Antimicrobial Susceptibility of Australian Virulent Rhodococcus Equi Isolates Collected Between 1991 and 2014. (September 2015) - Allen, J.L.
Abstract : Reasons for performing study: Bronchopneumonia caused by Rhodococcus equi is an important disease of young horses throughout the world. Although early diagnosis and treatment improves the prognosis, this also increases the amount of antimicrobial usage and therefore increases the likelihood of resistance developing. Objectives: The aim of this study was to determine the level of resistance to commonly prescribed antimicrobial agents of 97 virulent Rhodococcus equi isolates. Study design: Analysis of archived samples. Methods: Virulent Rhodococcus equi isolates were collected between 1991 and 2014 from clinically affected horses and from air samples collected in the breathing zone of foals. Antimicrobial susceptibility of these isolates was assessed using a disc diffusion assay with a panel of agents. The minimum inhibitory concentration (MIC) was determined for rifampicin, erythromycin, tetracycline, and neomycin using a novel resazurin‐based microtitre assay. Results: Resistance to rifampicin was detected in 3 of the isolates (2 collected in 2013 and one in 2014) by both methods. The MIC for these isolates was 64 μg/ml (n = 2) and 16 μg/ml (n = 1). All isolates collected prior to 2013 had MICs<0.125 μg/ml, which was the limit of detection in this assay. Although no isolates were resistant to tetracycline, there was a general increase in MIC in isolates collected in recent years. No isolates were resistant to either neomycin or erythromycin, with MIC values ranging between 0.25 and 2 μg/ml for neomycin and 0.125–1 μg/ml for erythromycin. Conclusions: The success of the macrolide‐rifampicin combination relies on the synergistic action of these 2 agents. Resistance to rifampicin will reduce the therapeutic efficacy of this treatment. It is of serious concern that the resistant isolates were all recently collected. Hopefully, recent research will lead to fewer asymptomatic foals receiving antimicrobials which will in turn reduce the likelihood of ongoing development of resistance. Ethical animal research: All organisms in this study were received by the laboratory from diagnostic accessions.Sources of funding: Funding for the study was provided by Rural Industries Research and Development Corporation and the University of Melbourne. G. Herbert was the recipient of a RN McCarthy scholarship from the Faculty of Veterinary Science.Competing interests: None declared.