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Ovarian cysts are nonfunctional, fluid-filled cysts that develop spontaneously near the ovaries throughout the female guinea pig’s reproductive cycle.
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Cysts are associated with elevated hormones, such as estrogen, leading to irregular reproductive cycles, persistent heat, hair loss, and infertility.
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Fertility is reduced in affected females older than 15 months.
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Potentially serious uterine disorders (eg, leiomyomas, uterine cancer) can occur in conjunction with cysts.
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Permanent treatment requires ovariohysterectomy or ovariectomy. Some guinea pigs may respond to medical management with specific hormone injections, sometimes coupled with palliative fluid removal from the cysts.
Introduction
Ovarian serous cystadenoma or, more specifically, cystic rete ovarii is a commonly seen medical condition of guinea pigs. Reports in the literature have described from 66% to 75% of sows between 3 months and 5 years of age can be affected.
Sows aged 2 to 4 years are the most commonly affected and serous cysts can spontaneously develop throughout the estrus cycle. Size of cysts is highly variable, but one report showed a diameter of 0.5 to 7 cm with an increase in size as the animal ages; however, it did not show any correlation between reproductive history and prevalence of cysts (Figs. 1 and 2).
In most cases, both ovaries are affected although the right ovary is more commonly afflicted in unilateral cases. One study suggested serous ovarian cysts are a normal component of the cyclic guinea pig ovary.
The most common clinical sign, often noted by owners, is progressive hair loss over the flank region (Fig. 3) and abdomen, without pruritus or abnormal appearance to the skin. The author has observed alopecia in other locations of the body without skin disease that resolved with ovariohysterectomy in guinea pigs with cystic ovaries (Fig. 4). Crusting of the skin around the nipples is commonly observed as well. Many clinicians describe the pear-shape appearance of the body conformation looking from above the guinea pig because of abdominal enlargement with a normal-sized chest area. Clients may report mounting, aggression, and other sexual behaviors although these can also be observed in unaffected guinea pigs in estrus. Nonspecific clinical signs of appetite loss, lethargy, or vocalization when handled are reported and these require appropriate diagnostic evaluation. Some animals may be asymptomatic and show no signs of disease and cysts are found incidentally at necropsy. Microscopic appearance of the large ovarian cysts suggests reproductive performance in these guinea pigs could be compromised and breeding records indicate that fertility is markedly reduced in affected females older than 15 months of age.
Diagnosis with plain radiography is difficult but possible in some cases in which fluid-filled cysts and/or space-occupying masses can be identified. Clinicians may recommend spaying with a presumptive diagnosis and then confirm suspicion at surgery. The guinea pig should be spayed even if large cysts are not found. Routine diagnostic laboratory work, such as a complete blood count, biochemical profile, and urinalyses, are nonspecific but required for presurgical screening and to rule out concurrent medical conditions.
Histopathology
Embryologically, rete cells are from the mesonephros and differentiate into the rete testis in male and rete ovarii in female guinea pigs in which they form a vestigial structure within the ovary. These cells function in phagocytosis of degenerating oocysts and do not produce hormones. The pathogenesis of cyst formation is unclear; however, one theory relates to a defect in ion pumps leading to fluid accumulation.
Three types of ovarian cysts are seen in guinea pigs differentiated only by histopathology: serous cysts (cystic rete ovarii), follicular cysts, and parovarian cysts. Their significance relates mainly to response to medical therapy.
The most common are serous cysts (cystic rete ovarii), which remain present throughout the estrus cycle (Figs. 5 and 6). Serous cysts are lined with a simple cuboidal-to-columnar epithelium composed of cells with solitary cilia or tufts of cilia. Cells of these cysts do not have the ultrastructural characteristics of steroid-synthesizing cells nor do they possess 3β-hydroxysteroid dehydrogenase activity. This means these cysts are incapable of steroidogenesis and do not respond to surges of luteinizing hormones (LHs) similar to follicular cysts. Keller and colleagues
identified cystic endometrial hyperplasia, mucometra, or endometritis in appropriate placental tissue, or leiomyofibroma in 21 of 54 guinea pigs with cystic ovaries, but in only 1 of 17 guinea pigs without cystic ovaries. Histologic appearance and location of the cysts within the ovary were consistent with cystic rete ovarii.
Follicular cysts are the second most common and are derived from preovulatory follicles that fail to ovulate. This aberrant structure reaches ovulatory size, fails to ovulate, and alters normal ovarian cyclicity. The wall of these cysts is lined by granulosa cells.
Parovarian cysts are the rarest and are cysts of the parovarium, which is a vestigial structure that is associated with the ovary and consists of mesonephric tubules and a portion of the mesonephric duct.
Treatment
Treatment of ovarian cysts is by either surgical or medical management with ovariohysterectomy being the recommended treatment of choice. For patients unable to undergo surgery because of concurrent medical conditions, financial issues, or owner’s hesitation due to possible risks, the medical options are limited to hormonal therapy with the use of human chorionic gonadotropin or gonadotropin-releasing hormone (GnRH). If hormonal therapy fails to cause regression of the ovarian cysts, the patient has serous cysts (cystic rete ovarii) or parovarian cysts; follicular cysts are the only type that will generally respond to hormone therapy. Additionally, ultrasound-guided aspiration of the cysts provides a temporary solution in some cases; however, aspiration usually needs to be followed by medical treatment to prevent the rapid reaccumulation of fluid.
Human Chorionic Gonadotropin
Human chorionic gonadotropin is the drug most commonly used for this condition at a dose of 1000 IU per guinea pig intramuscularly (IM), repeated in 7 to 10 days. Depending on the concentration, this is often a large dose to give as an IM injection and some guinea pigs will resent treatment. Human chorionic gonadotropin will also stimulate an antibody response, making subsequent doses potentially less effective and could lead to an allergic reaction with repeated use.
This drug is used to treat ovarian cysts in cattle and to induce estrus in cats. Unlike human chorionic gonadotropin, GnRH does not stimulate an immune response. Recommended dose is 25 μg per guinea pig every 2 weeks for two injections. The commercially available form, Cystorelin, is available in a multidose vial for injection and the volume of the injection is significantly less than that recommended for human chorionic gonadotropin, making it much more tolerable for guinea pigs.
Other unproven hormonal therapies
Lupron
Lupron has been proposed as another option for treating ovarian cysts in guinea pigs. Leuprolide acetate acts as an agonist at pituitary GnRH receptors, thereby interrupting the normal pulsatile stimulation of, thus desensitizing, the GnRH receptors. The result is it indirectly downregulates the secretion of LH and follicle-stimulating hormone (FSH), leading to a reduction in estradiol and testosterone levels. Veterinary uses include management of adrenal endocrinopathy in ferrets and chronic egg laying in birds. To date, the author is unaware of any studies of its use for treating ovarian cysts and personal experience has been unrewarding.
Deslorelin
Deslorelin acetate is an injectable GnRH agonist, also known as an LH-releasing hormone agonist, which stops the production of sex hormones. It is available as a subcutaneous implant that can last up to 6 months. It is currently used to induce ovulation in mares, to stabilize high-risk pregnancies of livestock, and for the treatment of adrenal endocrinopathy in ferrets. Unlike other GnRH agonists that are used to inhibit LH and FSH, deslorelin is used for the initial effect on the pituitary gland and its associated surge of LH secretion. One report in the European literature found no reduction in ovarian cyst size in guinea pigs with the use of deslorelin implants.
Practitioners unfamiliar with guinea pig abdominal surgery may find the ovariohysterectomy particularly challenging. This is often due to the location of the ovaries and the guinea pig gastrointestinal system that consists of a large, full cecum and sometimes gas-filled intestines. Ovariohysterectomy is performed via a ventral midline incision as described for the routine surgical approach in any patient. Practitioners performing this surgery should be familiar with anesthetic protocols, patient anatomy, and important presurgical and postsurgical requirements. It is not recommended to fast guinea pigs before surgery and attention to clearing the oral cavity and monitoring excessive salivation is important. Guinea pig intubation is difficult-to-impossible and most are maintained on facemask. Intravenous catheterization is also challenging, the cephalic veins are most commonly used.
Ovariectomy is another option when there is no confirmed disease of the uterus and is performed via a dorsolateral approach.
Incisions are made on each side ventral to the erector spinae muscle and caudal to the last rib. Blunt dissection through the muscle will provide access to the ovary, which can be slightly exteriorized and ligated ensuring removal of the entire oviduct around the ovary. No advantage has been shown for performing ovariohysterectomy instead of ovariectomy unless there is uterine disease (complication rates are lower with ovariectomy in canine patients). In guinea pigs, advantages of ovariectomy are small and dorsal incisions, less gastrointestinal disruption, and a more rapid recovery.
Postoperative care
Most guinea pigs will not eat post surgery. Postoperative care requires analgesia, assisted feedings, fluid therapy, and patient-hygiene monitoring. Many clinicians report higher postoperative mortality rates in guinea pigs when compared with other pets undergoing the same procedure; owners should be warned of this risk. It is believed that a more socialized guinea pig is likely to do better.
Summary
Ovarian cysts are a common medical condition in guinea pigs and one that should not be mistaken for other diseases (eg, dermatophytosis). Diagnosis and treatment are rewarding because surgery is curative. Recommendation for early spaying of guinea pigs is not routine but may become so in veterinary medicine.
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