Avibacterium paragallinarum: Pathogenesis Mechanisms and Subunit Vaccine Development

Avibacterium paragallinarum (A. paragallinarum) is the primary causative agent of infectious coryza in chickens. Infection often leads to growth retardation in broilers and a 10% reduction in egg production, reaching over 40% in laying hens. The problem is particularly severe under intensive farming conditions, significantly jeopardizing global poultry health and farming profitability. From a ‘One Health’ perspective, this not only disrupts the stability of the food supply chain, but also increases antibiotic usage due to disease prevention and control needs, thereby aggravating antimicrobial resistance (AMR) and posing a global public health challenge. This review systematically summarizes advances in the pathogenesis of A. paragallinarum and the protective immunity induced by subunit vaccines. It focuses on the infection mechanisms of A. paragallinarum, emphasizing its colonization strategies in the infraorbital sinus and nasal epithelium of chickens, and analyzes the roles of key virulence factors such as hemagglutinin and capsule in adhesion, colonization, and immune evasion. We integrate the tissue-specific pathogenesis of A. paragallinarum with the role of respiratory commensal microbiota in facilitating infection, providing an in-depth analysis of the bacterium’s key immune evasion strategies, thus offering novel insights into host–pathogen-microbiome interactions. Concurrently, to the best of our knowledge, this review provides the first comprehensive overview of current developments in subunit vaccines and their immunoprotective properties, with special attention to limitations in eliciting mucosal immune responses. By delving into the pathogen-host interaction mechanisms, this review aims to inform the optimization of subunit vaccine design and immunization strategies. Ultimately, it seeks to establish a theoretical basis and practical framework for precise control of A. paragallinarum.