Antimicrobial resistance (AMR)

Selection pressure and transmission pathways

Antibiotic exposure creates selection pressure: susceptible bacteria are suppressed while resistant organisms or resistance determinants can persist and spread (Marshall & Levy, 2011). Reviews of food-animal use describe elevated resistance in bacteria associated with animals receiving non-therapeutic antimicrobials and identify potential transmission through direct contact, the food chain, water, air, and manure- or sludge-amended soils (Marshall & Levy, 2011). These pathways differ by organism, drug class, production system, sanitation, occupational exposure, and local antimicrobial use (Marshall & Levy, 2011; Muloi et al., 2018).

Use is substantial at global scale (Tiseo et al., 2020). A modelling analysis using sales data from 41 countries estimated 93,309 tonnes of antimicrobials were sold for chickens, cattle, and pigs in 2017 and projected an 11.5% rise by 2030 under its assumptions (Tiseo et al., 2020). These are modelled sales estimates rather than direct measurements from every country, and reporting was more common among countries with larger export markets (Tiseo et al., 2020).

Evidence from reducing use

Intervention evidence most directly addresses resistance within animal populations after use restrictions (Tang et al., 2017). A systematic review identified 181 eligible studies; interventions restricting antibiotic use were commonly associated with absolute reductions of 10–15 percentage points in the prevalence of antibiotic resistance in animals, although results varied by antibiotic, bacterium, and specimen type (Tang et al., 2017). In the smaller set of human studies, pooled resistance prevalence was 24% lower in intervention groups, with a stronger association among people in direct contact with food-producing animals (Tang et al., 2017).

Evidence about direction of transmission is less definitive (Muloi et al., 2018). A systematic review focused on antimicrobial-resistant Escherichia coli found that studies often supported exchange between animal and human populations but that methodological limitations prevented robust conclusions about directionality in many settings (Muloi et al., 2018). Evidence that animal antibiotic use selects resistance is stronger than evidence assigning a precise fraction of human resistant infections to livestock in a particular place (Tang et al., 2017; Muloi et al., 2018).

Public-health recommendations

WHO recommends an overall reduction in medically important antibiotics in food-producing animals and complete restriction of their use for growth promotion and disease prevention without diagnosis (WHO, 2017). The same guidance retains therapeutic use for sick animals and recommends diagnosis, prudent drug selection, vaccination, hygiene, and changes in housing and husbandry as components of reduced routine use (WHO, 2017). Thus, reducing routine antibiotic use is a mainstream public-health priority, while implementation also requires disease-prevention measures that protect animal health (WHO, 2017).