An Analysis of Cross-Protection of Commercial IBV Vaccines against Locally Isolated Field Strains of Infectious Bronchitis Virus in Pakistan

Vaccine failure against infectious bronchitis (IB) due to emergence of nephro-pathogenic strains in field is a major problem. This study evaluated cross protection of commercial IBV vaccines against field isolates. A total of 160, day-old chicks were equally divided into four groups. Group A was vaccinated with a single dose of H120 strain IBV vaccine, group B with two doses (days 02 and 15) of H120 strain, group C with two doses of heterologous strains (H120 on day 02 and 4/91 on 15) while group D kept unvaccinated. Antibody titer was evaluated using a commercial ELISA kit. The difference in antibody titer of vaccinated groups was non-significant (P value > 0.05) till two weeks post-vaccination. At 04 weeks of age, the highest antibody titer was observed for group B, followed by group C and A. Challenge with a field isolate of IBV induced a rise in antibody titer in all groups. Group D had the highest score (severe) of clinical signs and mortality (n=04/10) followed by group A, showing a moderate score of clinical signs and mortality (n=02/10). In contrast, group C was the most protected group showing mild signs and no mortality. Nephro-pathogenic gross lesions were predominant in all groups except group C which had the lowest score. It was concluded that nephro-pathogenic strains are involved in field outbreaks of IB. Further, a vaccination program only with a classical strain could not provide full protection while priming with a classical strain in first week and boosting by a variant strain vaccine after second week may provide better protection against the disease. Protective-typing is also recommended for development of vaccine from locally isolated strains.