Post-breeding endometritis (i.e., inflammation/infection of the endometrium), is a physiological reaction taking place in the endometrium of mares within 48 h post-breeding, aimed to clear seminal plasma, excess sperm, microorganisms, and debris from the uterine lumen in preparation for the arrival of an embryo. Mares are classified as susceptible or resistant to persistent breeding-induced endometritis (PBIE) based on their ability to clear this inflammation/infection by 48 h post-breeding. Mares susceptible to PBIE, or those with difficulty clearing infection/inflammation, have a deficient immune response and compromised physical mechanisms of defense against infection. Molecular pathways of the innate immune response known to be involved in PBIE are discussed herein. The role of the adaptive uterine immune response on PBIE remains to be elucidated in horses. Advances in the pathobiology of microbes involved in PBIE are also revised here. Traditional and non-traditional therapeutic modalities for endometritis are contrasted and described in the context of clinical and molecular aspects. In recent years, the lack of efficacy of traditional therapeutic modalities, alongside the ever-increasing incidence of antibiotic-resistant microorganisms, has enforced the development of non-traditional therapies. Novel biological products capable of modulating the endometrial inflammatory response are also discussed here as part of the non-traditional therapies for endometritis.

Degenerative changes of the endometrium are directly related to age and fertility in mares. Chronic degenerative endometritis (CDE) is correlated with uterine fluid retention and reduced ability to clear uterine inflammation. Recent research in the areas of equine surgery and sports medicine has shown that platelet-rich plasma (PRP) treatment acts as an immunomodulator of the inflammatory response. Therefore, the aim of this study was to determine if the uterine infusion of PRP could modulate the local inflammatory response and modify the intrauterine NO concentrations after artificial insemination (AI) in both normal mares and those with CDE. Thirteen mares with endometrium classified as grade III on the histology (mares with CDE) and eight mares with endometrial histological classification I or II-a normal mares were selected to investigate the effect of PRP therapy. The mares were inseminated with fresh semen in two consecutive cycles in a crossover study design. Thereby, each mare served as its own control and the treatment was performed with intrauterine PRP infusion four hours after AI. The percentage of neutrophils in uterine cytology (CIT, %), uterine fluid accumulation observed on ultrasonography (FLU, mm) and nitric oxide concentration of uterine fluid (NO, μM) were analyzed before and 24 hours after AI. The results reported that mares with CDE (CIT, 68.3 ± 3.27, FLU, 10.7 ± 1.61) have a higher (P < 0.05) intrauterine inflammatory response after AI than normal mares (CIT, 24.4 ± 3.56, FLU, 0), but NO concentrations did not differ (P > 0.05) between categories of mares. In treated cycles with PRP, the intrauterine inflammatory response decrease (P < 0.05) in CDE mares (CDE: CIT, 31.4 ± 6.48, FLU, 5.5 ± 1.28; normal mares: CIT, 13.5 ± 4.31, FLU, 0) when compared with nontreated cycle (CDE: CIT, 68.3 ± 3.27, FLU, 10.7 ± 1.61; NM: CIT, 24.4 ± 3.56, FLU, 0), but did not modify NO concentrations in uterine fluid. Thus, we can conclude that PRP was effective in modulating the exacerbated uterine inflammatory response to semen in mares with CDE but did not reduce NO concentrations in intrauterine fluid.

Persistent mating-induced endometritis (PMIE) results in decreased fertility in horses, thereby causing a significant impact in the horse market. Platelet-rich plasma (PRP), a modulator of the inflammatory response, has been largely used in veterinary medicine. Here, we investigated the effects of PRP on uterine inflammation, conception rate, endometrial polymorphonuclear neutrophil (PMN) migration, and COX-2 protein levels in the endometrial tissue. Thirteen PMIE-susceptible mares were used for artificial insemination (AI). The mares were inseminated with fresh semen in three consecutive cycles in a cross-over study design. The following cycle classifications were used: control cycle, no pharmacological interference; pre-AI, 20 mL of PRP was infused 24 h before AI; and post-AI, 20 mL of PRP was infused four h after AI. Follicular dynamics were monitored daily by transrectal ultrasound. When a follicle larger than 35 mm was detected, ovulation was induced with deslorelin acetate (1 mg, im). AI was performed 24 h after ovulation induction. Intrauterine fluid (FLU) was evaluated by ultrasonography before and 24 h after AI. PMNs in uterine cytology (CYT) and biopsy (HIS) were also observed before and 24 h after AI. Pregnancy was determined within 14 days after ovulation. Number of COX-2 positive cells was evaluated by immunohistochemistry. Both PRP treatments resulted in a decrease of PMNs in the CYT after breeding when compared to controls. FLU did not differ between cycles; however, the conception rates were significantly higher in the PRP mares. Mares positive for endometritis decreased in both treatment groups, and a more intense positive COX-2 labeling was observed in the control group when compared to the two treatment groups. In conclusion, PRP beneficially reduces inflammatory response in PMIE mares independent of when treatments were administered, thus increasing chances of successful pregnancy.

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