AASRP Research Summary
Phenotypic and genotypic antimicrobial susceptibility of non-aureus stephylococci in dairy goats
| Date/Time: |
9/12/2025
16:15
|
| Author: |
Michelle P Buckley |
| Clinic: |
Texas Tech School of Veterinary Medicine |
| City, State, ZIP: |
Amarillo, TX 79106 |
Michelle P. Buckley, DVM, MS
1
;
Fazal Sattar, BS
2
;
Ganwu Li, MS, PhD
2
;
Orhan Sahin, DVM, MS, PhD, DACVM
2
;
Roselle C. Busch, DVM, PhD
3
;
Amanda J. Kreuder, DVM, PhD, DACVIM (LA)
1
;
Paul J. Plummer, DVM, PhD, DACVIM (LA)
1
;
Patrick J. Gorden, DVM, PhD, DABVP-Dairy, DACVCP
1
;
1Department of Veterinary Microbiology and Preventive Medicine, Iowa State University, Ames, IA, 50011 USA
2Department of Veterinary Diagnostic and Production Animal Medicine, Iowa State University, Ames, IA, 50011, USA
3Department of Agriculture and Natural Resources, University of California at Davis, Davis, CA, 95616 USA
Introduction:
Subclinical mastitis, most commonly due to non-aureus staphylococci (NAS), results in significant production losses within the dairy goat industry while also shortening the productive life of the animal due to the resultant chronic changes within the udder. The prevalence of this group of pathogens may be up to 50% in some dairies. Dry-off is a common production point to address these infections through the extra-label use of long-acting intramammary antimicrobial products which are approved for dairy cattle in the US. However, the impacts of this treatment on antimicrobial resistance development are unknown in goats. The aim of this study was to evaluate the impact of targeted dry therapy on phenotypic and genotypic antimicrobial resistance of NAS in dairy goats.
Materials and methods:
Antimicrobial sensitivity testing was performed using a commercially available broth microdilution plate containing drugs approved in the US for intramammary treatment of mastitis in dairy cattle. Sensitivity testing was performed on dry-off and post-kidding samples for isolates that failed to cure or developed new NAS infections after treatment with one of two 𝛽-lactam (cephaparin, CEPH; cloxacillin, CLOX) dry cow therapies or in non-treated control animals. Whole genome sequencing was also used to identify antimicrobial resistance genes commonly associated with NAS species.
Results:
The most prevalent NAS associated with non-cured infections for all three treatment groups was Staphylococcus epidermidis, while the most common NAS associated with new infections after kidding was S. equorum. There were small variations in MIC for multiple drugs when data was evaluated by treatment group and location, though these changes did not form a consistent pattern and are likely due to the small sample size and farm-level differences in pathogens. The minimum drug concentration of penicillin required to inhibit growth in 50% of isolates (MIC50) and 90% of isolates (MIC90) did not change for non-cured isolates after antimicrobial therapy across the entire study population. Presence of the blaZ gene was highly correlated with increased penicillin MIC. Isolates with the highest MIC values (>4 µg/mL) were all positive for the gene, while 79% of isolates with penicillin MIC <0.12 𝛍g/mL did not contain the blaZ gene.
Significance:
This study did not identify any significant changes in antimicrobial resistance gene profiles or MIC of treated isolates compared to non-treated to indicate resistance gene selection for 𝛽-lactams after intramammary dry treatment in dairy goats. Based on this data, there is no evidence to support the hypothesis of wide-spread selection of resistant NAS to penicillin after a single intramammary administration of CEPH or CLOX.
