Abstract
Infectious coryza (IC) is an acute respiratory disease caused by Avibacterium paragallinarum (Av. paragallinarum). In recent years, there have been frequent outbreaks of IC in chickens vaccinated with an inactivated vaccine, causing huge losses to the poultry industry. In this study, the protective efficacy of the trivalent inactivated IC vaccine (PT Medion Farma Jaya) against the field isolates of three serovars of Av. paragallinarum was verified. After vaccination, the hemagglutination inhibition antibody titers in double-vaccinated groups (A2, B2, and C2) were higher than those in single-vaccinated groups (A1, B1, and C1). The highest antibody titer was 213.1 at 3 weeks after the booster vaccination in group A2. Consistent with the trend in hemagglutination inhibition antibody titers, the protective efficacy of double vaccination was better than that of single vaccination. The clinical symptoms and pathological changes were alleviated, or the bacterial shedding was significantly reduced with double vaccination after challenge with field isolates of three serovars (p < 0.05). In particular, the chickens with double vaccination showed no clinical symptoms, pathological changes, or bacterial shedding after challenge by the serovar C strain. There was no significant difference in body weight and egg production between the double-vaccinated groups and the negative control group (p > 0.05). Therefore, we recommend that the commercial IC vaccine should be double-vaccinated in clinical applications.