Abstract
Background:
Being a ubiquitous, highly contagious virus with a continuous mutation and a large number of evolutions worldwide, the infectious bronchitis virus (IBV) continues to wreak problems among Egyptian chickens and generate economic losses. The commonly applied IBV vaccination protocols in broilers include alternatives to classic and/or variant attenuated live virus vaccines.
Aim:
The current study targeted to assess the protective efficacy of concurrent and successive Ma5 and 4/91 vaccine strain regimens against the field variant II IBV strain (IBV-EGY-ZU/Ck-127/2021) in chickens.
Methods:
Commercial broiler chickens were vaccinated with Ma5 and 4/91 strains simultaneously at 1 and 14 days of age. The evaluation parameters included clinical protection and humoral and early innate immunity aspects in the renal tissues of vaccinated and infected birds.
Results:
The vaccine regimen ameliorated the clinical and histopathological lesions against variant II IBV and enhanced body gain as well as it succeeded in preventing tracheal shedding and minimizing cloacal shedding of the field virus. The IL-1β mRNA gene expression was evident as early as 24 hours, with highly significant upregulation at 48 hours post-vaccination and 24 hours post-challenge in vaccinated birds. Remarkable upregulation was observed in OAS expression 48 hours post-challenge in vaccinated and unvaccinated infected birds. The vaccinated birds developed a significant antibody titer of 704.0±111.98 at 28 days of age, with a consistent antibody titer increase after the challenge.
Conclusion:
Overall, a combination of heterologous protectotype commercial vaccines achieved good protection against the Egyptian variant II IBV strain. This vaccine program could be an effective protocol against the threat posed by IBV viruses circulating in the Egyptian field.

Key words: Infectious bronchitis virus (IBV), Variant II, Vaccine efficacy, Pathology, Immunity