Is antimicrobial administration to food animals a direct threat to human health? A rapid systematic review

Background: Large quantities of antimicrobials are given to food animals, particularly in feed, potentially increasing antimicrobial resistance in humans. However, the magnitude of this effect is unclear. Methods: We searched PubMed, Embase and Web of Science for studies on interventions that limited antimicrobial use in food animals, in any setting and context, to reduce antimicrobial resistance 1) in those food animals; and 2) in humans. We validated our strategy by testing whether it identified known relevant studies. Data from included studies were extracted into pre-designed and pilot-tested forms. Results: We included 104 articles containing 93 studies. Heterogeneity (different animal species, environs, antimicrobial classes, interventions, administration routes, sampling, and methods), was considerable, precluding meta-analysis. The evidence was therefore synthesised narratively. A total of 89 studies (3 directly, 86 indirectly) addressed whether limiting antimicrobial exposure in food animals led to decreased antimicrobial resistance in those animals. The evidence was adequate to conclude this, although the magnitude of the effect could not be quantified. Four studies (1 directly, 3 indirectly) examined whether withdrawal of antibiotics changed resistance of potential pathogens in retail food for human consumption, and in bacteria of humans themselves. The direct (observational) study of broiler hatchery in ovo antimicrobial injection found a credible effect in terms of size reduction and time sequences. Interpretation: Limiting antimicrobial use in food animals reduces antimicrobial resistance in food animals, and probably reduces antimicrobial resistance in humans. The magnitude of the effect cannot be quantified.