Bacterial Isolates and Antimicrobial Susceptibility Profiles in Mares with Endometritis in Zhangye, China

Equine endometritis impairs fertility, but regional data remain scarce in Zhangye, Gansu Province, China, a major production base for equine serum-derived biological products. This cross-sectional study investigated endometritis and associated bacterial isolates in 985 mares from five intensive horse farms. Mares with reproductive abnormalities underwent clinical, ultrasonographic, and cytological evaluation. Laboratory investigations included culture of low-volume uterine flush samples, biochemical testing, 16S rRNA gene sequencing, phylogenetic analysis, and antimicrobial susceptibility testing. Among the 156 mares selected for further evaluation, 35 were diagnosed with endometritis. This corresponded to an apparent prevalence of 3.6% in the full surveyed population (35/985; 95% confidence interval, 2.5–4.9%) and a detection rate of 22.4% in the selected subgroup (35/156; 95% confidence interval, 16.2–29.8%). Clinical manifestations were generally subtle. Thirty culture-positive samples yielded 38 isolates across six genera. Escherichia coli, Streptococcus equi subsp. zooepidemicus, and Staphylococcus aureus predominated. A single bacterial taxon was recovered from 22 (73.3%) culture-positive mares, whereas multiple bacterial taxa were recovered from eight (26.7%). Escherichia and Streptococcus isolates showed relatively high resistance, whereas Staphylococcus and Bacillus isolates exhibited lower overall resistance. Limited geographical coverage, small isolate numbers, incomplete antimicrobial-use records, and unexamined resistance genes constrained interpretation. These findings support early screening of mares with reproductive disorders and antimicrobial selection guided by culture and susceptibility testing.