Efficiency of transvaginal oocyte aspiration (TVA), post-mortem ovary harvest (OH), and Intracytoplasmic Sperm Injection (ICSI) in a Quarter Horse mare population in North America

In a clinical setting, Intracytoplasmic Sperm Injection (ICSI)-derived equine blastocysts can be obtained either by transvaginal oocyte aspiration (TVA) or post-mortem ovary harvest (OH). Most peer-reviewed retrospective reports have described the suitability of these techniques for obtaining blastocysts, pregnancies, and live foals, particularly in Warmblood horse populations in Europe. In this study, retrospective data from Quarter Horse client-owned mares subjected to TVA-ICSI (n = 105; 151 cycles) or OH-ICSI (n = 42) at an academic institution in North America between 2023 and 2025 were analyzed. The efficiency of TVA- or OH-ICSI, defined as cumulus -oocyte complex (COC) recovery; in vitro oocyte maturation; embryo cleavage (day 5 post-ICSI); blastocyst production (days 7–10 post-ICSI); number of blastocysts per session; pregnancy on days 14, 28, and 45; and foaling rates, was compared between TVA-ICSI and OH-ICSI. In addition, the effects of mare age, transferring fresh versus vitrified/warmed blastocysts, the day of development after ICSI (i.e., day 7, 8, 9 vs. 10), and the embryo transfer facility on pregnancy and early embryonic death rates were studied. Overall, a higher COC recovery rate was observed with OH (78 ± 16%) than with TVA (60 ± 15%; P < 0.05). In vitro oocyte maturation (65 ± 19% vs. 49 ± 22%), cleavage (61 ± 21 vs. 52 ± 30%), blastocyst per mature oocyte (24 ± 22 vs. 16 ± 22%), blastocyst per cleaved embryo (37 ± 33% vs. 22 ± 29%), and average number of blastocysts per cycle (1.6 ± 1.3 vs. 0.9 ± 1) were higher in TVA-ICSI than in OH-ICSI (P < 0.05). A total of 167 blastocysts were transferred in six commercial embryo transfer facilities. Differences between TVA-ICSI (n = 146 blastocysts) and OH-ICSI (n = 21 blastocysts) groups on pregnancy rates at days 14 (79 vs. 76%), 28 (62 vs. 57%), and 45 (58 vs. 48%), early embryonic loss rate at day 28 (22 vs 25%) or cumulative embryonic loss at day 45 (27 vs. 38%), and foaling rates (54 vs. 53%) were not observed (P > 0.05). Mare age (young [3 – 10-years-old] vs. middle-aged [11 – 17-years-old] vs. old [≥ 18-years-old]) affected the average number of follicles (14 vs. 16 vs. 15), COCs (9 vs. 11 vs. 9), MII oocytes per cycle (5 vs. 7 vs. 5), and pregnancy rates at day 28 (64% vs. 72% vs. 42%), and 45 (57% vs. 68% vs. 42%; P < 0.05), while an effect of mare age on IVM (59 – 63%), cleavage (57 – 61%), blastocyst rate (21 – 23%), and average number of blastocysts per cycle (1.4 – 1.6) was not observed (P > 0.05). Differences in pregnancy rates at day 14 (82 vs. 76%), 28 (65 vs. 58%), and 45 (60 vs. 54%), as well as embryonic loss rates at day 28 (20 vs. 24%) and 45 (28 vs. 29%) were not observed between fresh versus vitrified/warmed transferred blastocysts (P > 0.05). Pregnancy rates on days 14, 28, and 45 were higher for day 7 (90%, 78%, and 73%) than for day 9 (69%, 47%, and 41%) or day 10 (60%, 35%, and 35%) blastocysts (P < 0.05). An effect of the embryo transfer (ET) facility on pregnancy or embryonic loss rates was not detected (P > 0.05). Based on logistic regression analyses, for the population of mares in this study, the number of injected MII oocytes (> 5 MII; OR: 1.98; CI: 1.35 – 4.01) and mare age (≥ 18-years-old; OR: 0.43; CI: 0.12 – 0.87]) increased the likelihood of producing a blastocyst per ICSI session or negatively affected the likelihood of pregnancy at day-28, respectively.