Female: hormonal therapy

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Induction of ovulation

Significance
  • See also mare - hormonal analysis  [Endocrine: hormone assay - female]  .
  • Induction of ovulation decreases the number of inseminations/mare and conserves the stallion's libido, semen quality and energy.
  • Decreasing number of insemination events minimizes the chances for uterine infection in high risk mares.
  • It is common for the first ovulation in the breeding season to occur at the ends of a very long estrus period with follicles persisting many days before ovulation.
  • Hastening of these ovulations is frequently desirable, not always possible.
  • Follicular atresia with ovulation failure may occur at any stage during the breeding season  [Ovary: ovulatory failure]  non-responsive to hormone treatment.

Treatment

Human chorionic gonadotropin (hCG)

  • hCG has LH-like activity and is obtained from the urine of pregnant women.
  • It is the most efficient agent (as deslorein) for the induction of ovulation in the mare.
  • hCG has no effect on an inactive ovary or one with smaller follicles (<30-35 mm in diameter). Therefore, it must be used as part of a complete fertility management program for optimal benefit.
  • Doses of 1500-3000 iu   →   ovulation within 48 h (follicle >30-35 mm diameter).
  • Can be given IV or IM.
  • Higher dose rates may be required early in the breeding season and the response is more varied.
  • hCG is routinely used on some studs to correlate breeding with ovulation.
  • It may be used to hasten ovulation therapeutically where failure to ovulate is anticipated upon rectal examination.
  • Due to the large molecular size of the protein in hCG, antibodies may be produced and anaphylaxis is a possibility.
  • hCG needs to be given for 3-5 estrous cycles, however, before significant antibodies develop and antibodies have not been shown to persist until the following breeding season.
  • There is no cross-reactivity with natural LH.

Luteinizing hormone (LH)

  • Extracted from the pituitary glands of domestic animals.
  • Less consistent results than with hCG.

Gonadotropin-releasing hormone (GnRH)

  • 0.04 mg is administered intramuscularly 6 hours before mating.
  • Therapy with GnRH may be indicated where serial rectal palpations    suggest that a previously maturing follicle has become arrested in its development (GnRH may be more successful than hCG in the induction of ovulation in such follicles, and also in large, thick-walled follicles).
  • Deslorelin is a GnRH analog available as a biocompatible short-term implant.
  • Has been effective in inducing ovulation within 48 h in mares with follicles greater than 30 mm and showing characteristic signs of behavioral or physiologic estrus.
  • Smaller molecular size suggests they should be less antigenic.
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