New antimicrobial susceptibility panels for MIC testing of respiratory pathogens in cattle
Respiratory infections are by far the most important indication for antibiotic treatment in calves and beef cattle. These infections can be caused by a number of different viruses and bacteria. For bacterial infections, antimicrobial susceptibility testing (AST) is often necessary for selection of an optimal antibiotic treatment and avoid treatment failure.
Unfortunately, the AST panels used previously by Danish diagnostic laboratories for bovine respiratory pathogenic bacteria comprised many antimicrobials that were no longer relevant for treatment of cattle, while several antibiotics relevant for the treatment of cattle were not included. The main reason was the lack of interpretation values, i.e. neither clinical breakpoints nor epidemiological cut-off values (ECOFFs) were available for several of these antibiotics.
In a project funded by the Cattle Levy Foundation, University of Copenhagen, Technical University of Denmark, and the Laboratory for Veterinary Diagnostics (Danish Agricultural and Food Council) aimed at developing new practice-relevant AST panels for MIC testing, including the development of new interpretation values necessary for the clinical usage.
Initially, we identified all the antimicrobials relevant for treatment of these infections under Danish conditions and for which interpretation values are lacking. This work was supported by representatives from the Danish Veterinary Association, and representatives from ViNordic were also consulted. In order to establish new ECOFFs, we designed a customized AST panel with extended concentration ranges for the project. Some antimicrobials that are more relevant for treatment of pigs were also included, because the aim was to develop a panel that could be used for both porcine and bovine pathogens. The panel includes oxytetracykline, doxycykline, benzylpenicillin, sulfamethoxazole-trimethoprim, tulathromycin, tylosin, lincomycin-spectinomycin, and streptomycin.
The three bacterial pathogens most often isolated from calves were tested on the project panel: Mannheimia haemolytica (n=71), Histophilus somni (n=73), and Pasteurella multocida (n=50). Further, Trueperella pyogenes (n=39) was included because it is one of the most frequently isolated bacteria from calves, in particular from navel infections. AST with the customized panel was performed according to the CLSI methodology. Additionally, the EUCAST methodology was used in parallel for a subset of the isolates, to support the development of EUCAST-approved ECOFFs, and to investigate if the available ECOFFs were applicable in laboratories using the CLSI methodology.
The resulting MIC distributions were used together with published MIC distributions for proposal of new ECOFFs. When possible, data from at least 3 laboratories was included, following the EUCAST guidelines for development of tentative ECOFFs. For T. pyogenes, sufficient published data were not available.
Subsequently, taking into consideration the new ECOFFs and tentative ECOFFs, a new panel for diagnostic AST was developed. This panel includes oxytetracycline, doxycycline, florfenicol, benzylpenicillin, amoxicillin, ceftiofur, trimethoprim/sulfamethoxazol (1:19), gamithromycin, tulathromycin, tilmicosin, tildipirosin, enrofloxacin, lincomycin/spectinomycin (1:8), streptomycin, and tiamulin. Since the end of this project, AST with this new panel has been offered to veterinarians submitting samples to the Veterinary Laboratory in Kjellerup. It can be used to investigate antibiotic resistance in Mannheimia haemolytica, Histophilus somni and Pasteurella multocida.
Further details from the project are available (in Danish) in the final report for the research project.
John Elmerdahl Olsen and Peter Damborg have represented University of Copenhagen for this project. Elaboration of project results can be obtained free of charge by contacting project responsible Vibeke Frøkjær Jensen (vfje@lf.dk). A publication for the journal Dansk Veterinær Tidsskrift is under preparation. The results of the MIC analyses for all four pathogens will also be made available for EUCAST, for future development ECOFFs.