Abstract
Background: Septicaemia caused by Riemerella anatipestifer (R. anatipestifer) is a serious problem in the duck industry worldwide, and it is currently one of the major concerns for duck farming in Vietnam.
Aim: This study was conducted to identify the causative agent of septicemia in ducks in Vietnam. The antimicrobial susceptibility and serotypes of R. anatipestifer isolates were also determined to provide valuable information for disease treatment and vaccine development.
Methods: R. anatipestifer was isolated using blood agar and chocolate agar media. The commercial API 20NE microtest system and partial nucleotide sequence analysis of the 16s rRNA were used to identify R. anatipestifer strains. Serotypes were determined by slide agglutination test using standard antisera against R. anatipestifer. The disc diffusion method was utilized to investigate the antimicrobial susceptibility of R. anatipestifer isolated strains.
Results: A total of 408 samples were collected from ducks with typical symptoms of septicemia for R. anatipestifer isolation. Sixty-nine R. anatipestifer strains were identified. Serotyping results showed that 30 out of 69 bacterial strains were classified as serotypes 1, 6, 8, 10, and 20, with serotype 10 being the most prevalent. The antimicrobial susceptibility test revealed that 100% of the bacterial isolates were susceptible to Amoxicillin/clavulanic acid and Imipenem. In contrast, the majority of R. anatipestifer strains were resistant to Nalidixic acid (89.9%), Streptomycin (75.4%), and Norfloxacin (72.5%).
Conclusion: This is the first ever report in term of identification, serotyping and antimicrobial susceptibility tests of R. anatipestifer causing septicaemia in ducks of Vietnam, providing useful scientific informations for treatment as well as vaccine development to control the disease.

Key words: antimicrobial susceptibility, ducks, R. anatipestifer, serotyping