Trends in antibiotic use and susceptibility of <i>Edwardsiella ictaluri</i> : An 18-year retrospective from the Delta region of western Mississippi

Abstract Objective Antibiotic resistance is a persistent concern for the global aquaculture industry. Monitoring trends in key bacterial species can help to preserve antibiotic effectiveness and sustain treatment success. Methods At Mississippi State University’s Aquatic Research and Diagnostic Laboratory (ARDL) in Stoneville, Mississippi, the susceptibility of bacterial pathogens to approved antibiotics is estimated by disk diffusion assays. Susceptibility records, documented as zone of inhibition (ZOI) diameters (mm), along with antibiotic feed directives from the period 2007–2024, were assessed to monitor changes in the in vitro susceptibility of Edwardsiella ictaluri (the causative agent of enteric septicemia of catfish) and patterns of medicated feed use in the region. Susceptibility to florfenicol (FFC), oxytetracycline (OTC), and sulfadimethoxine–ormetoprim 5:1 is assessed for all case submissions to ARDL. According to ARDL standards, which are based on client feedback, isolates are considered susceptible (responsive) when the ZOI measures 20 mm or greater. Results In total, 3,911 diagnostic records were included in this retrospective, with mean ZOI diameters (±SD) for susceptible isolates measuring 47.98 mm (±7.42) for FFC, 36.54 mm (±7.79) for OTC, and 35.53 mm (±6.23) for sulfadimethoxine–ormetoprim. For FFC, ZOI diameters were distributed bimodally, showing a population divide and potential epidemiological cutoff value above ARDL’s clinical breakpoint of 20 mm. A high percentage (16.18% [625/3,870]) of isolates were unresponsive (ZOI = 0 mm) to OTC, which correlated to smaller FFC ZOIs. Data revealed a period spanning approximately 9 years (2014–2022) during which the prevalence of case isolates with reduced susceptibility to at least one antibiotic was elevated. This trend aligns with increased directed antibiotic use starting in 2013. After initiation of an experimental E. ictaluri vaccine program in 2019, there were marked reductions in antibiotic use in the region, which correlated with a return to baseline levels of susceptibility observed at ARDL. Conclusions This study demonstrates the impact of antibiotic use on susceptibility trends, highlighting the complex selective influences underlying the dynamics and instability of antimicrobial resistance within these bacterial populations and illustrating the potential to effectively combat antimicrobial resistance in aquaculture systems through vaccination and judicious antibiotic use.