Epidemiological investigation of Riemerella anatipestifer in large-scale chicken farms in 29 provinces of China from 2021 to 2024.

Riemerella anatipestifer causes septicemia in waterfowl, especially in ducks worldwide, while it is becoming a threat to chicken farming in China. In this study, a total of 27,136 samples from septicemic chickens and dead embryos were collected in 29 provinces from 2021 to 2024 to characterize the transmission dynamics of R. anatipestifer and guide evidence-based control strategies. A total of 752 R. anatipestifer strains were isolated and identified by PCR, showing rapid geographical expansion (from 10 to 19 provinces) and rapidly increasing isolation rates (from 1.03 % to 4.56 %), with peaking in winter/spring. Infection shifted toward younger chickens, infection rates in 3-6-week-old white-feathered broilers increased dramatically from 0.99 % to 13.24 % (mainly leg lesions), while 50-100-day-old layers showed 2.16 % salpingitis. Tissue-specific colonization showed high loads in oviducts (21.68 %) and hocks (10.89 %). Although R. anatipestifer was detected in 20.18 % of early "jelly-like embryos," it was not found in semen/ovarian follicles. Serotyping showed dominance of type 1 (70.82 %) and type 10 (15.86 %), with 2.83 % untypable strains. Alarmingly, R. anatipestifer had a severe resistance to enrofloxacin (91.54 %), polymyxin (88.22 %), and amikacin (86.10 %). As the first large-scale survey of R. anatipestifer in chickens, these findings highlight its cross-species adaptation, vertical transmission risk, and increasing prevalence in young broilers, which contributes to the understanding of the epidemiology of R. anatipestifer in chickens and provides scientific guidance for the treatment and control of the disease.