Minimum Inhibitory Concentration Determination of Equine Corynebacterium pseudotuberculosis Isolates From 1996 to 2012

Equine Corynebacterium pseudotuberculosis has not developed appreciable changes to minimum inhibitory concentrations (MIC) patterns over time. Authors’ addresses: William R. Pritchard Veterinary Medical Teaching Hospital, (Rhodes); Department of Medicine and Epidemiology, (Magdesian, Spier, Edman); Department of Clinical Pathology, Microbiology and Immunology, (Byrne); and Department of Population Health and Reproduction (Kass), School of Veterinary Medicine, University of California, Davis, One Shields Ave., Davis, CA 95616; e-mail: sjspier@ucdavis.edu. 

1. Introduction

Few reports exist regarding the antimicrobial susceptibility patterns of C. pseudotuberculosis. The purpose of this study was to evaluate for temporal trends in the in vitro antimicrobial activities of 23 antimicrobials against equine C. pseudotuberculosis isolates. A secondary aim was to determine if a relationship exists between MIC and abscess location.

2. Materials and Methods

The study was retrospective and cross-sectional. Medical records were reviewed to obtain clinical and MIC data. Two hundred and seven C. pseudotuberculosis isolates from 196 horses were included. Minimum inhibitory concentrations were determined by the microdilution technique for isolates with missing MIC data. MIC values over three periods were compared (1996–2001, 2002–2006, 2007–2012). 

3. Results

There were 140 horses with external abscesses, 20 with internal abscesses, 23 horses with both external and internal abscesses, and 4 with ulcerative lymphangitis. Commonly used antimicrobials with MIC₅₀ and MIC₉₀ values below the susceptibility break points used in this study included the amino-glycosides, doxycycline, erythromycin, azithromycin, chloramphenicol, enrofloxacin, rifampin, tetracycline, and trimethoprim-sulfamethoxazole. Ceftiofur, clarithromycin, ampicillin, and penicillin have variable break points depending on the micro-organism used, complicating interpretation. There were no significant changes in MIC values over the study period, and there was no relationship between MIC patterns and abscess location.

4. Discussion

C. pseudotuberculosis shows susceptibility to many antimicrobials and has not developed resistance over time; however, it is important to consider pharmacokinetics and pharmacodynamic properties of antimicrobials when formulating a treatment plan.

Acknowledgments

The authors would like to thank and recognize Dr. Eline Britz for providing the patient clinical data and Dr. Russ Sakai for data entry. This research was supported by the Center of Equine Health, University of California, Davis.

Conflict of Interest

The Authors declare no conflicts of interest.