Streptococcus equi subspecies zooepidemicus endometritis in mares: culture, cytology, and antimicrobial susceptibility tests

Infectious endometritis is a significant cause of subfertility in broodmares, leading to devastating economic and emotional effects on horse owners and breeders each year. Aims of this observational, retrospective study were to report the percentage of positive uterine cultures and specific pathogens, evaluate the relationship between uterine culture and cytology results, and describe the results of antimicrobial susceptibility tests for Streptococcus isolates at an equine reproduction center in Colorado. We hypothesized that the most common bacterial pathogen would be Streptococcus sp. and that all Streptococcus isolates would be sensitive to beta-lactam antibiotics (i.e. penicillin G and ceftiofur), but not all isolates would be sensitive to other antibiotics. A double-guarded uterine swab and brush were used to collect 622 paired samples for culture and cytology, respectively, from mares at Colorado State University between 2017 and 2019. Swabs were plated onto Spectrum TM 4-Part (Colorado) plates (Vetlab Supply, Palmetto Bay, FL), which consisted of MacConkey agar, Tryptic Soy agar with 5% sheep blood, Chromogenic Gram-positive agar, and Chromogenic Gram-negative agar. Antimicrobial susceptibility tests were performed using the Kirby-Bauer disk diffusion method, incorporating amikacin, ceftiofur, ciprofloxacin, enrofloxacin, gentamicin, and penicillin G. Cytology slides were stained with a modified Wright-Giemsa stain (Astral Diagnostics Inc., West Deptford, NJ) and evaluated under 400 and 1000 x microscopy. A total of 513 cultures (82.5%) exhibited no growth of microbial pathogens, whereas 109 cultures (17.5%) had growth of 1 or more pathogens. Most common bacterial pathogens were Streptococcus spp. (73.4% of positive cultures), Escherichia coli (34.9%), Klebsiella spp. (5.5%), Pseudomonas spp. (3.7%), Staphylococcus spp. (0.9%), and Enterococcus faecalis (0.9%). A total of 20 positive cultures (18.3%) exhibited growth of more than 1 pathogen. A positive cytology was noted on 32.3% of samples collected from mares with a Streptococcus spp. culture and 42.1% of samples associated with an Escherichia coli culture. Antimicrobial susceptibility tests were performed on 38 isolates of Streptococcus spp. Percentages of susceptible isolates were as follows: amikacin (29.7%), ceftiofur (100%), ciprofloxacin (75%), enrofloxacin (61.1%), gentamicin (66.7%), and penicillin G (100%). In summary, Streptococcus spp. and Escherichia coli were the most common bacterial pathogens isolated. All Streptococcus isolates were susceptible to beta-lactam antibiotics. Use of antimicrobial agents to treat equine uterine infections should be based on antibiotic stewardship principles.

Keywords: Mare, uterine, culture, cytology, endometritis, antibiotics

This manuscript was originally published in the journal Clinical Theriogenology Vol 12(3) Sept 2020.  Clinical Theriogenology is the official journal of the Society for Theriogenology (SFT) and the American College of Theriogenologists (ACT).  This content has been reproduced on the IVIS website with the explicit permission of the SFT/ACT.