Clinical Research Abstracts of the British Equine Veterinary Association Congress 2015.
Authors: Allen J L, Herbert G, Muscatello G, Gilkerson J R
Journal: Equine veterinary journal
Summary
# Editorial Summary: Antimicrobial Resistance in Rhodococcus equi Rhodococcus equi bronchopneumonia remains a significant threat to foal health globally, and the standard macrolide-rifampicin combination therapy has long been the cornerstone of treatment; however, increasing antimicrobial usage driven by early diagnostic capabilities now risks selecting for resistant strains. Allen and colleagues examined 97 virulent R. equi isolates collected between 1991 and 2014 using disc diffusion assays and a resazurin-based microtitre method to determine minimum inhibitory concentrations (MICs) for rifampicin, erythromycin, tetracycline, and neomycin. Alarmingly, rifampicin resistance emerged in three isolates collected in 2013–2014 with MICs of 64, 64, and 16 µg/ml respectively—a substantial increase from the <0.125 µg/ml detected in all pre-2013 samples—whilst tetracycline MICs showed a general upward trend in recent years, though no true resistance developed; reassuringly, neomycin and erythromycin remained effective with low MIC values. The emergence of rifampicin-resistant strains threatens the efficacy of the combination therapy approach that depends on synergistic action between these agents, suggesting that the equine industry must urgently reconsider diagnostic and treatment protocols for suspected R. equi disease to minimise unnecessary antimicrobial exposure in asymptomatic foals and arrest the trajectory of resistance development.
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Practical Takeaways
- •Monitor for treatment failures in foals with R. equi bronchopneumonia treated with macrolide-rifampicin combinations, particularly in recent cases, as resistance is emerging
- •Consider antimicrobial stewardship practices to reduce prophylactic treatment of asymptomatic foals, which may slow development of further resistance
- •Tetracycline, neomycin, and erythromycin remain viable alternatives currently, though tetracycline susceptibility should be monitored
Key Findings
- •Rifampicin resistance detected in 3 of 97 isolates (3.1%), all collected in 2013-2014, with MICs of 16-64 μg/ml compared to <0.125 μg/ml in pre-2013 isolates
- •No resistance to tetracycline, neomycin, or erythromycin detected, though tetracycline MICs showed general increase in recently collected isolates
- •Emergence of rifampicin resistance in recent years is concerning given reliance on macrolide-rifampicin combination therapy for R. equi bronchopneumonia