Association between antimicrobial usage and resistance on commercial broiler and layer farms in Bangladesh

Antimicrobial resistance has emerged as a significant health problem worldwide, including in Bangladesh, where chickens are an important protein source for human nutrition. One of the factors accelerating the development of antimicrobial resistance is the inappropriate use of antimicrobials on commercial chicken farms. A cross-sectional study was conducted in 2019 on 140 commercial chicken farms in the Chattogram district of Bangladesh to investigate the association between antimicrobial use and resistance in <i>Escherichia coli</i> and <i>Salmonella</i> spp. cultured from cloacal swabs of chickens and from the poultry shed environment. All <i>E. coli</i> and <i>Salmonella</i> spp. isolates were resistant to multiple antimicrobial classes, including those categorized as "Highest Priority Critically Important Antimicrobials" for human medicine. Notably, resistance was observed in <i>E. coli</i> isolates from farms that did not use these antimicrobial classes in the current production cycle. For example, although quinolones were not used on 43.9% of <i>E. coli</i> positive farms, 95.7% of these farms had quinolone-resistant <i>E. coli</i> isolates. The results of the path analysis revealed that there was a "direct effect" of the frequency of antimicrobial usage on "high" resistance, with resistance increasing when antimicrobials were administered more frequently (<i>β</i> = 0.28, <i>p</i> = 0.002). There was a "direct effect" of the purpose of antimicrobial use on "low" resistance, with resistance marginally decreasing when antimicrobials were administered solely for therapeutic use (<i>β</i> = -0.17, <i>p</i> = 0.062), but increasing when they were used prophylactically. Overall, the study results could be used to educate farmers on better practices for antimicrobial administration, and to guide government agencies to update policies on antimicrobial use and resistance surveillance in the poultry sector of Bangladesh.