ISSN 2398-2977      

Female: hormonal therapy

pequis

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   Urogenital: rectal palpation  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.

Estrus synchronization

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Use of prostaglandins

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Transitional mares

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Termination of pregnancy

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Silent heat

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • McCue P M, Logan N L & Magee C (2007) Management of the transition period: hormone therapy. Equine Vet Educ 19 (4), 215-221.
  • Irvine C H G, Sutton P, Turner J E & Mennick P E (1990) Changes in plasma progesterone concentrations from days 17 to 42 of gestation in mares maintaining or losing pregnancy. Equine Vet J 22 (2), 104-106 PubMed.
  • Lofstedt R M (1988) Control of estrus cycle in the mare. Vet Clin North Am 4, 2 PubMed.
  • Penzhorn B L, Bertschinger H J & Coubrough R I (1986) Reconception of mares following termination of pregnancy with prostaglandin F2alpha before and after day 35 of pregnancy. Equine Vet J 18 (3), 215-217 PubMed.

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