Abstract

Respiratory viral infections, particularly Infectious Bronchitis Virus (IBV) and Newcastle Disease Virus (NDV), remain major threats to poultry health and productivity worldwide. Despite widespread vaccination, co-infection and immunological interference between these live attenuated RNA viruses continue to cause economic losses. This study aimed to evaluate the effects of different NDV–IBV vaccination schedules on immune response, tissue pathology, and growth performance in broiler chickens. A total of 200 Ross 308 chicks were randomly assigned to four groups (ND-IB-1, ND-IB-5, ND-IB-8, and ND-IB2-1) and vaccinated with combinations of NDV Clone 30 and IBV 4/91 (and Ma5) at different ages via the ocular route. Growth performance was monitored up to day 35, and samples were collected for serological (IgY ELISA), cytokine (IFN-γ ELISA), histopathological, and molecular (RT-qPCR) analyses. Results showed that delaying IBV vaccination to day 8 (Group ND-IB-8) significantly improved body weight gain (2107.8 ± 45.2 g), enhanced NDV-specific IgY titers (18115 ± 275 U/ml), and reduced IFN-γ levels (45.6 ± 1.9 pg/ml) compared with earlier schedules. Histopathology revealed minimal tracheal and Harderian gland lesions (score = 1.5 ± 0.3–0.4) in ND-IB-8 birds, while early co-vaccination induced severe epithelial damage. RT-qPCR confirmed the absence of viral RNA at day 35, indicating full vaccine clearance. In conclusion, optimal scheduling, specifically delaying IBV 4/91 vaccination until day 8, minimizes immunological interference, reduces inflammation, preserves respiratory tissue integrity, and enhances overall growth and immune performance. The findings provide a practical framework for synchronizing NDV–IBV vaccination to improve health and productivity in commercial broiler production.