Equis ISSN 2398-2977

Female: diestral ovulation

Contributor(s): Terry Blanchard, David Dugdale, Graham Munroe, Sarah Stoneham, Madeleine L H Campbell

Introduction

  • Ovulation in the presence of a functioning corpus luteum, without signs of estrus, is relatively common in cycling mares.
  • Diestral ovulations may occur between days 2 and 15 of the luteal phase.
  • These ovulations are frequently detected if mares in diestrus are examined regularly ultrasonographically.
  • Ovulation during diestrus can prolong diestrus as the recently developed corpus luteum is refractory to prostaglandin F2-alpha (which normally ends diestrus by causing luteolysis).
  • Cause: development of an ovarian follicle which ovulates during diestrus.
  • Signs: behavioral signs consistent with being in diestrus, ie not receptive to 'teasing'.
  • Diagnosis: ultrasonography, palpation.
  • Treatment: exogenous prostaglandins.
  • Prognosis: good to excellent.

Pathogenesis

Etiology

  • Development of an ovarian follicle which ovulates during diestrus.

Pathophysiology

  • Part of the normal female reproductive cycle Reproduction: management - female in mares.
  • Normal diestrus has an average duration of 16 days, and extends from ovulation to luteolysis.
  • Following ovulation, plasma progesterone concentration Endocrine: hormone assay - female increases from 0 ng/ml to 8 ng/ml by day 5. Peak concentration of 5-20 ng/ml is achieved between days 5 and 8 and maintained until days 14-15.
  • Progesterone exerts a negative feedback on gonadotropin releasing hormone (GnRH) which in turns decreases the concentration of luteinizing hormone (LH).
  • Follicle stimulating hormone (FSH) is unaffected and often peaks mid-diestrus.
  • This FSH peak can lead to development of follicles which ovulate in diestrus without signs of behavioral estrus.
  • Pregnancy is possible following AI of mares with diestral ovulation.
  • If normal prostaglandin release occurs when a new corpus luteum forms (days 11-15 of estrous cycle) as a result of a recent diestral ovulation, either complete luteolysis does not occur or luteolysis of the diestrous ovulation does not occur (because it is refractory to PGF2-alpha).
  • The immature corpus luteum is unaffected by prostaglandin and contributes more progesterone in the face of destruction of the older corpus luteum leading to prolongation of diestrus.
  • Similar circumstances may be responsible for failure of exogenous prostaglandin therapy (because the corpus luteum arising from the diestrus ovulation it is refractory to PGF2-alpha).

Diagnosis

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Treatment

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Outcomes

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Further Reading

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