Background and Aim: Egg production is a critical indicator of reproductive efficiency in laying hens. High environmental temperatures and humidity expose laying hens to heat stress, adversely affecting egg production, egg quality, feed intake, blood chemistry, health, and behavior. Despite the global economic significance of egg production, research on the impact of heat stress in tropical climates, particularly in locally adapted breeds, remains limited. This study investigates the effects of heat stress on egg production in a new synthetic breed of laying hens raised under hot and humid conditions. Materials and Methods: This study utilized secondary data from the Ligor chicken development project, covering the period from January 1 to December 31, 2023. A total of 1293 daily records of egg production from 872 laying hens, aged between 20 and 70 weeks, were analyzed. Egg production parameters, including hen day production (HDP) and average egg weight (AEW), were recorded daily. Environmental data, including temperature and humidity, were collected at hourly intervals using a data recorder. The heat stress index (HSI) was calculated based on temperature and relative humidity values. Statistical analyses, including regression modeling, were performed to assess the relationship between HSI and egg production. Results: The findings demonstrated that heat stress negatively impacted egg production. A statistically significant negative correlation was observed between HSI and both HDP and AEW (p < 0.05). For each unit increase in HSI, HDP decreased by 1.29%, and AEW declined by 0.18 g. These results indicate that heat stress adversely affects the productivity of laying hens in tropical climates. Conclusion: Heat stress significantly reduces egg production and egg weight in laying hens under hot and humid conditions. The findings highlight the need for adaptive breeding strategies and improved environmental management to mitigate the adverse effects of heat stress. Future research should focus on genetic selection for heat resistance and the integration of precision farming techniques to optimize poultry production in tropical regions. Keywords: egg production, heat stress, hot and humid conditions, laying hens.

The emergence of colistin-resistant Escherichia coli (E. coli) in livestock poses a major public health concern due to its zoonotic potential and multidrug resistance (MDR). The study aimed to detect mobilized colistin resistance genes (mcr-1 to mcr-9) in E. coli isolates, along with characterizing their antimicrobial susceptibility, resistance genes, virulence genes, and whole genome sequencing. We investigated E. coli contamination in a swine slaughterhouse in Nakhon Si Thammarat Province, Thailand. A total of 200 fecal samples were collected and screened for E. coli using selective media supplemented with colistin. A total of 200 fecal samples were collected from a swine slaughterhouse and screened for E. coli using selective media supplemented with colistin. Presumptive E. coli isolates were confirmed by PCR, followed by molecular detection of mcr-1 to mcr-9 genes. Antimicrobial susceptibility testing was performed using the disk diffusion method. Selected isolates were further analyzed for additional antimicrobial resistance genes and virulence associated genes by PCR. Whole genome sequencing was conducted on representative isolates exhibiting high levels of antimicrobial resistance. Our results showed that out of 200 fecal samples, 124 presumptive E. coli isolates were recovered from a swine slaughterhouse using selective media containing colistin. PCR confirmation identified 112 isolates (90.32%) as E. coli. Molecular detection of mobilized colistin resistance (mcr) genes (82 isolates, 73.21%) demonstrated that mcr-1 (50.89%) was the most prevalent, followed by mcr-9 (25.89%) and mcr-3 (24.11%). Overall, the 82 mcr E. coli isolates showed the highest level of resistance to ampicillin (97.56%), followed by tetracycline (95.12%), piperacillin (73.17%), and chloramphenicol (65.85%). For non-mcr E. coli isolates, the highest resistance percentage was observed for ampicillin (96.67%), followed by piperacillin (80%) and tetracycline (73.33%). Among the isolates, 75% exhibited MDR phenotypes, showing 22 distinct resistance profiles. The most common MDR pattern was AMP-PIP-TE-C-S (12.5%). Additional antimicrobial resistance genes, including aadA, ampC, and bla_TEM, were detected in over 60% of a subset of 30 tested isolates. The virulence gene analysis revealed that eae (74.10%), associated with enteropathogenic E. coli (EPEC), was the predominant pathotype. Whole genome sequencing of five selected isolates confirmed the presence of multiple antimicrobial resistance and virulence determinants. In conclusion, this study reveals a high prevalence of MDR E. coli harboring colistin resistance genes (mcr-1 to mcr-9) in a swine slaughterhouse in southern Thailand. The findings highlight the potential risk of zoonotic transmission of antimicrobial resistant E. coli through the food production chain and emphasize the importance of continuous genomic surveillance and prudent antimicrobial use in livestock production systems.