Seasonal runoff-driven transport of oxytetracycline from swine feces and its effects on soil bacteria and antimicrobial resistance risk

InroductionThe environmental dissemination of veterinary antibiotics through livestock feces represents a relevant concern within the One Health framework due to its potential effects on ecosystem functioning and antimicrobial resistance (AMR) selection. Oxytetracycline (OTC), widely used in swine production systems, is excreted largely as parent compound and may reach surrounding environments through rainfall-driven runoff. This study evaluated the seasonal persistence and runoff-mediated transport of OTC from swine feces under natural field conditions.MethodsExperimental fecal deposits fortified with OTC were exposed outdoors for 30 days across four seasons, and runoff water generated after natural precipitation events was collected and analyzed by HPLC-UV. In parallel, culturable aerobic mesophilic bacteria were quantified in exposed soils to assess potential microbiological responses. Meteorological variables were integrated to evaluate their influence on OTC mobilization and environmental behavior.Results and discussionOTC residues were detected in runoff water in all seasons and persisted until late-stage rainfall events, indicating sustained environmental availability and continuous hydrological transport. Higher concentrations were generally associated with increased precipitation intensity and erosivity, whereas cumulative precipitation and temperature emerged as key explanatory variables in regression models. Sediment production followed similar seasonal patterns, highlighting the potential relevance of particle-associated transport pathways that warrant further quantitative investigation. OTC-exposed soils showed significantly higher culturable bacterial counts than controls, indicating shifts in microbial abundance under antibiotic exposure, which potentially reflects the selection of tolerant bacterial populations. Importantly, all detected concentrations exceeded the predicted no-effect concentration for resistance selection (PNECMIC), resulting in consistently high risk quotients across all seasons, and indicating a high potential risk for resistance selection. These findings demonstrate that runoff represents a relevant pathway for the environmental dissemination of OTC from swine feces under realistic field conditions. The persistence of residues and the consistently high AMR selection risk highlight the need to integrate environmental monitoring and resistance risk assessment into livestock waste management strategies.