Aerobic uterine isolates and antimicrobial susceptibility in mares with post-partum metritis

Abstract

Background: The potential for life-threatening complications of metritis requires prompt initiation of antimicrobial treatment, often before microbiological test results are reported. However, published studies to guide first-line antibiotic selection are scarce.

Objectives: To report the most frequent bacterial species, antimicrobial susceptibility and prevalence of multidrug resistance (MDR) in cases of equine metritis.

Study design: Retrospective analysis of uterine bacterial culture and antimicrobial susceptibility test (AST) results from mares with metritis.

Methods: Data from uterine culture and AST from 45 mares and 88 bacterial isolates were analysed. The frequency of single and mixed infections, bacterial species and susceptibility to antimicrobials was reported.

Results: Mixed growth (62.2% mares) was more frequent than pure growth. The most commonly isolated bacterial species was Escherichia coli (30.7%) and mares with mixed growth most commonly grew a combination of Gram-negative and Gram-positive bacteria (65.5%). Gentamicin with penicillin was an appropriate choice for 65.1% of the mares. Trimethoprim/sulfonamide was effective in only 48.8% of the mares. Effective antimicrobial therapy was provided by the combination of penicillin with amikacin (90.7%) or with enrofloxacin (81.4%). Some organisms were also sensitive to tetracyclines, cephalosporins and chloramphenicol. MDR was more frequent in Gram-negative (85.4%) than Gram-positive bacteria (23.5%).

Main limitations: Low number of bacteria, limited number of veterinary specific interpretive criteria for equine metritis.

Conclusions: Mixed infections were common in equine metritis. Gram-negative bacteria and enterococci were often associated with resistance to the most frequently used antimicrobials. Potentiated sulfonamides are not an appropriate first choice for mares with metritis. While the combination of penicillin and gentamicin may be an appropriate first-line treatment for some mares, use of amikacin or enrofloxacin instead of gentamicin may be preferential.

Keywords: antibiotics; horse; metritis; multidrug resistance; post-partum; susceptibility.