Field Efficacy of an Attenuated Infectious Bronchitis Variant 2 Virus Vaccine in Commercial Broiler Chickens.

Egyptian poultry suffer from frequent respiratory disease outbreaks associated with Infectious Bronchitis Virus (IBV) variant 2 strains (Egy/VarII). Different vaccination programs using imported vaccines have failed to protect the flocks from field challenge. Recent studies confirmed a successful protection using homologous strains as live attenuated vaccines. In this study, a newly developed live attenuated IB-VAR2 vaccine representing the GI-23 Middle East IBV lineage was evaluated in day-old commercial broilers in an IBV-endemic area. A commercial broiler flock was vaccinated with the IB-VAR2 vaccine at day-old age followed by IB-H120 at day 16. The vaccinated flock was monitored on a weekly basis till the slaughter age. The health status and growth performance were monitored, and selected viral pathogen real-time RT-PCR (rRT-PCR) detection was conducted on a weekly basis. Finally, the flock was compared to a nearby farm with only the classical IB-H120 vaccination program. Results showed that the IB-VAR2 vaccine was tolerable in day-old broiler chicks. The IBV virus rRT-PCR detection was limited to the trachea as compared to its nephropathogenic parent virus. Respiratory disease problems and high mortalities were reported in the IB-H120-only vaccinated flock. An exposure to a wild-type Egy/VarII strain was confirmed in both flocks as indicated by partial IBV S1 gene sequence. Even though the IB-VAR2-vaccinated flock performance was better than the flock that received only IB-H120, the IBV ELISA (enzyme-linked immunosorbent assay) and log2 Haemagglutination inhibition (HI) antibody mean titers remained high (3128 ± 2713 and ≥9 log2, respectively) until the 28th day of age. The current study demonstrates the safety and effectiveness of IB-VAR2 as a live attenuated vaccine in day-old commercial broilers. Also, the combination of IB-VAR2 and classical IBV vaccines confers a broader protective immune response against IBV in endemic areas.