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
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