Molecular Characteristics and Factors Associated with Antimicrobial Resistance in Campylobacter Isolates from Small-Scale Chicken Farms in Southern Thailand

Background/Objectives:Campylobacter spp. are leading causes of foodborne gastroenteritis, and small-scale poultry farms are important reservoirs of antimicrobial resistance (AMR). This study investigated the prevalence of Campylobacter spp., AMR profiles, virulence genes, and associated risk factors of Campylobacter isolates in small-scale chicken farms in southern Thailand. Methods: Cloacal swab samples were collected from 10 randomly selected birds at three time points across 12 farms. Isolates were identified by multiplex PCR. Antimicrobial susceptibility was determined by agar dilution. Resistance-associated genes (gyrA, 23S rRNA A2075G/A2074C, cmeABC, tet(O)) and virulence genes (flaA, flhA, cadF, cdtABC) were detected using PCR. Risk factors were analyzed using logistic regression. Results: Of 358 cloacal samples, 151 were positive for Campylobacter spp. (overall weighted prevalence: 53.56%). A total of 153 isolates were recovered from positive samples of 142 C. jejuni (weighted prevalence: 50.04%) and positive samples of 11 C. coli (crude prevalence: 3.07%), as two samples contained mixed infections of both species. For C. jejuni, weighted resistance was high for CIP (91.44%), NAL (90.10%), and TET (64.22%), while erythromycin resistance was absent (0.00%), and MDR was low (0.70%). For C. coli, resistance reported as crude proportions was 81.82% for CIP, 72.73% for NAL, 45.45% for TET, 0.00% for ERY, and 9.09% for MDR. The gyrA Thr-86-Ile mutation was associated with quinolone and fluoroquinolone resistance, and the A2074C mutation with tylosin resistance. The cmeABC operon was found in all isolates. The cdt virulence cluster was present in all C. jejuni, but largely absent in C. coli. Multivariable modelling showed that prevalence was associated with larger flock size, inadequate manure management, and overall antimicrobial use, whereas AMR was associated with farm location, owner age, and specifically tetracycline use. Conclusions: High prevalence and molecular diversity of Campylobacter highlight the need for improved antimicrobial stewardship and biosecurity in small-scale poultry systems.