Relationship between Clinical Uterine Findings, Therapy, and Fertility in the Mare

SIMPLE SUMMARY: After breeding, mares develop a natural inflammatory reaction of the uterus which then leads to reduced fertility in about 15% of the population. In our study, we analyzed the effects of the inflammatory reaction on fertility of the mares, but also the clinical uterine findings at the time of artificial insemination. Our results show that age and reproductive status of the mare, together with cervical tone, influence mare fertility. Moreover, using cooled-stored semen and performing the artificial insemination after ovulation leads to increased pregnancy rates. Accumulation of intrauterine fluid 12 h after insemination decreased chances of pregnancy, while treatment with oxytocin could slightly improve fertility of affected mares. Taken together, age and reproductive status, together with clinical findings, are useful parameters for assessment of fertility in mares. ABSTRACT: Persistent breeding-induced endometritis (PBIE) is a major cause of subfertility in horses and the susceptibility is increased by several factors. The aim of this study was to determine the effects of clinical uterine findings and PBIE therapies, respectively, on pregnancy rate in mares. The analysis included records from 220 mares (390 cycles) inseminated at an artificial insemination (AI) center in Switzerland. Gynecological examinations were performed repeatedly before and after AI to determine cervical tone, uterine edema, and intrauterine fluid accumulation. Pregnancy rate was lower (p < 0.001) in barren mares compared to mares of other reproductive status. A more flaccid cervix (p = 0.009) was observed at the time of ovulation in pregnant cycles, but there was no difference (p > 0.05) regarding uterine edema. Intrauterine fluid accumulation reduced pregnancy rate (p = 0.002). Oxytocin administration had beneficial effects on pregnancy rate (p = 0.015), especially for barren mares, while uterine lavage did not have any effect (p > 0.05). The results show that cervical tone and intrauterine fluid accumulation, but not its degree, are useful parameters for assessment of fertility in mares. Oxytocin treatment improved pregnancy rates in mares with PBIE while uterine lavage had a limited effect.