Generation of equine-specific local antibiograms: reproductive tract of mares and fillies in Western Australia.

Objective Overview

• This study generated a local antibiogram specific to the reproductive tract bacterial isolates from mares and fillies in Western Australia to guide veterinarians in selecting effective first-line empirical antimicrobials.
• It assessed the antimicrobial susceptibility patterns of bacterial infections commonly found in the equine reproductive tract, aiming to promote responsible antimicrobial use and reduce resistance development.

Background and Importance

• Antimicrobial resistance (AMR) is a growing global concern that threatens the success of treatments for bacterial infections.
• Antimicrobial stewardship (AMS) programs are designed to optimize antimicrobial use, ensuring continued effectiveness of these drugs.
• There was a lack of specific, evidence-based data on antimicrobial susceptibilities for bacteria isolated from reproductive infections in mares, particularly in Western Australia.
• This gap made it difficult for veterinarians to choose the most appropriate empirical (initial) treatments before culture and sensitivity results are available.

Study Aims

• To retrospectively analyze culture and sensitivity data from reproductive tract samples collected from mares and fillies in Western Australia over a five-year period (2015-2020).
• To generate an algorithmically derived antibiogram specific to these isolates, providing data-based guidance on antimicrobial selection for the equine reproductive tract.

Methods

• Retrospective review of 767 isolates cultured from reproductive tract swabs or fluid samples of mares and fillies between July 2015 and June 2020 in Western Australia.
• Data included bacterial species identification and sensitivity test results to various antimicrobials.
• An algorithm developed in R software was used to create an antibiogram, which summarizes susceptibility percentages of bacterial isolates to different antibiotics.
• The data were categorized into Gram-negative and Gram-positive bacteria to better analyze susceptibility patterns.

Key Results

• Most common isolates:
  • Escherichia coli (E. coli): 36% of isolates (277/767)
  • Streptococcus species: 31% of isolates (240/767)
• Gram-negative bacterial isolates’ susceptibility rates:
  • Enrofloxacin: 84% susceptible
  • Ceftiofur: 75% susceptible
  • Gentamicin: 72% susceptible
  • Tetracycline: 65% susceptible
• Gram-positive bacterial isolates’ susceptibility rates:
  • Ampicillin: 86% susceptible
  • Penicillin: 83% susceptible
  • Ceftiofur: 83% susceptible
  • Trimethoprim-sulfonamide (TMS): 76% susceptible
• A declining trend in susceptibility to ceftiofur was observed between the two analyzed time periods (2015-2017 vs 2018-2020):
  • Gram-negative susceptibility decreased from 79% to 69%
  • Gram-positive susceptibility decreased from 91% to 75%

Interpretation and Recommendations

• Ceftiofur and enrofloxacin, classified as critically important antimicrobials, showed high efficacy against Gram-negative isolates but should be reserved for use only after performing culture and sensitivity testing.
• To support antimicrobial stewardship and prevent further resistance, these drugs should not be used empirically unless justified.
• For empirical first-line treatment when culture results are not immediately available:
  • Gram-negative infections suspected to involve E. coli may be treated with tetracycline
  • If the causative agent is uncertain, gentamicin is recommended
  • Gram-positive infections may be treated empirically with ampicillin, penicillin, or trimethoprim-sulfonamide (TMS)
• There is a need for continual updating of the antibiogram to reflect evolving resistance patterns and to guide veterinarians in evidence-based treatment choices.

Implications for Veterinary Practice

• This study equips equine veterinarians in Western Australia with local susceptibility data, improving empirical antimicrobial selection specifically for reproductive tract infections in mares and fillies.
• By using this local antibiogram, practitioners can make more informed decisions, improving treatment success and minimizing unnecessary use of broad-spectrum or critically important antimicrobials.
• Regular antibiogram updates are essential to monitor trends in resistance and ensure ongoing relevance of the empirical treatment guidelines.