Antimicrobial Susceptibility of Australian Virulent Rhodococcus Equi Isolates Collected Between 1991 and 2014

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