Evidence-based interpretation of uterine cultures in mares: Linking bacterial growth patterns to endometrial inflammation

We evaluated uterine bacterial growth patterns and their association with endometrial inflammation (EI) in equine samples collected using uterine swabs (US) or low-volume uterine lavage (UL). A database including 1,545 US and 2,066 UL with cytological and bacteriological results was retrospectively analysed. Endometrial inflammation was defined as ≥2 polymorphonuclear cells per high-power field, and bacterial culture was considered positive when aerobic growth occurred within 48 h, with the number of bacterial isolates per sample recorded (0, 1, 2, or ≥3). Data were analysed using generalized mixed-effects models including the number of isolates and bacterial species as fixed effects. The prevalence of EI was 4.8% for US and 36.6% for UL. For both techniques, EI prevalence was lower in samples with negative bacterial cultures (1.0 ± 0.3% for US and 10.7 ± 1.7% for UL) compared with samples yielding ≥1 bacterial isolate. In UL samples, isolation of a single bacterial species was associated with greater probability of EI (45.1 ± 2.6%) compared with samples yielding ≥3 isolates (33.3 ± 3.1%). For UL, the presence of Streptococcus sp. (β-haemolytic) and Staphylococcus aureus increased the probability of EI compared with their absence, whereas in US samples only Streptococcus sp. (β-haemolytic) increased EI probability. In conclusion, bacterial growth increased the likelihood of EI for both sampling techniques, with Streptococcus sp. (β-haemolytic) as the primary bacteria associated with evidence of EI. The integration of endometrial cytology with bacterial culture and pathogen identification improves interpretation of bacteriological findings and supports responsible antimicrobial use in mares.