Ovarian cysts

Etiology

• Follicular cysts:
  • Intraovarian cysts.
  • Form from failure of one or more Graafian follicles to ovulate or regress causing progressive dilation.
  • Likely to be hormonally active.
• Cystic rete ovarii:
  • Intraovarian cysts.
  • Arise from tubular structure extending from ovarian hilus to medulla.
  • Spontaneous formation and may be congenital.
  • Not thought to be hormonally active.
• Paraovarian cysts:
  • Extraovarian cysts.
  • Arise from mesometrium as remnants of mesonephric ducts.
  • Spontaneous or congenital.
  • Not thought to be hormonally active.

Predisposing factors

General

• Intraovarian cysts may develop from impaired ovulation due to stress, malnutrition or systemic disease.

Specific

• Intraovarian cysts:
  • Increasing age.
  • May develop concurrent to uterine disease, eg endometrial hyperplasia Endometrial hyperplasia or inflammatory uterine disorders.
• Extraovarian cysts: developmental abnormalities of the reproductive tract.

Pathophysiology

• Follicular cysts:
  • Rabbits are induced ovulators → lack of mating stimulus or systemic disease may impair LH surge.
  • One or more mature Graafian follicles develop → fail to undergo ovulation to form corpora lutea.
  • Progressive dilation resulting in ovarian enlargement → compression of surrounding structures.
  • Continued production of estrogens → secondary or concurrent uterine hyperplasia → inflammatory or neoplastic uterine disease Uterine adenocarcinoma may develop → impaired fertility and vulval discharge Vagina: discharge.
• Cystic rete ovarii:
  • Arise in the ovarian medulla or at tubal extremity of ovary → lined by ciliated columnar or cuboidal epithelial cells with external spindle-shaped stromal layer.
  • Expansion into ovarian stroma → ovarian enlargement → compression of surrounding structures and disruption of ovary.
  • Inciting cause and hormonal effects not well understood but tend to occur concurrent to uterine neoplasia or hyperplasia.
• Paraovarian cysts:
  • Arise in mesometrium and distinct from ovarian tissue → lined by epithelial tissue.
  • Mostly asymptomatic but may cause organ compression if very large.
  • Very rare in rabbits so cause is unclear.

Timecourse

• Intraovarian cysts:
  • Clinical signs develop in weeks to months depending upon cyst size, number and hormonal effects.
  • Often asymptomatic until very large.
• Paraovarian cysts: often asymptomatic so may be present lifelong.

Epidemiology

• Ovarian cysts are rare in rabbits but frequently develop concurrently with uterine disease and should always be suspected in rabbits with urogenital disorders.