Seasonal surveillance of Salmonella prevalence, antimicrobial resistance, and genetic relatedness in chickens from slaughterhouses and retail markets in Northeast Thailand

Background and Aim: Salmonella is a major foodborne zoonotic pathogen that threatens public health and poultry production worldwide. Although its prevalence and antimicrobial resistance (AMR) in Thai poultry have been widely reported, continuous comparative surveillance across slaughterhouses and retail markets with seasonal stratification remains limited, particularly in Northeast Thailand. This study aimed to systematically compare the prevalence, serotype distribution, AMR profiles, and genetic relatedness of Salmonella isolates recovered from chicken slaughterhouses and retail markets, with emphasis on seasonal and supply chain variations. Materials and Methods: A total of 689 swab samples were collected from two slaughterhouses and two retail markets in Khon Kaen Province, Thailand, across three seasons (summer, rainy, and winter) from March 2023 to February 2024. Isolation and identification of Salmonella were performed according to ISO 6579-1:2017. Serotyping was conducted using the White–Kauffmann–Le Minor scheme. Antimicrobial susceptibility testing was performed against 14 antibiotics using the disk diffusion method following Clinical and Laboratory Standards Institute guidelines. Genetic relatedness of selected isolates was assessed using pulsed-field gel electrophoresis. Statistical analyses included chi-square tests, relative risk estimation, and multivariable logistic regression. Results: The overall prevalence of Salmonella was 32.37% (223/689), with significantly higher contamination in retail markets than slaughterhouses. Prevalence peaked during the rainy season (45.29%), whereas multidrug resistance was highest in winter, particularly in slaughterhouse isolates. The predominant serotypes were Salmonella Agona (15.25%) and Salmonella Saintpaul (14.79%). High resistance rates were observed to streptomycin (43.05%), ampicillin (34.53%), and nalidixic acid (30.49%). Multidrug resistance was detected in 32.43% of isolates. Logistic regression analysis identified retail markets and rainy and winter seasons as independent risk factors for contamination. Pulsed-field gel electrophoresis demonstrated clonal relatedness among isolates within similar locations and seasons, with evidence of cross-contamination along the supply chain. Conclusion: This study provides a comprehensive, year-round comparative assessment of Salmonella contamination and AMR in slaughterhouses and retail markets in Northeast Thailand. The distinct seasonal dissociation between peak prevalence and peak multidrug resistance highlights the need for season-specific and site-targeted surveillance and intervention strategies within the poultry supply chain. Keywords: antimicrobial resistance, chicken, food safety, genetic relatedness, prevalence, poultry supply chain, seasonal variation, Salmonella.