Clinical Research Abstracts of the British Equine Veterinary Association Congress 2015.
Abstract: 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. Objective: The aim of this study was to determine the level of resistance to commonly prescribed antimicrobial agents of 97 virulent Rhodococcus equi isolates. Methods: 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.
© 2015 The Author(s). Equine Veterinary Journal © 2015 EVJ Ltd.
Publication Date: 2015-09-17 PubMed ID: 26375857DOI: 10.1111/evj.12486_6Google Scholar: Lookup
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Summary
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The research article investigates the level of resistance in different isolates of Rhodococcus equi, a significant cause of bronchopneumonia in young horses worldwide, to various commonly prescribed antimicrobials. The study found an increase in antibiotic resistance, raising concerns about the continued effectiveness of current treatments.
Research Objective
- The objective of the research was to assess the resistance level of Rhodococcus equi, a type of bacteria responsible for bronchopneumonia in horses, to some commonly used antibiotics.
Research Approach
- The team analyzed samples of virulent Rhodococcus equi that had been collected over a period of 23 years (1991 – 2014) from affected horses and air samples taken from the breathing zones of foals.
- The researchers tested the bacterial isolates’ susceptibility to antibiotics using a disc diffusion test and determined the minimum inhibitory concentrations (MICs) for four commonly prescribed antibiotics: rifampicin, erythromycin, tetracycline, and neomycin. This was done using a newly designed resazurin-based microtitre assay.
Results and Conclusions
- All samples collected before 2013 had MIC levels below the detection limit for the disc diffusion assay. However, three recent isolates (collected in 2013 and 2014) showed resistance to rifampicin.
- Although no isolates resisted tetracycline, there seemed to be a general increase in tetracycline MIC levels in recently-collected samples.
- None of the isolates were resistant to erythromycin or neomycin.
- Given the importance of the synergistic action of rifampicin and macrolide in treatment, rifampicin resistance could significantly compromise the effectiveness of current treatment options.
- The research underscores the urgent need for alternative treatment options. It also recommends reducing unnecessary use of antibiotics in asymptomatic foals to prevent the development of further antibiotic resistance.
Ethics and Funding
- The organisms used in the research were all obtained from diagnostic accessions. This means they were not specially harvested for the purpose of this study, hence ensuring ethical animal treatment.
- The study was funded by the Rural Industries Research and Development Corporation and the University of Melbourne. Additionally, researcher G. Herbert received a RN McCarthy scholarship from the Faculty of Veterinary Science.
Cite This Article
APA
Allen JL, Herbert G, Muscatello G, Gilkerson JR.
(2015).
Clinical Research Abstracts of the British Equine Veterinary Association Congress 2015.
Equine Vet J, 47 Suppl 48, 3.
https://doi.org/10.1111/evj.12486_6 Publication
Researcher Affiliations
- Centre for Equine Infectious Disease, Faculty of Veterinary and Agricultural Sciences, The University of Melbourne, Parkville, Victoria, 3010, Australia.
- Centre for Equine Infectious Disease, Faculty of Veterinary and Agricultural Sciences, The University of Melbourne, Parkville, Victoria, 3010, Australia.
- Faculty of Veterinary Science, University of Sydney, New South Wales, 2006, Australia.
- Centre for Equine Infectious Disease, Faculty of Veterinary and Agricultural Sciences, The University of Melbourne, Parkville, Victoria, 3010, Australia.
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