PS12-1. Reduced Tylosin Susceptibilities in Fusobacterium Necrophorum Subspecies of Liver Abscess Origin.

Abstract Liver abscesses are a prevalent health disorder in feedlot cattle and cause substantial economic losses due to liver condemnation, reduced carcass value, decreased feed efficiency, and lower average daily gain. In North America, tylosin is commonly included in feedlot diets to prevent liver abscesses. Tylosin, a macrolide antibiotic, inhibits peptide elongation during protein synthesis and targets Fusobacterium necrophorum, a key pathogen in liver abscess formation. Tylosin is administered in the feed throughout the finishing phase of the feedlot cattle; however, there is increased concern regarding antimicrobial resistance among liver abscess pathogens. Fusobacterium necrophorum, particularly the subspecies necrophorum and funduliforme, remain the primary etiological agents, yet limited data exist on the current antimicrobial susceptibility patterns of these subspecies despite long-term tylosin use in feedlot production systems. The objective of this study was to determine the antimicrobial susceptibility patterns of F. necrophorum subsp. necrophorum (n = 50) and F. necrophorum subsp. funduliforme (n = 50) against chlortetracycline, erythromycin, and tylosin. Minimum inhibitory concentrations (MICs) were determined using a microbroth dilution method following Clinical and Laboratory Standards Institute (CLSI) guidelines under anaerobic conditions. All antimicrobial susceptibility assays were conducted in duplicates and repeated twice. Most frequent observed MIC across repeated assays were used for analysis. Descriptive statistical analyses were performed using R 4.5.2, and log2(MIC) values were summarized as means with 95% confidence interval and then converted back to original scale. Both, MIC50 and MIC90 values were directly computed from MIC. For F. necrophorum subsp. necrophorum, mean MICs (µg/ml) were 2.2 (95% CI: 1.3–3.8) for chlortetracycline, 36.7 (95% CI: 28.2–47.9) for erythromycin, and 38.3 (95% CI: 31.3–46.9) for tylosin. Corresponding MIC50 and MIC90 values were 1.56 and 25 µg/ml for chlortetracycline, 25 and 100 µg/ml for erythromycin, and 25 and 100 µg/ml for tylosin. For F. necrophorum subsp. funduliforme, mean MICs (µg/ml) were 3.2 (95% CI: 1.8–5.8) for chlortetracycline, 25.5 (95% CI: 20.1–32.4) for erythromycin, and 45.3 (95% CI:36.6–56.1) for tylosin. The MIC50 and MIC90 values for F. necrophorum subsp. funduliforme were 2.34 and 50 µg/ml for chlortetracycline, 25 and 68.75 µg/ml for erythromycin, and 50 and 100 µg/ml for tylosin. Overall, chlortetracycline exhibited lower MIC values compared with the macrolides, erythromycin and tylosin for both subspecies. The relatively high MIC values observed for tylosin suggest reduced susceptibility among F. necrophorum isolates. These findings underscore the need to evaluate alternative or complementary prevention strategies to support antimicrobial stewardship in feedlot cattle production systems.