Effects of follicular aspiration and flushing, and the genotype of the fetus on circulating progesterone levels during pregnancy in the mare.

This study undertakes an examination of the effects of ultrasound-guided follicle aspiration (a procedure performed to obtain oocytes for in vitro fertilization) in pregnant mares, specifically looking at how the varying methodologies of the procedure can impact the levels of progesterone in the blood—a hormone critical to the maintenance of pregnancy. The research also delves into the difference between progesterone levels in mares carrying intra-species (horse) and inter-species (mule) fetuses.

Research Methodology

• The study featured the comparison of three different forms of transvaginal ultrasound-guided follicle aspiration or manipulation treatments where:
  • Treatment 1 had no aspiration and involved 4 mares.
  • Treatment 2 involved aspiration only of follicles that were greater than or equal to 20mm in diameter, involving 7 mares.
  • Treatment 3, also with 7 mares, involved aspiration of all visible follicles.
• A total of 18 mares took part in the study: nine of them carrying intra-species horse conceptuses and the other nine carrying inter-species mule conceptuses.
• All the follicle aspiration treatments were performed at peak follicular wave periods as determined by transrectal ultrasonography that was done on alternate days.

Research Findings and Observations

• The plasma progesterone profile of mares in Treatment 1 was found to be consistent with previous studies on pregnant mares.
• In Treatments 2 and 3, a decline in plasma progesterone levels was observed after Day 53 of gestation, suggesting that the follicular aspiration treatments interfered with the formation of secondary corpora lutea—the endocrine structure participating in maintaining the pregnancy.
• However, despite this observation, the drop in progesterone levels among these mares did not reach levels considered detrimental to the pregnancy.
• Mares carrying mule conceptuses exhibited inconsistently higher mean plasma progesterone concentrations at varied periods of gestation compared to those carrying horse conceptuses. Progesterone levels were higher between Days 39 and 45, equal from Day 46 to 66 and then lower from Day 67 to 150.
• Early progesterone level decline in mares carrying mule conceptuses correlates with the earlier disappearance of Gonadotropin from the blood circulation compared to mares carrying horse conceptuses.
• Hence, the results of the study suggest that the primary corpus luteum in a pregnant mare may have a higher sensitivity to mule conceptus Gonadotropin than to horse conceptus Gonadotropin.