Persistence and Dissemination of Enrofloxacin and Ciprofloxacin Residues: The Hidden Role of Litter and Droppings in the Emergence of Antimicrobial Resistance

The objective of this study was to evaluate the persistence and environmental dissemination of enrofloxacin (EFX) and its metabolite ciprofloxacin (CFX) residues in poultry systems, as well as their effect on antimicrobial resistance in <i>Escherichia coli</i>. The experimental design included three groups: one treated group (A) and two untreated groups (B.1 and B.2), located in pens adjacent to or within 30 cm of group A to assess residue transfer. Birds in group A received EFX orally (10 mg·kg^-1 every 24 h for 5 days) via orogastric tube. EFX and CFX residues persisted in litter and excreta for up to 18 days post-treatment, reaching initial concentrations of 20,968 µg·kg^-1 in litter and 884.8 µg·kg^-1 in droppings in group A. Significant differences were detected between groups (Kruskal-Wallis, <i>p</i> < 0.05), confirming greater accumulation in the treated group and environmental dissemination. <i>E. coli</i> isolates obtained from litter and droppings from group A showed 73% resistance after treatment and correlated positively with residue concentration (ρ = 0.53). While the untreated groups B.1 and B.2 showed resistance rates of 24% and 13%, respectively, the control group exhibited a resistance rate of 3.3%. This study shows the detection of low levels of EXF and CFX residues in the litter of untreated groups, indicating limited dispersion. These findings highlight the importance of proper byproduct management and targeted environmental monitoring within the One Health framework, as continuous environmental exposure over time, combined with the persistence of this compound, may contribute to the selection of resistant bacteria.