Deciphering defensive mechanisms in Riemerella anatipestifer infection: pathogen adaptation, host immune response and emerging strategies.

Riemerella anatipestifer (RA) is an economically important avian pathogen that has historically caused significant disease in waterfowls but is increasingly recognized in various poultry production systems and wild bird species, highlighting its expanding host range and adaptive potential. Despite the availability of vaccines and antimicrobial interventions, RA remains a persistent challenge due to serotype diversity, limited cross-protective immunity, evolving antimicrobial resistance patterns, and an incomplete understanding of the host-pathogen interactions that govern disease outcome. Recent advances in genomics, molecular epidemiology, immunology and vaccinology have generated new insights into RA evolution, pathogenicity, host adaptation, and immune protection. However, these advances remain fragmented, hindering the translation of mechanistic discoveries into broadly effective control strategies. Emerging evidence further indicates that disease progression is shaped not only by bacterial pathogenicity but also by the magnitude and regulation of host immune responses, where dysregulated inflammatory signaling and immune-mediated tissue damage contribute significantly to pathology. This review synthesizes current understanding of the dynamic interplay between pathogen adaptation, immune evasion, host defense responses, and immunopathogenesis that collectively determines disease susceptibility, progression and protection. Finally, we evaluate current vaccine platforms and alternative control approaches, identify major knowledge gaps, and highlight future directions for the development of durable, broadly protective and sustainable intervention strategies.