The limited fertility of jennies inseminated with frozen donkey semen remains a major challenge for donkey reproduction and conservation, contrasting with the more favorable outcomes observed in mares. Species-specific differences in post-breeding uterine inflammation have been proposed as a key factor underlying this disparity. This study compared the uterine inflammatory response and pregnancy outcomes of jennies and mares inseminated with frozen donkey semen. Twelve jennies and eleven mares were monitored over 48 and 44 estrous cycles, respectively. Artificial insemination (AI) was performed immediately after ovulation (T0) using 1 x 10 9 frozen–thawed spermatozoa deposited deep into the uterine horn. Endometrial edema (EE), intrauterine fluid (IUF), vascular perfusion, serum progesterone (P4), and acute-phase proteins (APPs) were assessed at T0, T6, T24, T48, T72, and T120. Endometrial cytology was conducted at T0, T6, T24, and T48, while uterine biopsies were obtained at T0, T12, and T48. Pregnancy was diagnosed on Day 14 post-AI. Jennies exhibited consistently lower ultrasonographic edema and intrauterine fluid accumulation than mares, yet developed a markedly stronger and more persistent endometrial inflammatory response. Neutrophil recruitment peaked in both species at 6 h post-AI, but inflammation resolved by 48 h only in mares. In jennies, inflammation persisted and was characterized by pronounced eosinophilic infiltration, a feature absent in mares. No differences were detected between species for APP concentrations or COX-2 expression. P4 concentrations were higher in jennies on Day 5 post-ovulation (13 vs. 6 ng/mL). Pregnancy outcomes differed sharply: mares achieved 54.55% pregnancy, whereas jennies showed 0%. In summary, jennies exhibit an intense and prolonged uterine inflammatory response to frozen donkey semen, distinguished by eosinophilic infiltration and delayed resolution. These species-specific differences likely contribute to the extremely low fertility observed in jennies and highlight the need for targeted reproductive strategies in asinine breeding programs. • Jennies showed delayed uterine inflammatory resolution. • Jennies exhibited marked uterine neutrophilia and eosinophilia. • Inflammation was local, without acute-phase protein changes. • Progesterone production was not impaired by inflammation. • Mares had higher pregnancy rates with frozen donkey semen.
Sancler-Silva et al. (Sun,) studied this question.