Tracking antimicrobial resistance pathways in a poultry-intensive catchment

Antimicrobial resistance (AMR) is an emerging environmental contaminant with direct implications for human and animal health, reinforcing the One Health premise that environmental integrity is foundational to health outcomes. We conducted a pilot catchment-scale study in western India to assess AMR prevalence and to distinguish dominant pathways associated with poultry expansion, poultry litter reuse, and human habitation. Four sub-catchments were selected to represent contrasting antibiotic pressure sources: i.e  dense poultry farming (4 farms km⁻²),  sparse poultry farming (2 farms km⁻²), agricultural fields receiving poultry litter as manure, and  habitation (village), along with a reference control. We evaluated antimicrobial resistance using culture-based enumeration of antibiotic-resistant bacteria (ARB) and isolation of multidrug-resistant (MDR) species, complemented by a multiple antibiotic resistance (MAR) index to compare contamination pressure across settings. Poultry litter contained high ARB loads, with 2.5 × 10^7 CFU g⁻¹ resistant to tetracycline and 1.7 × 10^7 CFU g⁻¹ resistant to erythromycin. Seven MDR bacterial species were identified in litter, and five species had MAR index values > 0.2, indicating substantial antibiotic selection pressure. In contrast, no evidence of AMR bacteria was detected in soil and water samples collected immediately surrounding poultry farms, suggesting that strict disinfection protocols afect the prevelance of AMR around farm premises. However, agricultural soils located approximately 0.5 km from the nearest poultry farm, where poultry litter was applied as manure, showed clear AMR signals, including seven MDR species and two species with MAR index values > 0.2. Soil and water samples from manured (poultry litter) fields exhibited markedly higher resistance than unmanured fields, particularly to erythromycin, ampicillin, vancomycin, penicillin, and ciprofloxacin. Resistance was highest for vancomycin in soil (9%) and penicillin in water (52%) from manured fields. By comparison, unmanured fields exhibited <1% resistance in soil and 15% in water. Escherichia coli and Enterobacter spp. were detected in village and control soils with MAR index values < 0.2, consistent with comparatively lower antibiotic contamination.
Overall, the results indicate that land application of untreated poultry litter, rather than proximity to poultry farms alone, can be a key pathway for spread of ARB and resistance determinants into receiving agricultural environments. The study demonstrates a source-differentiated catchment approach to establish baseline AMR assessment protocols that can help disentangle contributions from animal husbandry, manure management, domestic sewage, and background resistance. We recommend the development and implementation of treatment, handling, and disposal protocols for poultry litter to enable safer agricultural reuse and reduce further AMR propagation.