Molecular epidemiology and antimicrobial resistance of Salmonella at the human-macaque-environment interface in Thailand: A One Health surveillance study.

Background and aim: The close interaction between humans and free-ranging macaques in urbanized environments raises concerns about the potential transmission of antimicrobial-resistant zoonotic pathogens. This study applied a One Health approach to estimate the prevalence, serovar distribution, antimicrobial resistance (AMR), and genetic diversity of Salmonella spp. in long-tailed macaques (Macaca fascicularis) and environmental sources in Chonburi, Thailand.

Materials and methods: A total of 313 samples - including 224 rectal swabs from macaques and 89 environmental samples (pooled macaque feces, stray dog feces, soil, feed, and drain water) - were collected from Si Racha and Sattahip districts between April and July 2023. Salmonella isolation was conducted using conventional culture methods, followed by confirmation through serotyping and polymerase chain reaction targeting the invA gene. Antimicrobial susceptibility testing was performed against 14 agents using broth microdilution. Multi-locus sequence typing and 16S rRNA gene sequencing were conducted to assess phylogenetic diversity.

Results: The overall prevalence of Salmonella was 2.88%, with all positive samples detected in the Si Racha district. Environmental samples had a significantly higher prevalence (8.89%) than macaque rectal swabs (0.45%; odds ratio = 22; 95% confidence interval: 2.71-178.84; p = 0.0002). Six distinct serovars were identified, with Salmonella Corvallis predominating in macaque feces. Among the nine isolates, 77.78% exhibited resistance, primarily to tetracycline and ampicillin. Notably, 85.71% of AMR strains from environmental samples were multidrug-resistant (MDR), showing resistance to ≥6 antimicrobials. Phylogenetic analysis revealed genetic heterogeneity, with no clear clustering by source or serovar.

Conclusion: This study underscores the circulation of MDR Salmonella within macaques and their surrounding environments, implicating environmental reservoirs in potential zoonotic and reverse zoonotic transmission. The findings advocate for public awareness initiatives, environmental hygiene improvements, and integrative One Health strategies to mitigate AMR dissemination at the human-animal-ecosystem interface.