Current and future alternatives to surgical neutering in ferrets to prevent hyperadrenocorticism

SURGICAL NEUTERING of domestic ferrets (Mustela putorius furo) is common in the United States and various European countries. Female ferrets (jills) are induced ovulators, so they remain in estrus until they are mated or for as long as daylight lasts longer than 12 hours. In the early 1980s, several articles appeared concerning estrogen-induced bone marrow suppression in jills with prolonged estrus.1,2 Since then, preventive ovariectomy or ovariohysterectomy of jills has been advised. In male pet ferrets (hobs), there is no medical need for castration. The main reason to castrate hobs is to reduce aggression so that they can be kept in groups and to decrease the intensity of the musky odor produced by the sebaceous glands.3 In the United States, it is common to neuter ferrets at 6 weeks of age, before their delivery to pet shops.4

1. A 4-year-old neutered female ferret showing dorsal alopecia. No swelling of the vulva is noticed.

Hyperadrenocorticism, a common disease among pet ferrets, is characterized by signs of excessive production of sex steroids (androstenedione, 17-alpha-hydroxyprogesterone, dehydroepiandrosterone sulfate, and estradiol), such as symmetric alopecia (Figure 1), vulvar swelling in neutered jills, and recurrence of sexual behavior in neutered ferrets.4-7 In recent years, evidence has accumulated that hyperadrenocorticism in ferrets is mediated by an increased secretion of gonadotropic hormones after castration4,6:

• First, the initial signs of hyper adrenocorticism occur only during the breeding season,8 when plasma concentrations of gonadotropic hormones are high.

• Second, in the United States and in The Netherlands, where neutering of ferrets is common, hyperadrenocorticism is common.6,8 In contrast, adrenal gland tumors are considered unusual in the United Kingdom,9 where ferrets often remain intact.10

• Third, a significant correlation has been found between the age at neutering and the age at onset of hyperadrenocorticism.6

• Fourth, the gonadotropin-releasing hormone (Gn-RH) analogue leuprolide acetate has had beneficial effects in treating this disease.11

• Finally, luteinizing hormone (LH) receptors have been detected in the adrenal cortex in ferrets.12

Because evidence points to surgical neutering as an important risk factor for hyperadrenocorticism in ferrets, we reviewed the literature to find current and possible future alternatives to surgically neutering ferrets (Table 1). Most of the information we have provided here is based on species other than ferrets. Only after studies have been performed in ferrets can conclusions be drawn on whether these techniques are effective in this species.

Possible Alternatives to Surgical Neutering in Ferrets

Reproductive physiology

Gonadal activity is seasonal in both male and female ferrets, and more than 12 hours of light a day promotes reproductive activity.13 The pineal hormone melatonin seems to play a central role in regulating these changes.14 Plasma and pineal gland melatonin concentrations are significantly higher during the dark phase of the day (scotophase) than in the light phase (photophase). Therefore, it has been suggested that this hormone has an inhibitory effect on Gn-RH release.15

During the breeding season, Gn-RH stimulates the production of the gonadotropic hormones LH and follicle-stimulating hormone (FSH), which stimulate the gonads to produce either estradiol or testosterone. The latter two hormones exert a negative feedback on the hypothalamus and pituitary gland, thereby preventing excessive secretion of Gn-RH, LH, and FSH.

Definitions

In its classic definition, castration denotes the removal of gonads. Thus, the term covers removing the ovaries in females and the testes in males. In recent years, this definition has been extended by the introduction of new methods to create nonfunctional gonads, such as chemical and immunologic castration.16 The term castration is controversial since it is sometimes used to indicate gonad removal in males only. Neutering is a term that encompasses all means of eliminating gonadal function in both males and females,17 and we use this term in this article.

In this article, we do not discuss methods of contraception that do not affect the reproductive endocrinology, such as vaccination against zona pellucida proteins,18 vaccination against sperm proteins,19 or tubal ligation. Since these methods do not affect hormone production, they are expected neither to address the behavior of the ferrets during the breeding season nor to prevent hyper adrenocorticism in ferrets.

Alternatives to surgical neutering

• Administering progestagen

Principle. Although not fully understood, the probable mode of action of progestagens is suppression of gonadotropic hormone secretion, thereby preventing ovarian cyclicity.20

Method. Several progestagens are used in veterinary medicine: megestrol acetate, medroxyprogesterone acetate, and proligestone. Megestrol acetate can be given orally, medroxyprogesterone acetate can be given orally or by injection, and proligestone (14-alpha, 17-alpha propylidenedioxyprogesterone) is given by depot injection. The latter is recommended in the United Kingdom to prevent estrus in ferrets at a dose of 0.5 ml (100 mg/ml) given subcutaneously just before breeding season.21,22 Proligestone can also be used in jills in estrus.

Effect. In one study, return of estrus was reported in about 8% of ferrets two to five months after the initial dose of proligestone.22 In these cases, a second dose suppressed estrus for the rest of the breeding season. Megestrol acetate has been used in ferrets to prevent estrus but is not recommended because of the assumed risk of pyometra.23 This assumption may be based on data obtained from other species in which megestrol acetate has been associated with pyometra. In ferrets, however, pyometra and mucometra have only been reported in intact ferrets during the estrous season.24 Therefore, it is likely that the development of pyometra in ferrets is under the influence of estrogens rather than progesterones.

Remarks. Reported side effects associated with using progestagens in either dogs or cats are cystic endometrial hyperplasia, prolonged pregnancy, hypersecretion of growth hormone (GH), diabetes mellitus, and an increased risk of neoplastic transformation of mammary tissue.25 Of these side effects, only prolonged pregnancy (gestation = 51 days; normal = 38 to 44 days) has been reported in two ferrets after the use of proligestone. Proligestone had been given to these ferrets when they were in estrus and had been mated.21

No information on using progestagens in hobs exists. In other species, including people, progestagens have been used to suppress libido and fertility in males.26-28 Progestagens are rarely used for contraception in human males since they cause loss of libido and incomplete suppression of spermatogenesis.27 For this reason, the combination of progestagens and androgens is often used; it provides a better contraceptive effect than progestagens alone, and the libido is maintained.27 This combination would not be an option in ferrets because libido is an undesirable characteristic in hobs.

Delmadinone acetate is used to suppress libido in dogs.28 A recent study in beagles, however, revealed that this progestagen does not suppress plasma testosterone concentrations.29 Since it is unknown what the effect of delmadinone acetate is on other androgens, it is possible that the musky odor produced by the sebaceous glands in ferrets will not be reduced by this drug.

In people, cyproterone acetate and medroxyprogesterone acetate have been used to suppress libido in sex offenders.26 Although both drugs suppress plasma testosterone concentrations, it is unknown what the effect is on the gonadotropic hormones.

Studies with progestagens are needed to determine whether these drugs can be used to control libido and odor in hobs. In addition, the effect of progestagen administration on GH release should be studied in jills because progestin-induced expression of the mammary GH gene has been demonstrated in dogs and cats.29

• Sham mating

Principle. Ferrets, rabbits, and cats are induced ovulators. When ovulation is achieved without fertilization, pseudopregnancy will occur.24

Method. In rabbit does, the proximity of an intact male, mechanical stimulation of the vagina, or mounting by a female rabbit can induce ovulation.31 In cats, stimulation of the vagina will result in ovulation.20,25 In ferrets, both vaginocervical stimulation and neck gripping are necessary to induce ovulation.32 Because of this elaborate procedure, it is not practical for owners to try to induce ovulation in jills. As an alternative, vasectomized hobs are used in the United Kingdom to induce ovulation.23 One mating leads to cessation of estrus in about 75% of ferrets and two matings to cessation of estrus in 85% of ferrets.23

Effect. Estrus is terminated; pseudopregnancy lasts about 42 days.24

Remarks. During pseudopregnancy, jills may display nesting behavior and abdominal and mammary gland enlargement.24 The nesting behavior, which includes dragging cage mates around the cage and increased aggression toward the owners, does not make this an attractive option. Vasectomized hobs remain aggressive and have a musky odor similar to that of intact hobs.

• Administering hCG or Gn-RH

Principle. After mating, a preovulatory LH surge may last to up to 12 hours. This LH surge can be mimicked by administering human chorionic gonadotropin (hCG) or by stimulating endogenous LH release with the hypothalamic releasing hormone Gn-RH.

Method. Ten days after the onset of estrus, 20 µg Gn-RH or 100 IU hCG is given intramuscularly.23

Effect. About 35 hours after injection, the ferrets ovulate, resulting in the formation of corpora lutea. Vulvar swelling will start to decrease within one week of injection. Anestrus (pseudopregnancy) will last for 40 to 60 days.1

Remarks. One study found that multiple injections of Gn-RH may sensitize the ferret to the drug, resulting in anaphylactic reactions shortly after administration.24 Antihistamine administration ameliorates these reactions within minutes. We have not found any adverse reactions to hCG administration.

The consequences of pseudopregnancy have been mentioned in the previous section, and neutering ferrets with hCG or Gn-RH injections only applies to jills.

• Manipulating photoperiod and administering melatonin

Principle. As described above, a ferret's reproductive season starts when there are more than 12 hours of light a day. During the scotophase, melatonin concentrations in plasma are high.15 It has been speculated that keeping ferrets either under conditions with short photoperiods or giving them melatonin would suppress the hypothalamic- pituitary-gonadal axis.

Method. Provide a maximum of eight hours of light a day, or administer 1 mg melatonin subcutaneously eight hours after the onset of light daily.33

Effect. Ferrets kept under eight hours of light and 16 hours of darkness come into estrus only seven weeks later than ferrets exposed to long photoperiods (14 hours of light and 10 hours of darkness).33 Similar results were seen in ferrets kept under long photoperiods (14 hours of light) after receiving melatonin (1 mg/day) eight hours after the onset of light, illustrating that melatonin has the same effect as darkness.33 One study found that in the first year after ferrets were blinded, they came into estrus at the expected time, but thereafter estrus synchrony was lost.34 Estrus in blinded ferrets lasted from just a few weeks to up to 60 weeks.

Remarks. A limited light regimen and melatonin administration do not seem to have a lasting effect on inhibiting the hypothalamic-pituitary-gonadal axis in ferrets.

• Immunizing against Gn-RH

Principle. Several reports on using Gn-RH vaccines in mammals are available.35-38 Depending on the species, these vaccines prevent conception, prevent boar taint, or control hormone-dependent cancers, including breast and prostate cancers. In ferrets, a Gn-RH vaccine would serve two goals: inactivate gonads in both sexes and lower plasma concentrations of gonadotropic hormones.

Method. Depending on the study, Gn-RH inoculations were performed intramuscularly, subcutaneously, or intranasally.35-38

Effect. Decreased plasma gonadotropic hormone concentrations have been reported in male rats after immunization against Gn-RH.36 Testosterone concentrations decreased in bulls, boars, male rats, and dogs after immunization,35-37 while in female mice a sterilizing effect was seen.38 Reduced testes size has also been reported in boars and rats after immunization against Gn-RH.36,37

Remarks. In a pilot study conducted at Utrecht University, eight out of 12 ferrets immunized against Gn-RH became lethargic and anorectic after subcutaneous vaccination. Long-term supportive care did not result in any improvement, and the eight affected animals had to be euthanized (Schoemaker NJ, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands: Unpublished data, 1998). Postmortem examination disclosed nonspecific lymphocytic-plasmacytic infiltrations in multiple organs (liver, kidney, lung, and intestines), suggesting a nonspecific immune reaction. In the same experiment, 24 control ferrets were used (12 surgically neutered and 12 intact). Since these ferrets had not received any component of the Gn-RH vaccine, had been kept in the same area, and had not shown any signs of disease during the vaccination trial, it is assumed that a component of the vaccine was responsible for the nonspecific immune reactions. To confirm this assumption, further studies are necessary. Such a vaccine should not be used before the cause of these reactions has been unraveled.

• Immunizing with LH

Principle. Inhibit LH secretion by immunization with heterologous LH.39-41

Method. Heterologous LH is injected intramuscularly or subcutaneously.

Effect. Injecting bovine LH causes a 90% reduction in the weight of rabbit testes, genital atrophy in female rabbits, and loss of receptiveness to males.39 In another study with male rabbits, LH and testosterone plasma concentrations decreased significantly after immunization against LH; however, FSH concentrations increased significantly.40 Similar but less consistent effects of LH immunization were seen in dogs.42 In ewes, estrus and pregnancy were prevented for two years after immunization against LH, although plasma LH concentrations were not lower than those in control ewes.41 In these ewes, FSH concentrations were also increased.

Remarks. So far, no reports of LH vaccination in ferrets exist. Since not only LH but also FSH may influence the development of hyperadrenocorticism in ferrets, there is reason for caution with LH immunization in ferrets, especially since plasma LH concentrations were not reduced in the control ewes.

• Immunizing with LH receptor

Principle. Induce LH receptor dysfunction by immunization with heterologous LH receptor.

Method. Bitches were immunized with 0.5 mg bovine LH receptor encapsulated in a Silastic subdermal implant on the fifth day of vaginal bleeding, followed by intramuscular booster injections, which were given based on antibody titers.43

Effect. In bitches, immunization with bovine LH receptor suppresses serum progesterone concentrations for about one year, while serum concentrations of estradiol and LH are not affected. Although stimulation with Gn-RH in immunized dogs leads to a LH surge, serum progesterone concentrations do not increase. Thus, bitches immunized with bovine LH receptor do not ovulate or produce active corpora lutea.43

Remarks. The main drawback of this approach is that ferrets may fail to ovulate. Prolonged estrus can be expected, which may result in bone marrow suppression. In male mice, immunization against LH receptor reduced androgen production.44 In male ferrets, this might reduce aggressive behavior and decrease the intensity of their musky odor. Again, some caution is warranted because high LH receptor antibody titers in mice had an agonistic effect, resulting in hypertestosteronemia.44

• Administering depot Gn-RH agonist

Principle. Depot Gn-RH agonists, such as leuprolide acetate and deslorelin, increase the concentrations of gonadotropic hormones; this increase is followed by a desensitization of gonadotrope receptors, resulting in decreased LH and FSH plasma concentrations.45 The exact mechanism of the desensitization is still not clear.46

Method. Of the available formulations, leuprolide acetate (Lupron Depot 3.75 mg—TAP Pharmaceutical Products) is used to treat hyperadrenocorticism in ferrets.11 Ferrets weighing less than 1 kg receive an intramuscular dose of 100 µg at monthly intervals, and ferrets heavier than 1 kg receive 200 µg per month. A similar treatment protocol might also be effective for contraceptive purposes. Slow-release implants have been described in dogs,45,47 and a study with an implant containing the Gn-RH agonist deslorelin has been conducted in jills.48

Effect. The jills receiving a 4.7-mg deslorelin implant came into estrus within days after placement, but estrus ended within a week. The ovarian activity of these ferrets was suppressed for more than a year.48 In addition, leuprolide acetate has induced estrus in dogs.49

Remarks. Slow-release Gn-RH agonist implants suppress reproductive function in dogs.47 Gn-RH implants might therefore be an option for use in hobs. Deslorelin implants are currently under investigation at Utrecht University for their effectiveness in neutering hobs.

• Administering Gn-RH receptor antagonist

Principle. Competitive Gn-RH receptor occupancy with Gn-RH receptor antagonists results in a decreased release of gonadotropic hormones by the pituitary gland.46

Method. The available Gn-RH receptor antagonists have to be injected subcutaneously, but orally active nonpeptide Gn-RH antagonists are being developed for use in people.50

Effect. The initial increase in gonadotropic hormones, seen with Gn-RH agonists, are not seen with Gn-RH receptor antagonists. The use of these receptor antagonists will therefore result in an immediate decrease in gonadotropic hormone concentrations.

Remarks. Until now, only a few Gn-RH receptor antagonists have been registered for use in people.46 New and longer-acting drugs are being developed. Degarelix seems to be the most promising of these Gn-RH receptor antagonists.51 While older Gn-RH antagonists caused increased histamine release after injection, the newer drugs do not have this side effect. Never theless, local reactions at the injection site are still common.46 These drugs seem to be promising for future prevention and treatment of hyper adreno corticism in ferrets.

Conclusion

Several potential methods exist, other than surgical neutering, to influence reproductive function in ferrets. Progestagens seem practical and are already available for use in jills but need to be studied for use in hobs. Slow-release Gn-RH implants, such as deslorelin, are already registered for use in animals in some countries and may become available for use in ferrets within the next decade. Possible future alternatives may also be Gn-RH antagonists or immunization against Gn-RH. Detailed studies are needed before these techniques can be recommended for neutering ferrets. Once these techniques become available, it will be necessary to investigate whether they will actually reduce the incidence of hyperadrenocorticism in ferrets.

Nico J. Schoemaker, DVM, PhD, DECAMS, DABVP (avian practice)

Johannes T. Lumeij, DVM, PhD, DECAMS, DABVP (avian practice)

Ad Rijnberk, DVM, PhD, DECVIM

Department of Clinical Sciences of Companion Animals

Faculty of Veterinary Medicine

Utrecht University

Yalelaan 8, 3584 CM Utrecht, The Netherlands

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