Research Article

Majeed Khan Jadoon, Umama Butt, Aqeel Khan Tanoli, Muhammad Azhar

Published on : June 2025 | Volume : 2 Issue : 1

Abstract :

Infectious Bronchitis Virus (IBV) is a major threat to poultry health and production. The disease causes respiratory illness, poor growth, and increased mortality in broiler flocks. Control depends mainly on vaccination, but protection varies with strain matching. The Massachusetts (Ma5) strain and the 4/91 strain are widely used vaccines in Pakistan. Both vaccines induce immune responses, but their field performance has not been compared systematically. This study evaluated production outcomes and antibody responses in broiler flocks vaccinated with these two strains under commercial conditions. A total of 50 commercial broiler flocks were included. Twenty-five flocks were vaccinated with the 4/91 strain and 25 with the Ma5 strain. Flock size ranged from 22,000 to 31,000 birds. Performance parameters recorded at slaughter included bodyweight, feed conversion ratio (FCR), and mortality. Blood samples were collected to determine ELISA geometric mean titers (GMT) and coefficient of variation (CV%). Data were analyzed using descriptive statistics, non-parametric tests, and regression models. Results showed that bodyweight did not differ significantly between the two groups i.e. 2,189 g in the 4/91 group and 2,145 g in the Ma5 group. Feed conversion ratio was significantly lower in the 4/91 group (1.58) compared with the Ma5 group (1.69) (p <0.01). Mortality was also significantly reduced in the 4/91 flocks (5.2%) compared with 12.0% in the Ma5 group. ELISA GMTs were higher in the Ma5 group (10,595) compared with the 4/91 group (8,143). CV% was higher in the 4/91 group, though the difference was not significant. Logistic regression confirmed that the 4/91 strain reduced the odds of high mortality (OR 0.04, CI 0.00 – 0.23, p <0.01) indicating a strong protective effect. Linear regression showed that FCR was significantly improved by the 4/91 strain. ELISA GMT had a small positive association with FCR, while CV% showed no effect. This study demonstrates that the 4/91 strain provided superior flock performance compared with the Ma5 strain. Lower mortality and better feed efficiency were achieved with 4/91 strains. These findings also highlight the importance of strain matching over antibody levels alone.