Equine Infectious Endometritis—Clinical and Subclinical Cases

• We examined the prevalence of subclinical endometritis in mares before breeding.
• We compared uterine samples collected before and after breeding.
• A total of 28.6% of clinically normal mares had subclinical endometritis before breeding.
• Endometrial edema score can be used as a diagnostic indicator of endometritis.

Equine endometritis continues to be a major cause of subfertility and infertility in mares, and recently, the subclinical form of endometritis has been in focus. The purposes of this study were to investigate the prevalence of subclinical infectious endometritis, defined as mares without clinical symptoms indicating endometritis, but with bacterial growth and positive cytology from an endometrial sample, and the relation to clinical parameters before and after artificial insemination (AI). A total of 76 Standardbred mares submitted for AI were included in the study, in which a uterine sample was collected before (biopsy) and after AI (swab) for bacterial culture, cytology, and histology. The results showed that 28.6% of clinically normal mares, submitted for breeding with AI, had subclinical infectious endometritis before breeding. The endometrial edema score early in estrus was a strong diagnostic indicator of a subclinical infection with Streptococcus equi subsp. zooepidemicus (odds ratio, 5.48; P < .0001). An altered endometrial edema pattern in a mare showing increased edema could therefore imply a subclinical uterine infection, and therefore, further examination of the mare should be conducted to rule out infectious endometritis. Sensitivity, specificity, positive predictive value, and negative predictive value of the cytology smear compared with the presence of polymorphonuclear neutrophils (PMNs) on histology were 0.78, 0.84, 0.72, and 0.87, respectively. A moderate agreement (k = 0.60) was found between the cytologic response of the smear and the presence of PMNs on histology.