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.

Overview

• This study compares the efficiency of two methods for obtaining equine blastocysts in Quarter Horse mares—transvaginal oocyte aspiration (TVA) and post-mortem ovary harvest (OH)—both followed by Intracytoplasmic Sperm Injection (ICSI).
• Researchers evaluated various parameters including oocyte recovery, embryo development, pregnancy rates, and foaling success, focusing on factors such as mare age, embryo transfer timing, and type of embryo transfer.

Study Background and Purpose

• Equine ICSI allows conception by injecting a single sperm into an oocyte, useful for valuable or subfertile mares.
• Two common ways to collect oocytes for ICSI are TVA (collecting oocytes from live mare ovaries via needle aspiration) and OH (harvesting ovaries post-mortem and retrieving oocytes).
• Previous research mainly focused on Warmblood horses in Europe; this study focuses on North American Quarter Horses to assess method efficiencies and outcomes.

Methodology

• Retrospective analysis of 105 Quarter Horse mares undergoing 151 TVA-ICSI cycles and 42 mares undergoing OH-ICSI between 2023 and 2025.
• Measured parameters included:
  • Cumulus-oocyte complex (COC) recovery rates
  • In vitro maturation rates of oocytes
  • Embryo cleavage rates on day 5 after ICSI
  • Blastocyst production on days 7 to 10 post-ICSI
  • Blastocyst counts per session
  • Pregnancy rates at days 14, 28, and 45
  • Foaling outcomes
• Additional factors examined included:
  • Mare age groups (young: 3-10 years; middle-aged: 11-17 years; old: 18+ years)
  • Embryo transfer type (fresh vs. vitrified/warmed blastocysts)
  • Blastocyst developmental day at transfer (days 7, 8, 9, or 10)
  • Embryo transfer facilities

Key Findings: Oocyte Recovery and Embryo Development

• OH yielded a higher COC recovery rate (78%) compared to TVA (60%), indicating more oocytes retrieved from post-mortem ovaries.
• Contrarily, the in vitro maturation rate, cleavage rate, blastocyst production per mature oocyte, blastocyst per cleaved embryo, and blastocysts produced per cycle were significantly higher in TVA-ICSI than OH-ICSI.
• This suggests while OH recovers more oocytes, TVA produces higher quality oocytes capable of developing better embryos.

Influence of Mare Age

• Average follicle counts and oocytes obtained were similar across age groups.
• Pregnancy rates were significantly higher in young (64%-72%) and middle-aged mares (72%) compared to old mares (42%).
• Older mares (≥18 years) had a reduced chance of pregnancy at day 28.

Embryo Transfer and Pregnancy Outcomes

• Transfer of fresh vs. vitrified/warmed blastocysts showed similar pregnancy rates and early embryonic loss rates, indicating vitrification does not negatively impact pregnancy success.
• Blastocyst developmental day influenced pregnancy:
  • Day 7 blastocysts had the highest pregnancy rates (90% at day 14, 78% at day 28, 73% at day 45).
  • Later blastocysts (days 9 and 10) had lower pregnancy rates and higher risk of pregnancy loss.

Statistical Analysis and Predictors

• Logistic regression showed that producing more than 5 mature (MII) oocytes per ICSI session nearly doubled blastocyst production odds (OR 1.98).
• Advanced mare age (≥18 years) significantly lowered pregnancy likelihood at day 28 (OR 0.43), confirming age-related fertility decline.

Conclusions and Implications

• While OH recovers more oocytes, TVA leads to better embryo development outcomes in Quarter Horses, suggesting better oocyte quality or handling in TVA.
• Age is a critical factor impacting pregnancy success; younger mares have better reproductive outcomes.
• Embryo transfer of fresh or vitrified blastocysts is equally effective, allowing flexibility in clinical or breeding settings.
• Transferring blastocysts earlier in development (day 7) maximizes pregnancy rates.
• Clinicians should aim for higher mature oocyte numbers per cycle and consider mare age as a strong predictor of success in equine assisted reproduction.