Genetic diversity, serotype, and antimicrobial profiles of Riemerella anatipestifer isolated from ducks and chickens in Thailand

Riemerella anatipestifer, which causes New Duck Syndrome, poses a significant threat to poultry production, causesing respiratory distress, neurological signs, and septicemia primarily in ducks and occasionally chickens. R. anatipestifer outbreak control requires knowledge about their genetic diversity, serotypes, and antibiotic profiles, which are currently unavailable. In this study, R. anatipestifer was isolated from ducks and chickens in Thailand between 2021 and 2023 to characterize their genetic relatedness, antimicrobial profiles, and resistance genes. Seven different serotypes were identified in isolates from ducks. Non-typable strains were the most prevalent, followed by serotypes 7, 10, 1, 5, 11, and 17. However, only serotype 1 was identified in isolates from chickens. Antimicrobial susceptibility testing revealed high resistance to colistin and broad minimum inhibitory concentration (MIC) ranges for β-lactams, aminoglycosides, chloramphenicol, sulfamethoxazole, and trimethoprim. Of the 17 representative isolates analyzed by WGS, the most prevalent resistance genes were tet(X2) and lnu(I). Phylogenetic analysis based on core-genome single nucleotide polymorphisms (SNPs) categorized the isolates into four main clusters. Most of Thai isolates from both ducks and chickens clustered together, indicating the circulation of endemic strains within the region. This is the first comprehensive study of R. anatipestifer isolated from ducks and chickens in Thailand. This research illustrates the value of enhancing basic biosecurity and movement control among farms. The data also provide a valuable foundation for the development of antibiotic use guidelines and vaccines, which will enhance the judicious and minimal use of antibiotics in poultry production.