Emerging Riemerella anatipestifer infection in chickens: pathogenic characteristics and host immune response profiles

The increasing Riemerella anatipestifer infection in chickens presents an emerging threat to the poultry industry. In the present study, we analyzed R. anatipestifer infection in chickens, including commercial layers, broiler breeders, broilers, and layer breeders, in 2024. The chicken-derived strain SD24-17-C was isolated from infected broilers and characterized. In vitro assays revealed the enhanced capabilities of SD24-17-C for adhesion and invasion to primary chick embryo fibroblasts; moreover, its reduced susceptibility to chicken serum was detected. These findings indicate enhanced infectivity of the strain in chickens. Based on the in vivo infection assay and associated clinical symptoms, mortality, organ indices, and histopathological features, we detected that the pathogenicity of SD24-17-C was lower in chickens than in ducks. Analyses of immune-related cytokine expression indicated relatively higher IL-2, IL-8, IFN-γ, TLR3, and STING levels in chickens than in ducks, reflecting the potential roles of these cytokines in mediating immune responses and bacterial clearance during R. anatipestifer infection in chickens. These findings confirmed the susceptibility of chickens to R. anatipestifer and provided critical insights into associated host-pathogen interactions.