Characterization of Riemerella anatipestifer in broiler chickens from Shandong, China: serotype, antimicrobial resistance, and virulence gene profiles

Riemerella anatipestifer (RA) is an emerging pathogen in chickens, posing a severe threat to the poultry industry. This study aimed to characterize RA isolates from broilers in Shandong, China, focusing on epidemiology, serotype distribution, antimicrobial resistance, and virulence genes. From October 2024 to March 2025, 2,927 clinical samples were collected from 667 broiler flocks across Shandong. RA was detected by qPCR, and isolates were identified by bacteriological methods and 16S rDNA sequencing. Serotyping, antimicrobial susceptibility testing, and PCR detection of resistance and virulence genes were performed. Epidemiological results revealed an overall RA positive rate of 24.94 % (730/2,927), with serotype 1 being the most prevalent (95.16 %). The positive rate increased with age, reaching 57.75 % in 6-week-old chickens. Among clinical cases, the highest detection rate was observed in chickens with joint/leg disorders (41.77 %). All isolates were multidrug-resistant yet highly susceptible to florfenicol and doxycycline. The resistance genes ermF (95.16 %) and tet(X) (95.16 %) were prevalent. Key virulence genes (AS87_04050, fur, tbdR1, ompA) were present in all isolates, whereas sip was rare (3.23 %) and exclusive to serotype 10. These findings highlight the widespread occurrence and multidrug-resistant nature of RA in Shandong broilers, emphasizing the need for serotype-specific vaccination and prudent antimicrobial use. The study provides critical data for controlling RA in China and similar poultry production systems worldwide.