ISSN 2398-2977      

Ovary: large ovary syndrome

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Synonym(s): Enlarged ovary(s


Introduction

  • Cause: there are various causes of ovarian enlargement in the mare; it may be either unilateral or bilateral. 
  • Signs: an affected mare is frequently presented for signs related to hormone-dependent aberrant behavior and/or an abnormal reproductive cycle, but it is also common for ovarian enlargement to exist in the absence of external clinical signs. 
  • Diagnosis: depends upon careful history-taking, clinical examination and sequential ultrasound examinations, confirmed by appropriate laboratory analyses.
  • Treatment: depends on cause.
  • Prognosis: generally good.

Pathogenesis

Etiology

  • Physiologically enlarged ovaries: the presence of many multiple medium or large-sized follicles on an ovary may increase the overall size of that ovary. This is a normal, physiological phenomenon during vernal transition (when follicular development is being driven by follicle stimulating hormone) and during pregnancy (when development of follicles and secondary corpora lutea is being driven by equine chorionic gonadotrophin (eCG). Mares also sometimes happen to have several large follicles on one ovary during normal reproductive cyclicity.
  • Anovulatory hemorrhagic follicles: 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.
  • Ovarian neoplasia   Ovary: neoplasia - overview  : unknown.
  • Ovarian hematoma: excessive hemorrhage into the follicular lumen following ovulation: effectively a huge corpus hemorrhagicum.
  • Ovarian abscess: often associated with previous ovarian puncture/needle aspiration, eg for oocyte retrieval.

Predisposing factors

General
  • Physiologically enlarged ovaries: vernal transition   Ovary: ovulatory failure  ; pregnancy.
  • Anovulatory hemorrhagic follicles: not fully understood. More common in the spring and autumn than during the height of the breeding season. Use of exogenous human chorionic gonadotrophin (hCG) may be associated with an increased risk of AHF formation. Use of exogenous prostaglandin F2± (PGF2±) may be associated with an increased risk of AHF formation.
  • Ovarian neoplasia: none known.
  • Ovarian hematoma: none known.
  • Ovarian abscess: previous ovarian puncture/needle aspiration, eg for oocyte retrieval.

Pathophysiology

  • Physiologically enlarged ovaries: none - physiological.
  • Anovulatory hemorrhagic follicles: development of an hemorrhagic follicle which fails to ovulate means that the mare cannot conceive. AHFs are frequently also associated (if luteinized and producing progesterone) with a delayed return to estrus.
  • Ovarian neoplasia: 
    • GCTs originate from sex cord and ovarian stromal tissue. Most are benign and unilateral although bilateral and malignant forms are occasionally reported. Behavioral signs are dependent upon hormone production by the tumor. GCTs produce inhibin which suppresses FSH production by the anterior pituitary, which in turn suppresses activity in the contralateral ovary, which is characteristically small (rare cases of GCT in a cycling mare have been reported, it is thought that this is due to an early stage of GCT development and that suppression of follicular activity would follow in time). 
    • Teratomas are benign and arise from germ cells. 
    • Dysgerminomas are malignant tumors of the germ cells: aggressive, metastasize quickly. 
    • Cystadenoma: benign epithelial tumor. 
  • Ovarian hematoma the normal hemorrhage into the rupturing follicle at ovulation is excessive.
  • Ovarian abscess: infection.

Timecourse

  • Physiologically enlarged ovaries: mares in vernal transition may have bilaterally enlarged ovaries for several weeks, until the first ovulation of the year occurs and cyclicity becomes established. Pregnant mares have bilaterally enlarged ovaries once follicles and secondary corpora lutea begin to form under the influence of eCG, ie from c day 40 onwards.
  • Anovulatory hemorrhagic follicles: develop over one to three days at the end of estrus - variable duration.
  • Ovarian neoplasia: chronic.
  • Ovarian hematoma: occurs at the time of ovulation - resolves over weeks.
  • Ovarian abscess: chronic.

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.
  • Cuervo-Arango J & Newcombe J (2009) The effect of hormone treatments (hCG and cloprostenol) and season on the incidence of hemorrhagic anovulatory follicles in the mare: A field study. Theriogenology 72 (9),1262-7 PubMed.
  • Smith L J & Mair T S (2008) Unilateral and bilateral laparoscopic ovariectomy of mares by electrocautery. Vet Rec 163 (10), 297-300 PubMed.
  • Lloyd D, Walmsley J P, Greet T R et al (2007) Electrosurgery as the sole means of haemostasis during the laparoscopic removal of pathologically enlarged ovaries in mares: a report of 55 cases. Equine Vet J 39 (3), 210-214 PubMed.
  • McCue P M & Squires E L (2002) Persistent anovulatory follicles in the mare. Theriogenology 58, 541-543 VetMedResource.
  • Ramirez S, Sedrish S A, Paccamonti D L & French D D (1999) Ultrasound as an aid for diagnosis of ovarian abscesses in two maresVet Radiol Ultrasound 40 (2), 165-168 PubMed.
  • Pugh D G & Bowen J M (1985) Equine ovarian tumors. Comp cont Educ pract Vet 7, 710-716 VetMedResource.

Other sources of information

  • McCue P (1998) Review of Ovarian Abnormalities in the Mare. In: Procs AAEP. pp 125-133.
  • Bosu W T K & Smith C A (1993) Ovarian Abnormalities. In: Equine Reproduction. Eds: McKinnon A O & Voss J L. Lea & Febiger, USA. pp 397-403.

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