vaccination. When IB vaccine had been applied 1 week earlier, APV detection was delayed and much reduced. This interference by IBV resulted in a lower APV antibody response to vaccination. Following challenge with virulent APV, birds that had been vaccinated with APV alone were fully protected both clinically and virologically. Chickens that had received both vaccines were still protected clinically, but challenge virus could be detected in some pharyngeal swabs 4 days after challenge. In contrast, the APV vaccine had no effect on either the antibody response to the IB vaccine or the level of protection against IB challenge. It is concluded that IB vaccination interferes with the replication of APV, resulting in a reduction in the antibody response but with no adverse effect on the induction of protective immunity.