Abstract
Background:
Endometritis is a major cause of subfertility in mares and may occur in both clinical and subclinical forms. Although transrectal ultrasonography is routinely used to evaluate uterine health, its sensitivity for detecting mild inflammatory changes is limited. Endometrial cytology can identify inflammatory cells directly and may reveal uterine pathology even when ultrasonographic findings appear normal.

Aim:
The aim of this study was to determine the incidence of positive uterine cytology in mares with normal ultrasonographic findings and to evaluate the relationship between cytological inflammation, bacteriological culture results, and reproductive outcomes.

Methods:
The study included 43 mares aged 4–20 years (mean ± SD: 10.6 ± 3.8 years) presented for reproductive evaluation at the Equine Clinic of the University of Agricultural Sciences and Veterinary Medicine, Cluj-Napoca. Only mares with normal uterine ultrasonographic appearance were included. Endometrial cytology and uterine bacterial culture were performed during estrus. Cytological samples were classified based on the number of polymorphonuclear neutrophils per high-power field.

Results:
Cytological evidence of endometrial inflammation was detected in 6 of 43 mares (14.0%), despite normal ultrasonographic findings. Bacterial culture was positive in 5 mares (11.6%). A significant association was observed between positive cytology and bacterial culture (OR = 72; p = 0.0005). Pregnancy rates were significantly lower in mares with positive cytology (p = 0.028).

Conclusion:
Subclinical endometrial inflammation may occur in mares with normal ultrasonographic uterine appearance. These findings highlight the importance of incorporating endometrial cytology into routine reproductive evaluations to improve the detection of uterine inflammation and support better fertility management in breeding mares.

Key words: Endometritis; Mare; Subclinical inflammation; Ultrasonography; Uterine cytology.