Development and evaluation of an attenuated Avibacterium paragallinarum strain as a live vaccine candidate for infectious coryza.

Avibacterium paragallinarum (Av. paragallinarum), the causative agent of infectious coryza, is a significant pathogen responsible for substantial economic losses in the poultry industry. Current preventive strategies rely primarily on inactivated vaccines, which have limitations such as vaccine failure and limited cross-protection between serotypes. This study aimed to develop an attenuated strain of Av. paragallinarum as a potential live vaccine candidate. Using the Tn5-Kan transposon, we constructed a transposon mutant library and identified a mutant strain, designated 2019/HB64-40, which harbored a disrupted ksgA gene encoding a critical enzyme involved in ribosomal RNA methylation. Compared with the wild-type strain, the 2019/HB64-40 strain presented significantly reduced biofilm formation, lower hemagglutination titres, and impaired growth. Pathogenicity assessments in chickens demonstrated that the mutant strain displayed significantly attenuated virulence, characterized by fewer clinical symptoms and reduced bacterial shedding. Furthermore, following challenge, all unimmunized chickens presented severe clinical signs of infectious coryza at 2 dpi, with symptoms beginning to ameliorate by 5 dpi, culminating in a mean clinical sign score of 2.1. In contrast, only one chicken (1/10) in the immunized group displayed mild facial swelling and nasal discharge, with a mean clinical sign score of 0.1. The immunized group receiving the 2019/HB64-40 strain demonstrated 90% immunoprotection, highlighting the potential of this attenuated strain as a live vaccine candidate. While cross-serotype protection was not evaluated, the results suggest effective homologous protection and colonization capacity, underscoring its promising application in the prevention and treatment of infectious coryza.