ISSN 2398-2977      

Ovary: ovulatory failure

pequis

Introduction

  • Ovulation failure may take one of 3 forms in the mare:
    • Transitional estrus: ovulation failure in transitional mares both at the start and end of the breeding season   Female: reproductive cycle - normal  .
    • Follicular atresia with ovulation failure at any stage during the breeding season.
    • Formation of an anovulatory hemorrhagic follicle (AHF) at any stage during the breeding season   Ovary: large ovary syndrome  .
  • The incidence of ovulation failure in the mare may be as high as 8% of estrous cycles.

Pathogenesis

Predisposing factors

General
  • Early and late breeding season.
  • Age: older mares are more predisposed to ovulatory failure due to formation of AHFs.
  • Mares which develop one AHF during a breeding season have an increased chance of producing another in the same year.

Pathophysiology

Transitional estrus
  • During spring and autumn transition to/from winter anestrus, mares develop waves of medium sized follicles which regress rather than going on to ovulate. 
  • Large, tense, fluid-filled multiple ovarian structures.
  • Probably the result of abnormal gonadotropin output at the ends of the breeding season.

Follicular atresia

  • The 3 cm diameter stage is the critical maturation point for the follicle at which there is a three-fold increase in estrogens and androgens.
  • The follicle becomes palpably firmer and smaller instead of softer and larger as expected for a maturing follicle.
  • Follicles which undergo atresia are usually non-responsive to hCG.

Formation of anovulatory hemorrhagic follicles

  • Etiology not fully understood. May include insufficient gonadotrophin stimulation to induce ovulation; insufficient estrogen production from the dominant follicle, or hemorrhage into the lumen of the pre-ovulatory follicle.
  • May be associated with high circulating concentrations of luteinizing hormone.
  • Some authors believe that non-echogenic AHFs in the mare are analogous to ovarian follicular cysts in the cow, and echogenic, ie luteinized, AHFs are analogous to luteal cysts in the cow   Ovary: large ovary syndrome  .

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Meyers P J et al (1997) Use of the GnRH analogue, deslorelin acetate, in a slow-release implant to accelerate ovulation in oestrous mares. Vet Rec 140 (10), 249-252 PubMed.
  • Allen W R, Mathias S, Lennard S N & Greenwood R E (1995) Serial measurements of peripheral estrogen and progesterone concentrations in oestrus mares to determine optimum mating time and diagnose ovulation. Equine Vet J 27 (6), 460-464 VetMedResource.
  • Ginther O J & Bergfelt D R (1993) Growth of small follicles and concentrations of FSH during the equine oestrus cycle. J Reprod Fertil 99 (1), 105-111 PubMed.

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