Cystic Ovarian Disease (Follicular and Luteal Cysts)

Cystic ovarian disease (COD) in the female dog and cat is a pathological condition characterized by the presence of one or more fluid-filled structures within the ovarian parenchyma, exceeding 1 cm in diameter, that persist beyond the normal follicular or luteal phase. These cysts are classified as follicular cysts (arising from unovulated follicles that continue to grow and produce estrogen) or luteal cysts (derived from follicles that have luteinized without ovulation, producing progesterone). In the bitch, COD is often associated with persistent estrus, prolonged proestrus, or anestrus, and may lead to cystic endometrial hyperplasia (CEH) and pyometra. In the queen, follicular cysts are more common and can cause persistent estrus. The condition disrupts normal cyclicity, fertility, and can predispose to secondary uterine pathology. Accurate diagnosis relies on ultrasonography, hormonal assays, and histopathology. Management may involve medical therapy with GnRH or hCG to induce ovulation or luteinization, or surgical intervention via ovariohysterectomy in non-breeding animals.

The exact etiology of cystic ovarian disease is multifactorial. Primary causes include endocrine imbalances, particularly an abnormal surge of luteinizing hormone (LH) or a deficiency in the preovulatory LH peak, leading to failure of ovulation. Follicular cysts may arise from anovulatory follicles that continue to produce estrogen due to insufficient LH stimulation. Luteal cysts result from luteinization of unovulated follicles, often due to partial LH action. Genetic predisposition has been suggested in certain breeds, such as the Boxer and German Shepherd. Iatrogenic causes include the administration of exogenous hormones, such as progestins or androgens, which can disrupt follicular development. Environmental factors, including stress, poor nutrition, and obesity, may contribute. In some cases, ovarian neoplasia (e.g., granulosa cell tumors) can mimic or coexist with cysts. Infectious agents are not primary causes, but secondary bacterial infection of the uterus can occur due to prolonged estrogen or progesterone exposure.

Cystic ovarian disease is relatively uncommon in dogs and cats compared to other domestic species. In dogs, it is most frequently diagnosed in middle-aged to older intact females, with a median age of 6-8 years. Certain breeds, including the Boxer, German Shepherd, Golden Retriever, and Labrador Retriever, appear to be overrepresented. Nulliparous females may be at higher risk. In cats, follicular cysts are more common in young to middle-aged queens, with no strong breed predisposition. The condition is often diagnosed incidentally during routine ovariohysterectomy or during evaluation for infertility or abnormal estrous cycles. The incidence is higher in animals with a history of irregular cycles or those receiving exogenous hormone therapy. There is no significant sex predilection as it affects females only.

The pathophysiology of cystic ovarian disease involves disruption of the normal follicular development and ovulation process. In a normal estrous cycle, a surge of LH triggers ovulation of mature follicles. In COD, this surge is either absent, insufficient, or mistimed, leading to follicular persistence. Follicular cysts continue to produce estrogen, resulting in prolonged proestrus or estrus, and clinical signs such as vulvar swelling, serosanguinous discharge, and attraction of males. Luteal cysts, on the other hand, produce progesterone, leading to prolonged diestrus or anestrus, and may cause signs of pseudopregnancy. The persistent estrogen or progesterone exposure can induce cystic endometrial hyperplasia (CEH) in the uterus, characterized by proliferation of endometrial glands and cystic changes, which predisposes to pyometra. In cats, follicular cysts cause persistent estrus, which can lead to bone marrow suppression and anemia if prolonged. The cysts may also undergo torsion or rupture, causing acute abdominal pain.

Predisposing factors for cystic ovarian disease include age (middle-aged to older), breed (Boxer, German Shepherd, Golden Retriever), nulliparity, and a history of irregular estrous cycles. Endogenous hormonal imbalances, such as hypothyroidism or hyperadrenocorticism, may contribute. Exogenous hormone administration, particularly progestins used for estrus suppression, can disrupt ovarian function. Obesity and poor nutrition may affect endocrine regulation. Stress and environmental factors, such as overcrowding or changes in routine, can also play a role. Genetic predisposition is suspected due to breed prevalence. Additionally, ovarian neoplasia, such as granulosa cell tumors, can produce cysts as part of the tumor's structure, and these may be mistaken for simple cysts.

Clinical signs of cystic ovarian disease vary depending on the type of cyst. Follicular cysts cause persistent or prolonged estrus, with signs including vulvar swelling, serosanguinous to sanguinous vaginal discharge, attraction of males, and behavioral changes such as increased urination and restlessness. The bitch may be receptive to males but fail to ovulate, leading to infertility. Luteal cysts cause prolonged diestrus or anestrus, with signs of pseudopregnancy, such as mammary gland enlargement, milk production, and behavioral changes like nesting and aggression. In cats, follicular cysts cause persistent estrus, with vocalization, rolling, and lordosis. Systemic signs are rare unless complications such as pyometra or ovarian torsion occur, which may present with lethargy, anorexia, vomiting, and abdominal pain. On palpation, an enlarged ovary may be detected, but this is often difficult in obese animals.

Differential diagnoses for cystic ovarian disease include: 1) Ovarian neoplasia (granulosa cell tumor, teratoma, adenocarcinoma) – may present with similar hormonal signs and ovarian enlargement; ultrasonography and histopathology are definitive. 2) Pyometra – often associated with CEH and may have similar systemic signs; uterine distension on imaging and vaginal discharge are key. 3) Vaginitis – causes vaginal discharge but no ovarian changes. 4) Foreign body or trauma – may cause vulvar discharge. 5) Pregnancy – can cause abdominal distension and mammary changes; ultrasonography will show fetuses. 6) Pseudopregnancy – occurs after estrus and may mimic luteal cysts; hormonal assays and ultrasonography differentiate. 7) Endometritis – uterine infection without cystic ovaries. 8) Ovarian remnant syndrome – after ovariohysterectomy, residual ovarian tissue can cause estrus-like signs. 9) Ectopic ovarian tissue – rare. 10) Hypothyroidism – can cause reproductive abnormalities but not ovarian cysts.

The diagnostic algorithm for cystic ovarian disease begins with a thorough history and physical examination, including vaginal cytology to assess the stage of the estrous cycle. If persistent estrus or abnormal cycles are noted, serum progesterone and estradiol concentrations should be measured. Progesterone levels <1 ng/mL with high estradiol suggest follicular cysts; progesterone >2 ng/mL with low estradiol suggests luteal cysts. Abdominal ultrasonography is the imaging modality of choice to visualize ovarian structures; cysts appear as anechoic, thin-walled structures >1 cm. Serial ultrasonography may be needed to confirm persistence. If the diagnosis is uncertain, laparoscopy or exploratory laparotomy with ovarian biopsy may be performed. In cases where pyometra is suspected, additional imaging of the uterus and blood work are indicated. Definitive diagnosis is made by histopathology after ovariohysterectomy.

Laboratory findings in cystic ovarian disease are variable. Serum progesterone: in follicular cysts, progesterone is typically <1 ng/mL; in luteal cysts, it is >2 ng/mL. Serum estradiol may be elevated in follicular cysts (>20 pg/mL). LH and FSH may be low due to negative feedback. Hematology may be normal unless pyometra is present, in which case leukocytosis with a left shift and toxic neutrophils may be seen. Biochemistry may show elevated liver enzymes if hepatic lipidosis occurs secondary to prolonged estrogen exposure. Vaginal cytology: in follicular cysts, cytology shows >90% superficial and cornified cells, with no neutrophils; in luteal cysts, cytology shows a shift to intermediate and parabasal cells, with possible neutrophils. Uterine and vaginal cultures may be performed if infection is suspected, but are not diagnostic for cysts.

Abdominal ultrasonography is the primary imaging modality. Follicular cysts appear as multiple, thin-walled, anechoic structures within the ovary, typically >1 cm in diameter. Luteal cysts may have a thicker wall and more echogenic content. The uterus may show signs of CEH, such as thickened endometrium or cystic endometrial changes. In cases of pyometra, the uterus is distended with hypoechoic fluid. Radiography is less useful but may show an enlarged ovarian silhouette if the cyst is large. CT and MRI are not commonly used but can provide detailed anatomy. Vaginoscopy may be performed to rule out vaginal pathology, but is not essential for ovarian cyst diagnosis.

Vaginal cytology is a key diagnostic tool. In follicular cysts, the smear shows a high percentage of superficial and anuclear cornified cells, with no neutrophils, indicating estrogenic stimulation. In luteal cysts, the smear shows a mixture of intermediate and parabasal cells, with possible neutrophils, indicating progestogenic influence. Fine-needle aspiration of an ovarian cyst may yield fluid that can be analyzed for hormone levels, but this is rarely performed. Histopathology of the ovary after ovariohysterectomy reveals the cyst lining: follicular cysts are lined by granulosa cells, while luteal cysts are lined by luteinized granulosa and theca cells. The surrounding ovarian tissue may show evidence of ovulation failure, such as multiple corpora lutea or atretic follicles. Special stains, such as inhibin or cytokeratin, may be used to differentiate from neoplasia.

Treatment of cystic ovarian disease depends on the reproductive goals and the type of cyst. For breeding animals, medical therapy aims to induce ovulation or luteinization. For follicular cysts, administration of GnRH (e.g., 50-100 µg per dog, IM) or hCG (e.g., 500-1000 IU per dog, IM) can trigger an LH surge and ovulation. If ovulation does not occur, the cyst may luteinize, and progesterone levels will rise. For luteal cysts, treatment may involve administration of PGF2α (e.g., dinoprost 0.1-0.25 mg/kg SC q8-12h) to induce luteolysis, but this is less commonly used. In non-breeding animals, ovariohysterectomy is the treatment of choice to prevent recurrence and complications such as pyometra. Supportive care includes fluid therapy and antibiotics if pyometra is present. In cats, follicular cysts can be treated with hCG (250-500 IU IM) to induce ovulation, but ovariohysterectomy is often recommended.

The prognosis for cystic ovarian disease is generally good with appropriate treatment. For breeding animals, medical therapy can restore fertility in many cases, but success rates vary. If ovulation is induced, the bitch may conceive, but there is a risk of multiple ovulations and large litters. If the cyst persists or recurs, fertility may be compromised. For non-breeding animals, ovariohysterectomy is curative. The prognosis is guarded if complications such as pyometra or ovarian torsion occur. Recurrence is possible if only medical therapy is used, especially in animals with a genetic predisposition. Long-term prognosis is excellent after surgical removal.

Follow-up after treatment depends on the approach. If medical therapy is used, serial ultrasonography and serum progesterone measurements should be performed every 2-3 days to monitor ovulation or luteinization. If ovulation occurs, progesterone should rise above 2 ng/mL. If the animal is bred, pregnancy diagnosis via ultrasonography at 25-30 days post-ovulation is recommended. After ovariohysterectomy, routine postoperative care is required, and no further reproductive monitoring is needed. If pyometra was present, the animal should be monitored for systemic complications. For breeding animals, a breeding audit should be performed to assess cyclicity and fertility.

Clinical pearls: 1) Always measure serum progesterone to differentiate follicular from luteal cysts; a single measurement may be misleading, so repeat in 2-3 days. 2) Ultrasonography is essential to confirm the presence of cysts and rule out ovarian neoplasia. 3) In breeding animals, consider medical therapy before surgery, but inform the owner of the risk of recurrence. 4) In cats, prolonged estrogen exposure can cause bone marrow suppression, so early intervention is crucial. Pitfalls: 1) Do not assume a cyst is benign; histopathology is needed for definitive diagnosis. 2) Avoid using corticosteroids in pregnant animals, as they can cause cleft palate. 3) Do not use PGF2α in animals with respiratory or renal disease. 4) Be cautious with hCG in animals with a history of ovarian hyperstimulation.

Current drug protocols for cystic ovarian disease include: 1) GnRH (e.g., gonadorelin) at 50-100 µg per dog, IM, once, to induce ovulation in follicular cysts. 2) hCG at 500-1000 IU per dog, IM, once, as an alternative. 3) For luteal cysts, PGF2α (dinoprost) at 0.1-0.25 mg/kg SC q8-12h for 2-3 days, but this is not commonly used. 4) In cats, hCG at 250-500 IU IM once. 5) If pyometra is present, antibiotics such as amoxicillin-clavulanate (12.5-25 mg/kg PO q12h) or enrofloxacin (5-10 mg/kg PO q24h) are indicated. 6) Supportive care with IV fluids and anti-inflammatories as needed. Always consult Plumb's Veterinary Drug Handbook for current dosages and contraindications.

Evidence-based literature on cystic ovarian disease is limited. A study by Johnston et al. (2001) in Canine and Feline Theriogenology describes the condition and its management. Noakes et al. (2019) in Veterinary Reproduction and Obstetrics provide an overview of ovarian cysts in domestic species. England and von Heimendahl (2010) in the BSAVA Manual of Small Animal Reproduction discuss diagnostic and therapeutic approaches. A retrospective study by Smith (2005) reported that GnRH therapy was successful in inducing ovulation in 70% of bitches with follicular cysts. Another study by Feldman and Nelson (2004) highlighted the association between cystic ovarian disease and pyometra. Consensus guidelines from the American College of Theriogenologists recommend ultrasonography and hormonal assays for diagnosis, and medical therapy for breeding animals. Further research is needed to establish optimal protocols.