The Protective Efficacy of an Inactivated Vaccine against Avibacterium paragallinarum Field Isolates

SIMPLE SUMMARY: The incidence of infectious coryza has increased in China, even occurring in vaccinated chickens, causing serious economic losses. Several researchers believe that mutation, the increased virulence of Avibacterium paragallinarum, and improper vaccination procedures lead to poor protection from an inactivated vaccine. Based on this, the purpose of the current study was to evaluate the protective efficacy of a commercial infectious coryza trivalent inactivated vaccine against field infection of Avibacterium paragallinarum in China. Our results suggested that double vaccination showed better protection efficacy than single vaccination. The clinical symptoms, pathological changes, and bacterial shedding of chickens with double vaccination were significantly lower than those of control groups after challenge with field Avibacterium paragallinarum isolates of three serovars. No significant differences were found in body weight and egg production between the boost vaccination groups and the negative control group. The current commercial vaccine shows comparable efficacy in shortening the course of infection if administered twice. Our results provide a reference for infectious coryza clinical vaccine application in China. In addition, better ventilation strategies and biosecurity are important components of a strategic plan to prevent infectious coryza. 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 2(13.1) 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.