Non-Surgical Methods of Contraception and Sterilization :Current and Future Options in Fertility Control of Dogs and Cats
Nonsurgical contraceptive measures include permanent or temporary pharmaco-castration of males, reversible and nonreversible estrus prevention in females, estrus suppression in females, and pregnancy prevention or termination after unwanted mating. As for females, no new products for estrus prevention or suppression have been introduced in the last decade, other than revised formulations and new brands of progestins previously marketed. In fact, contraceptive options in bitches have decreased in some countries with the recent withdrawal of the androgen mibolerone (Cheque Drops©) as an estrus preventative. However, an implant product providing down-regulating amounts of a potent GnRH-agonist has recently been approved as male dog contraception in New Zealand and Australia, and efforts are underway to obtain approval as a contraceptive in bitches as well, as well as application to cats. Depending on price and regulatory hurdles, the product is likely to become available in many countries in the next decade. There are significant advances in protocols and products available for termination of unwanted pregnancies, including extra-label uses of prostaglandin, of PG analogs, of dopamine agonists, and of dexamethasone in selected situations; additionally, the anti-progestin aglepristone is approved and sold with an indication of pregnancy termination in some countries. The goal of non-surgical contraception in males has advanced on two fronts: the above-mentioned GnRH-agonist implant technology, and a new product that provides testis-necrosing doses of a metallic salt injected directly into the testes.
The use of fertility inhibitors is gaining acceptance to control populations of companion animals and wildlife. For dog population management, nonsurgical sterilization is increasingly advocated as deserving priority for development because of its potential to be more costeffective than surgical sterilization. For dog population management, nonsurgical or chemical sterilization is increasingly advocated as deserving priority for development because of its potential to be more cost effective than surgical sterilization. Chemical sterilization methods so far employed include hormonal methods, immunocontraceptives and Inorganic Chemo-sterilants such as calcium chloride-CaCl2, zinc gluconate neutralized by arginine (Neutersol), sodium chloride-NaCl solution).
If nonsurgical fertility control is chosen to manage dog populations or their impact, social acceptance, humaneness, effectiveness, feasibility, costs, and sustainability of this method should be evaluated at an early planning stage. This framework is based on the assumption that a set reduction of dog population size or the elimination of a disease such as rabies, within a predefined timeframe can be achieved by using nonsurgical fertility control as an additional tool to education and vaccination. The trend of using calcium chloride should be adopted to the community; training should be given to the practitioners in the way how to inject the chemical into the testicle; the government should take into consideration in policy making in the application of calcium chloride for dog population control; further research should be conducted to reduce side effects of calcium chloride.
Contraception serves to reduce the size of animal population and suppress physiological sexual activities. Furthermore it is carried out because of medical indications as well as behavioral problems. The most frequently mentioned undesirable male behaviors are aggression towards other males or people, roaming, fighting with other males, inappropriate sexual behavior (mounting of other animals or people), fear of inanimate stimuli and urine marking in the house The contraception also prevents sexually transmitted diseases, such as Brucella canis or canine transmissible venereal tumour (Sticker tumour). Furthermore contraception becomes more often a prerequisite for adoption of animals from shelters. Early sterilization of dogs and cats is frequently conducted, resulting in accelerated adoption procedures . When choosing a method of contraception one should be guided by the main objective for contraception – i.e. whether it should be a permanent or a temporary, reversible contraception. Among surgical methods the most widely used procedure is removal of the testes by castration (orchiectomy, orchidectomy). Surgical sterilization of dogs and cats has been a part of the arsenal used to prevent unwanted breeding for decades. Innovations in methods and procedures have led to mobile sterilization vans and, more recently, to growing acceptance of the sterilization of puppies and kittens.
Contraception is the method used to prevent the occurrence of pregnancy in female animals and can be achieved through the use of a medication, device, or procedure. Choices of contraceptive methods should address factors, such as efficacy, safety, availability, acceptance, and affordability (Bansode et al., 2019). Contraceptive agents can be used in different forms depending on the types (pills or injectable forms) and the method of use. Some contraceptive agents induce contraception by altering the hormonal activities, especially the hormone-based contraceptive like diethylstilbestrol, progestin, medroxyprogesterone acetate (MPA), proligestone (PROL), and mibolerone (Asa, 2018). While the use of implants is impaired through the use of an intrauterine device (IUD), such as the Depo-proveras and melengestrol acetate (MGA) implants (Boutelle and Bertschinger, 2010). The use of the non-surgical contraceptive method is more accurate when owners request to breed their animals in the future (Massei and Miller, 2013) unless in the case of some diseases, such as rabies, where permanent sterilization is preferable. Most of the contraception techniques should be implemented by trained veterinarians, except for oral contraceptives. Most of the new devices are not licensed and require expert and trained personals to use them, and their cost is very high (Taylor et al., 2017). Female animals treated with hormone-based contraceptives may experience some side effects, such as hormonal in-balance, uterine infection, pyometra, mammary tumor, diabetes mellitus, and other potentially lifethreatening side effects that may arise from the drugs. Therefore, animals treated with hormone-based contraception require daily monitoring by veterinarians for the prevention of the mentioned complications (Asa, 2018). The advantages of contraception include a longer duration before reverse, fewer side effects, suppression of sexual behavior, easy administration, a low cost, and also applicable for humans (Cathey and Memon, 2010). On the other hand, the disadvantages of this method are the need for repetition, quick reverse when discontinued using the repeated dose, possible slow onset of activity in some drugs, possible chance of a reaction at the injection site
There is no question that spaying and neutering has had an enormous impact on reducing the numbers of unwanted puppies, kittens, dogs and cats at animal shelters in many areas of the country. On the other hand, targeting difficult-to-reach communities and feral populations of cats and dogs would be easier if a simpler, nonsurgical method was available. The dream has always been to render a dog or cat sterile with a single pill or injection that could be administered by a nonveterinarian.
Medical control of reproductive cyclicity in dogs and cats can be achieved using a variety of different drugs. Gonadal steroids such as progestogens and androgens have historically been used for a long time, and their action will be reviewed here. A recent development is the use of long acting GnRH agonists.
Progestogens
Synthetic analogues of progesterone, also termed progestins or progestogens (PG), are pharmaceutical compounds commonly used to control the reproductive cycle of domestic animals .
The following PGs are commonly used in dogs and cats for temporary (starting the treatment shortly before proestrus onset) or prolonged (starting in anestrus) postponement of estrus, or for suppression of estrus (starting the treatment after proestrus onset) :
medroxyprogesterone acetate (MPA), megestrol acetate (MA), proligestone (PR), chlormadinone acetate (CMA), delmadinone acetate (DMA), norethisterone acetate (NTA) and melengestrol acetate (MGA). From the clinical point of view all these product act in the same way through a block of the production and/or release of GnRH from the hypothalamus. These compounds show a variety of action on the reproductive and endocrine system (such as hyperplasia of the endometrium, hyperplasia of the mammary parenchyma, decreased production of adrenocorticosteroids, increased secretion of prolactin and growth hormone, insulin resistance) as well as some local skin reactions at the injection site and behavioral modification (increased appetite and weight, polydipsia, slight depression, decreased libido in males). In pregnant bitches and queens the use of PGs may cause masculinization of female fetuses if administered early in pregnancy (during organogenesis, up to day 25 after ovulation) or delayed parturition if administered in the last decade of pregnancy.
Clinical Considerations For a Safe Use of Progestogens
The above cited effects are not always present, are reversible and do not generally cause problems in healthy young to adult animals treated for not too long and using the recommended dosage. In general, a treatment period of 12 months is considered adequate in most individuals, although longer treatments can also be safe provided that the female is given a rest of a few months every year or so. While most bitches and queens may tolerate well treatment periods of more than 6 months, animals with a pre-existing disease such as subclinical diabetes, microscopic mammary lesion/tumor or cystic endometrial hyperplasia may see their condition worsen rapidly as a result of the PG treatment. The following is a series of considerations on patient selection and type of presenting complaint for which a PG treatment should or should not be used.
Do not use long acting compounds (such as MPA or PR or any other compound marketed for long term use) prior to puberty in felines, as this may cause the queen to develop a long-lasting mammary hypertrophy which could become a life-threatening situation. In prepuberal animals it is best to use initially a short acting compound (such as MA) per os for 1-2 weeks and then change to a long acting PG once potential side effects have been ruled out.
Do not treat pregnant females, as this may cause fetal developmental defects as well as delayed parturition, thereby causing fetal death in utero due to placental ageing and detachment.
Do not treat pseudopregnant bitches. During a PG treatment clinical signs of pseudopregnancy will disappear but will recur once treatment is discontinued, and the problem may worsen.
Do not treat a female during diestrus. The stage of the reproductive cycle should always be identified using vaginal cytology and/or serum progesterone assay, and the bitch or queen should best be treated during anestrus. Diestrus should be ruled out in felines too, as approximately 30% of queens ovulate spontaneously, maintaining thereafter a 30-45 day-long diestrus.
Do not treat females with uterine haemorrhage. Prolonged sanguineous vulvar discharge following parturition in the bitch can be a critical problem which should either be treated with a uterine contractive drug (i.e., as ergonovine) or sent to surgery. Milder bloody vulvar discharge can be caused by uterine neoplasia, cystic endometrial hyperplasia with superimposed endometrial inflammation, pyometra, metritis. None of these conditions will benefit from administration of a progestogen.
Do not treat diabetic patients. Although not always necessary, it would be wise to measure blood glucose before and/or after a prolonged treatment to confirm health status with regard to glucose metabolism.
Do not use PGs in females with prolonged heat. A prolonged heat may be due to ovarian cyst(s), a granulosa cell tumor, or may be due to a split heat (in the bitch) or to a misinterpretation of normal estrous signs by the owner. For none of these categories is a progestogen treatment indicated (although in some cases a young, health bitch with an ovarian cyst may temporarily benefit from administration of a progestogen). Therefore, bitches or queens with a prolonged heat should not be treated with a progestogen, unless a diagnosis of cystic ovarian disease has been carefully confirmed and surgery or administration of GnRH or hCG are not a valid therapeutic option.
Choosing the Right Candidate
The ideal candidate is an adult postpuberal female in anestrus. Before a female gets treated with long acting compounds she should be evaluated for normality of uterine and mammary conditions as well as of glucose metabolism. In fact, a long acting progestogen might precipitate a subclinical uterine, endocrine or mammary condition (such as diabetes, cystic endometrial hyperplasia-pyometra in the bitch or mammary hyperplasia in the queen) which often are not clinically evident in the early stages, and which have been reported (albeit rarely) also in young animals. If one of the above conditions is present the administration of a long acting progestogen prior to diagnosis may pose a serious health threat on the female. A minimum database of clinical information to be gathered prior to administering a long-acting compound should include:
Collecting a thorough reproductive history to rule out occurrence of estrus within the last 1-2 months (which would mean that the female is in diestrus)
A complete clinical exam
Palpation of the mammary gland to rule out presence of mammary nodules
A vaginal smear to rule out presence of oestrus
Table 1 shows the suggested dosages of the most commonly used progestogen-based compounds in the bitch and queen.
Table 1. Suggested dosages of the 3 most commonly used progestogen compounds in bitches and queen for the control of estrous.
| Suggested dosage | Dog | Cat |
| Medroxy-progesterone acetate | 2.5-3.0 mg/kg IM every 5 months | 2.0 mg/kg IM every 5 months |
| Megestrol acetate | <2.0 mg/kg administered for <2 weeks in proestrus, or <2.0 mg/kg administered for a longer duration of time in anestrus. A typical dosage for estrus suppression is 2.0 mg/kg/day for 8 consecutive days, while a typical dosage for temporary postponement is 0.5 mg/kg/day in late anestrus. | In anestrus: 5 mg/cat every 2 weeks or 2.5 mg/cat/week (better if divided into 2 administrations every 3.5 days) in proestrus: 5 mg/cat/day for 4 days, then 5 mg every 2 weeks. |
| Proligestone | 10-33 mg/kg SC every 3,4,5,5 months | 10 mg/kg SC every 3,4,5,5, months |
Androgens
Androgens are also widely used for the control of estrous cycle in dogs and cats, although less information is available on most of the active principles commercially available for veterinary use. In the male, androgens cause a block of spermatogenesis (due to degeneration of seminiferous tubular epithelium), an increase in libido, a higher incidence of priapism, and growth of prostatic tumors. In the female, the main reproductive effect of androgens is to cause suppression of ovarian activity thanks to a negative feedback on the pituitary which decreases gonadotropin secretion; the ovaries of dogs treated with an androgen generally contain primary and secondary follicles but few that mature to ovulatory size. Androgens will also cause atrophy of mammary gland/endometrium and lactation arrest.
Mibolerone (originally marketed by Upjohn as Cheque drops®) was until recently the only androgen approved for estrus suppression in bitches in the United States. Other androgens marketed in Europe for human or veterinary use such as testosterone, methyltestosterone nandrolone and stanozolol are sometimes used in bitches, although recommended dosages are not available for each one of these compounds. Androgens are not recommended for use in breeding animals. Estrus suppression can be achieved starting the treatment at least 30 days before onset of the next proestrus and for as long as estrus suppression is desired. Prolonged postponement can be achieved in bitches for up to 2-5 years. Return to estrus averages about 70 days, with a range of 7 to 200 days. There are no published reports describing fertility after treatment with androgens. Although most bitches seem to exhibit apparently normal fertility, we have occasionally observed cases of prolonged anestrus in bitches treated with androgens. The most commonly reported side-effect of androgens in female dogs is clitoral hypertrophy, which occurs to some degree in 15 to 20% of dogs treated with mibolerone. Other reported side-effects of androgens include creamy vaginal discharge, vaginitis, increased mounting and aggressive behaviour, anal gland inspissations, musky body odor, obesity, and epiphora. Androgens are contraindicated in potentially pregnant bitches, as they may cause masculinization of female fetuses; in prepuberal bitches, in which they may precipitate premature physeal closure, and in dogs with renal or hepatic diseases (Olson et al., 1986a). Presence of intranuclear hyaline bodies in hepatic cells and, rarely, changes in liver function tests have been described in dogs after treatment with mibolerone; clinical significance of hepatocellular changes is unknown. Testosterone also has been described for estrus suppression in bitches. Successful regimens reported include injection of 100 mg testosterone proprionate once weekly, oral treatment with 25 to 50 mg methyl-testosterone twice weekly, and subcutaneous implantation of at least 759 μg/kg. Testosterone, methyltestosterone and nandrolone are currently used with indications such as oestrus suppression, false pregnancy, lack of libido as well as with other non-reproductive indications (renal insufficiency, anemia, post-surgery etc.). Testosterone propionate (100 mg once weekly) and methyltestosterone (25-50 mg twice weekly per os) are currently marketed in some European countries for oestrus suppression. Canine false pregnancy can safely be treated with androgens as it does not recur following cessation of treatment unlike what happens with progestins.
Long-Acting GnRH Agonists
A recent development in the field of the control of reproduction in the bitch is the use of long-acting GnRH agonists such as deslorelin, which have become commercially available as veterinary drugs in Europe during 2008. In a recent study (Romagnoli et al., 2009) to evaluate clinical efficacy of deslorelin for inhibiting reproduction in the bitch, 10 adult, healthy bitches or bitches with mammary neoplasia for which owners were requesting suppression of cyclicity without performing gonadectomy were administered a 4.7 mg or a 9.4 mg deslorelin implant subcutaneously. The study design included history, physical exam, uterine ultrasound, haematology, serum biochemistry and progesterone (P4) assay to be performed prior to treatment, with physical exam and serum P4 to be repeated every 2.5 months until the treatment was discontinued. The first implant of deslorelin was administered in anestrus (N=5) or in diestrus (N=5).Treatment was repeated every 5 months for as long as necessary based on the clinical situation of the dog and owner’s desires. Some of the bitches implanted in anestrus came in heat within 4-15 days after treatment, while none of the bitches implanted in diestrus showed heat during treatment. Suppression of reproductive cyclicity was successfully achieved in 6/10 bitches for 1-4 years. No behavioural and local/general side effect were observed in any of the treated bitches. The 4.7 mg deslorelin implant may work well for suppression of cyclicity provided that it is administered in diestrus and at intervals of 4.5 months. The 9.4 mg implant may be more suitable for this use although its efficacy may also be shorter than 12 months. Owner compliance is an important limiting factor.
GnRH agonists can be used to control reproduction also in male dogs as well as in male and female cats (Romagnoli et al., 2005). We have administered the 4.7 mg deslorelin implant to adult dogs and adult and prepubertal male and female cats. Adult dogs respond well to the treatment, and we have observed a sharp reduction of fertility and aggressiveness in all (4/4) treated dogs, while libido has not always been affected completely in these animals as they were adults and well trained in breeding bitches in heat (Romagnoli, unpublished data). Results in male cats have been less reliable, with some animal responding well (loss of penile spikes, loss of libido, loss of aggressiveness) while others have kept showing good (albeit decreased) libido, good fertility and unchanged aggressiveness. Based on our ongoing studies in adult queens, the 4.7 mg deslorelin implant seems to be effective in inhibiting reproductive cyclicity for >12 months, and good efficacy has also been observed in delaying puberty in male and female cats implanted at 2-4 months of age (Romagnoli, unpublished data).
The high cost of surgical sterilization will probably always limit its role as a method of pet population control, particularly in non-affluent parts of the world. Additionally, some people view such surgeries as unaccepable infringements on animal rights.
It is clear that an inexpensive and non-surgical method for permanent contraception would be of enormous benefit. A variety of approaches to this goal have been explored, but to date, none have shown sufficient efficacy to be widely deployed.
Injectable Chemical Sterilization
Several investigators have explored injection of irritating chemicals into the testes or epididymides to sterilize dogs and cats. Epididymal injection of such compounds as chlorhexidine gluconate, dilute formaldehyde or zinc tannate have, at times, appeared a promising approach to sterilization. However, when larger scale trials were performed, sterility was not consistently achieved and a significant number of animals developed unsatisfactory inflammatory reactions at the site of injection. This technique is rarely used.
Intratesticular and intraepididymal injection of zinc gluconate neutralized with arginine has shown greater promise than other chemical sterilants, and has been marketed for chemical castration of young dogs (Neutersol). This technique has been shown to be highly effective in eliminating fertility in dogs. However, it does not dramatically reduce secretion of testosterone from the testes, and therefore does not afford the beneficial effects seen with castration. Additionally, sterilizing young male dogs using this technique has not provided substantial cost savings over surgical castration.
Antifertility Vaccines
Several studies have been conducted to evaluate immunization against LH (luteinizing hormone) as a method for contraception or sterilization. By using potent adjuvants, such vaccines were effective in some dogs. In general, however, the level and duration of immunity obtained was so variable that LH vaccines have not been pursued. Similar approaches to immunize against gonadotropin-releasing hormone have also failed to achieve effective contraception.
A large amount of research has been expended to develop vaccines against zona pellucida proteins, and some of this work has focused on dogs and cats. The idea is that antibodies to the zona pellucida may inpair oocyte development, impair ovulation or block binding of sperm to the zona pellucida; any of these effects could reduce or abolish fertility. The image to the right shows pig oocytes that have been stained with a fluorescently-tagged control versus anti-zona pellucida antibodies – the bright fluorescence of the two oocytes on the right is due to binding of anti-zona pellucida antibodies.
To date, an effective zona pellucida vaccine for pets has not been announced, and some studies have demonstrated significant side effects (e.g. development of cystic ovaries) after such treatment of dogs.
Megestrol acetate (MA) works as a contraception and prevents female cats from going into heat and becoming pregnant. MA has long been prescribed by American veterinarians to treat various conditions in both male and female cats, but before now, it has not been widely used in the United States as a contraceptive.
The use of MA for other purposes, and its use as a contraceptive overseas, have given veterinarians an increased and now solid level of comfort in recommending the use of MA as a contraceptive during the COVID-19 pandemic.
Whether it’s a cat you are fostering or a new stray who has wandered into your neighborhood, many of us experience the uncomfortable position of having an unspayed female cat in our lives when access to surgical spay is limited. Given this unpleasant reality, MA is a welcome addition for cat caregivers.
How Much & How to Give Megestrol Acetate:
- Experts recommend that one 2.5 mg dose of MA be given orally to each unspayed female cat once a week. The simple guideline is “one cat one dose once a week.” For female cats already showing signs of heat, the dose should be given at 5 mg per cat per day for three days only. After that, it should be given in a 2.5 mg dose once a week.
- MA is best given in liquid form added to food that is served on a plastic plate. If served on a paper plate, some or all of the MA might be absorbed by the paper.
- It is recommended that cats be given MA for no more than 30 weeks.
- MA requires a commitment on the part of the caregiver to give the medicine each week in the right amount to each cat. This is most easily accomplished in a household with only one cat, and more difficult in scenarios in which there are multiple cats. If you are giving MA to more than one cat, each cat must be watched to ensure she doesn’t eat her own food with its dose and then wander over to eat another cat’s dose as well.
Nonsurgical methods of contraception in dogs and cats: Where are we now?
The population of unowned, free-roaming cats and dogs in the world is unknown, but we know an overpopulation of these animals exists.The population of unowned, free-roaming cats and dogs in the world is unknown, but we know an overpopulation of these animals exists. They present a health threat to people and pets. Rabid dogs are the main source of rabies deaths in people worldwide. Free-roaming dogs and cats may also have a negative impact on wild species and the environment.
Furthermore, the welfare of free-roaming animals is debatable and likely varies with location, climate, and people’s attitudes. Some evidence indicates that free-roaming cats have shorter life spans than owned cats. One study showed that 87 of 169 (51%) free-roaming kittens died before 6 months of age, and the most common cause of death was trauma from dog attacks or motor vehicle accidents.2
METHODS OF POPULATION CONTROL
The mainstay of population control for dogs and cats is surgical sterilization via ovariohysterectomy and orchiectomy. However, many reasons why surgical sterilization may not be effective as the sole method for population control exist. It requires anesthesia, medical equipment, a suitable surgical facility, adequate recovery time, and the advanced training of a veterinarian. It carries the risks inherent in any surgical procedure. Many people are unwilling to subject their pets to what they perceive to be a painful and invasive procedure. The cost of surgery is prohibitive for many owners, particularly in developing countries.
In addition, many dogs and cats are not owned; thus, shelters and other animal organizations are responsible for financing the surgeries. Neutering these animals involves some method of capturing or trapping the animals, which can be labor-intensive. Trap-neuter-return programs for feral cats and dogs are one such method.3
Nonsurgical forms of contraception are a promising additional method of population control (Table 1). According to the Alliance for Contraception in Cats and Dogs, an ideal nonsurgical product would be safe, effective, affordable, permanent, delivered in a single injection or treatment, available for males and females and for dogs and cats, and have documented effects on behavior and health.4 In this article, we will examine some of the nonsurgical methods currently being researched or developed that have the best chances of fulfilling these criteria.
PHYSIOLOGY REVIEW
A brief review of basic reproductive physiology is necessary to understand the mechanisms of action of these nonsurgical methods. Gonadotropin-releasing hormone (Gn-RH) is a decapeptide hormone produced in the hypothalamus. It is responsible for regulating the release of the glycoproteins luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior lobe of the pituitary gland. In females, LH causes ovulation, corpora lutea formation, and progesterone secretion in the ovary. FSH is responsible for follicular development and estradiol synthesis. In males, LH stimulates testosterone production in the Leydig cells of the testes. FSH is necessary for Sertoli cell function.
CONTRACEPTIVE DRUGS AND CHEMICALS
These agents include implants that contain a Gn-RH-agonist, injections that contain a Gn-RH antagonist, and the chemicals vincyclohexene diepoxide or zinc gluconate.
Gn-RH agonists
Initially, Gn-RH agonists stimulate the release of FSH and LH, thus causing estrus and ovulation, which is a disadvantage of this method. Contraception can be achieved by sustained exposure to Gn-RH agonists, which causes Gn-RH receptor downregulation of pituitary cells and decreased FSH and LH release. Thus, Gn-RH agonists can be used in males and females. These agonists have low bioavailability orally since they can be broken down by peptidases; therefore, they must be administered parenterally.5 Current formulations involve the use of slow-release devices.
Gonazon. Gonazon (Intervet/Schering-Plough Animal Health) contains azagly-nafarelin in a controlled-release device.6,7 It received regulatory approval in the European Union in 2006. The implant is inserted subcutaneously in the umbilical area in dogs and in the neck in cats. Gonazon shows promise as a safe, long-term contraceptive in female dogs and cats, and, possibly, male dogs.
In a placebo-controlled clinical trial of 124 female dogs, Gonazon safely prevented estrus for about one year.6 In a follow-up trial, some of the bitches had the first implant removed and a second one placed. The second implant was left in place for 18 months and was 92% effective at preventing estrus.6 Gonazon reduces testosterone concentration in male dogs.
In another placebo-controlled clinical trial, Gonazon was implanted in 10 4-month-old beagle bitches for one year, and none of them displayed estrus until after the implant was removed, whereas all of the control bitches displayed estrus during this period. The implantation was safe, as it was nonpainful and no inflammatory reactions occurred.
In a trial with queens, Gonazon was effective for about two years. The implantations were painless, and no side effects were noted.
Suprelorin. Suprelorin (Peptech Animal Health) is a controlled-release implant containing the Gn-RH agonist deslorelin. It is administered subcutaneously in the interscapular region. It was approved for use in dogs for testosterone suppression in Australia in December 2004 and in New Zealand in September 2005.It was originally approved at a six-month dose but is now also available at a 12-month dose. Suprelorin is available in some European countries.Although Suprelorin is not labeled for use in female dogs, in one study estrus was delayed for up to 27 months.A similar response was found in female cats for 14 months, and there were no negative side effects.
Gn-RH antagonists
Gonadotropin-releasing hormone antagonists function in contraception by binding to the Gn-RH receptors in the pituitary and preventing Gn-RH from binding. The advantages of antagonists are their immediate effect, and they do not cause post-treatment stimulation and estrus in females. The disadvantages are that antagonists require higher doses than agonists do, controlled-release systems have not been developed, and antagonists are more expensive.
Acyline. Acyline, a third-generation Gn-RH antagonist made up of 10 amino acids, is administered subcutaneously. In male dogs, it causes a slight decrease in scrotal diameter, decreased volumes of the second and third fractions of ejaculate, decreased sperm count and motility, increased sperm abnormalities, and decreased libido and erection without local or systemic side effects.In female dogs, acyline prevents normal estrus and ovulation when it is administered in proestrus and terminates pregnancy in bitches about one week after it is administered at 30 to 35 days of gestation. Unfortunately, the duration of action in male dogs is only about two weeks.More research must be done before acyline is considered a viable option for long-term contraception.
4-Vinylcyclohexene diepoxide
The industrial chemical 4-vinylcyclohexene diepoxide (VCD) destroys the primordial and primary follicles in the ovaries of rats and mice via apoptosis.15 SenesTech is developing a VCD product called ChemSpay that would have the same effect in cats and dogs.
In a study on the Navajo Reservation, 16 dogs were injected subcutaneously with VCD once daily for six days. VCD-treated bitches had significantly decreased heat behavior in the medium and high concentration dose groups vs. control. The dogs were not mated to determine whether pregnancy was prevented. At day 30 after the treatment, the bitches underwent ovariohysterectomy. Ovarian histology showed a statistically significant reduction in primordial follicles in the high-dose group for the total population and in all dose groups for puppies compared with control dogs. Blood chemistry analyses did not show any abnormalities, and the dogs were healthy 2.5 years later. This product shows promise as a safe and irreversible method of contraception.
Chemical castration
Neutersol, an intratesticular injection that contains zinc gluconate neutralized by arginine, causes testicular sclerosis and permanent sterility. It was the first drug approved by the Food and Drug Administration for sterilization of companion animals.It was approved for use in the United States in 2003 for puppies 3 to 10 months of age with testicles measuring 10 to 27 mm in width. The injection volume is based on testicular width. It has been used in larger adult dogs in an extralabel manner. Since 2005 it has not been available in the United States after the patent holder and marketing company discontinued their relationship.
EsterilSol is an identical product available through Ark Sciences in Mexico.4 Infertile (Rhobifarma Indústria Farmacêutica), which contains zinc gluconate and DMSO (dimethylsulfoxide) for intratesticular injection, was launched in Brazil in March 2009.
Neutersol has the advantages of being permanent after one administration and does not require general anesthesia.Outpatient clinics in the United States using Neutersol have been very successful.The product is appealing to owners who do not want their dogs to have surgery or who want their dogs to retain the “masculine” look and presence of testicles. Since there is no noticeable difference in testicular size in adult dogs treated with Neutersol, permanent identification such as microchipping or tattooing is necessary to indicate that the animal has been sterilized.
A large field trial to evaluate Neutersol in 10,000 dogs was conducted in three states in Mexico. Some dogs experienced moderate pain in the first 12 hours after injection.18 In 2.6% of the dogs, severe ulcers developed at the injection site. The ulcers were related to poor injection technique, and the incidence decreased after veterinarians began using separate needles to draw up and administer the solution as well as separate sterile needles for each intratesticular injection in each dog.
In another study on Isabella Island of the Galapagos, 3.9% of 103 dogs given zinc gluconate developed necrotizing injection site reactions. By comparison, 3.4% of 58 dogs experienced wound dehiscence after surgical castration. The zinc gluconate reactions were more severe and required more extensive surgical repair than the traditional surgical complications. The dogs that experienced zinc gluconate reactions were large, mature dogs that received near maximum-volume doses.
IMMUNOCONTRACEPTION
The goal of immunocontraception is to develop a vaccine against a target in the reproductive system. The vaccine would induce antibody formation, and the immunity would then suppress normal reproductive function. The current vaccine targets are the zona pellucida (ZP), Gn-RH, and the LH receptor. Extensive research has been conducted in the field of immunocontraception for humane control of wildlife overpopulation.20,21 Consequently, these strategies have been applied to population control of dogs and cats.
ZP vaccines
The ZP, a mucoprotein layer surrounding the oocyte, consists of three glycoproteins: ZP1, ZP2, and ZP3. Spermatozoa must bind to the ZP before fertilization can occur. Vaccines against ZP are, therefore, only effective in preventing pregnancy in females. Hormonal cycling is not affected. Continued reproductive behaviors may not be acceptable for free-roaming cats and dogs.
A porcine zona pellucida (pZP) vaccine has been effectively used in many wildlife species, including horses, deer, and elephants. However, single-dose pZP vaccines have not been effective in cats. A recent study in which cats were vaccinated with feline ZP subunits showed antibody production specific to feline ZP and lower conception rates compared with the control group.
In dogs, recombinant dog ZP3 was conjugated to a diphtheria toxoid carrier to create a vaccine.It was effective at preventing pregnancy in three out of four bitches, but they required multiple immunizations intramuscularly to keep titers high. Further research is being performed in this area, including the development of a fusion protein of ZP3 and rabies virus glycoprotein expressed in insect cells as well as a DNA vaccine expressing the same fusion protein.
Gn-RH vaccines
The principle of Gn-RH vaccines is that antibodies to Gn-RH prevent its binding to Gn-RH receptors, shutting down the release of LH and FSH. Gn-RH must be coupled with a large carrier in order to be immunogenic. Some disadvantages of Gn-RH vaccines are variations in individual responses, the need for repeated immunization to keep titers high, and the need for an adjuvant. The Gn-RH vaccines have the advantages of inhibiting hormonal cycling and associated behaviors, and being effective in both females and males.
GonaCon, a vaccine containing Gn-RH coupled to keyhole limpet hemocyanin and the mycobacterial adjuvant AdjuVac, was originally developed for the control of white-tailed deer populations by scientists at the USDA Wildlife Service’s National Wildlife Research Center. GonaCon has been tested in male and female cats.There was a wide variation in response and duration of action. It was about 75% effective in female cats, and some cats had estrus prevention for up to 3.5 years after a single dose.28 However, some of the females developed a granuloma at the injection site after 24 months, likely caused by the mycobacterial adjuvant.
GonaCon is not effective for inducing long-term infertility in dogs, and it resulted in severe injection site reactions that persisted for a year until surgical removal. New formulations are being developed to prevent this problem in dogs.
Another Gn-RH vaccine consisting of the antigen IPS-21, a commercially available adjuvant, and the immuno-stimulant dimethyl dioctadecyl ammonium bromide was tested in four male and 10 female cats.30 IPS-21 is a recombinant protein made up of eight tandem repeats of Gn-RH fused to a fragment of leukotoxin A from Mannheimia haemolytica. It was shown to be effective as evidenced by immunoneutralizing titers to GnRH for almost two years, but required several immunizations. None of the females exhibited estrous behavior or became pregnant. All the cats, including the cat injected with a placebo, showed nonpainful, palpable tissue reactions that resolved by day 28. There were no serum chemistry profile abnormalities.
LH receptor vaccines
Female dogs and cats have been vaccinated with bovine LH receptor (LH-R) encapsulated in a silastic subdermal implant, followed by four booster injections of LH-R intramuscularly.This resulted in the formation of LH-R antibodies with subsequent suppression of serum progesterone concentrations and lack of estrus for about one year. None of the females in these studies was mated to determine whether pregnancy was prevented. Further studies are needed to determine the sufficient LH-R antibody titer to cause infertility.
Advantage and disadvantage of surgical Castration methods
Advantage of surgical castration methods:
Surgical sterilization of dogs is one of the most commonly performed procedures in veterinary practice, and is done as a method of contraception to aid in the pet overpopulation problem, as well as to prevent diseases of reproductive system, such as benign prostatic hyperplasia and to modify undesirable behavior, such as internal aggression and mounting of other dogs, Will not be able to reproduce, will not get ovarian or uterine cancer, Will not have dangerous uterine infections, Will not mark territory by urinating or spraying, lessens tendency to fight with other animals despite castration is almost the sole method for control of pet’s overpopulation globally
Disadvantages and postoperative complications:
For many years, surgical castration has been considered a standard gold tool for sterilization of male animals. However, several drawbacks have been associated with this procedure such as high cost, time consumption, need for postoperative care and management, risk of post-operative complications, small-scale application, the requirement of anesthesia, medical equipment, a sterile surgical suite, a trained veterinarian, recovery time, and incision site observation (Jana and Samanta, 2007). Surgical castration also do have complication such as, hemorrhage, wound dehiscence, infections, and scrotal swellings, requires anesthesia has some morbidity and mortality and expensive and technical also not available in much of world (Adin, 2011).
Advantages and disadvantage of non- surgical castration methods
Advantage of non-surgical castration methods:
An ideal chemical sterilizing agent would be one that effectively arrests spermatogenesis and androgenesis as well as libido with absence of toxic or other side effects (Wiebe et al., 1989). Advantages of chemical castration are apparent reduction in pain and stress as well as elimination of hemorrhage, hernia, infection, myiasis and other surgical sequela. It is also suited for mass-scale sterilization, simple and inexpensive (Ibrahim et al., 2016). This method may offer savings in cost, time, and facility requirements, thus helping animal welfare organizations sterilize more animals and/or redirect resources to other lifesaving projects. It also presents an option for pet owners who would prefer to sterilize their dog without surgery. The low cost, ease of use, and cultural acceptance of a sterilization method that does not require removal of the testes make inorganic chemo-sterilants a valuable tool for largescale sterilization campaigns, particularly in areas lacking clinical facilities or skilled staff (Levy et al., 2008)
Disadvantage of non-surgical castration methods:
As a result of these chemical injections, side effects have been documented. The dog will experience pain in his scrotum for three to five days after the injections. There was swelling, redness, and irritation. Lethargy and diarrhea are known side effects that are usually temporary (Threlfall and Immegart, 2000).
Current and Potential Future Products
ZeuterinTM (zinc gluconate/arginine) is the only FDA-approved non-surgical sterilant available for companion animals in the US. It is labeled for use in male dogs from 3-10 months of age. An intratesticular injection causes permanent and irreversible sterility, and the requisite FDA study for product registration showed that testosterone levels of treated dogs were 41-52% lower than intact dogs. The product is available to licensed veterinarians who are trained in the Zeuterin™ injection technique. Zeuterin appeals to a demographic of clients who have concerns about surgery, or simply want their dogs to retain their testicles; it also has found a niche with a scattering of low-cost sterilization programs throughout the US. Another intratesticular injection, calcium chloride, has been reported to cause permanent and irreversible sterility in male dogs when mixed with ethyl alcohol. The product is not approved by the FDA, and its use is considered experimental. Suprelorin® (deslorelin) has been shown to provide effective long-term contraception in bitches and to effectively suppress ovarian activity in cats, but it has not been approved for contraceptive use beyond male dogs. It is approved in the US to treat adrenal disease in ferrets. Another noteworthy application is its current use (via experimental drug license) by Dr. Judith Samson-French and her team through the Dogs With No Names project, in which implants are applied to female dogs living on the First Nations Reserves in Canada. As referenced above, the GnRH vaccine (GonaCon) induces a long-acting contraceptive response in a number of mammalian species. Efforts are underway to fully explore its contraceptive potential in free-roaming cats. A previous study demonstrated the safety and efficacy of GonaCon in female cats in a laboratory setting (Levy et al., 2011). Fifteen female cats were injected intramuscularly with 0.5 ml GonaCon vaccine, and 5 control cats were injected with 0.5 ml sham vaccine. A breeding trial, starting on study Day 120 post-vaccination, demonstrated that vaccinated cats had a longer time to conception (median 39.7 months) compared to shamtreated cats (4.4 mo; P < 0.001). A total of 93% of vaccinated cats remained infertile for the first year following vaccination, and 73%, 53%, and 40% were infertile for 2, 3, and 4 years, respectively. At study termination (5 years after a single GonaCon vaccine injection), four cats (27%) remained infertile. Serological testing showed GnRH antibody titers declined more rapidly in short-term responding cats with < 2 years of infertility (n = 4), compared to long-term responding cats that experienced fertility control for >2 years (n = 11) (P < 0.05). Given the promise of the laboratory study, a field study is now underway at the University of Illinois that will test the safety and efficacy of a similar GnRH-based vaccine under field-like conditions that simulate a free-roaming cat colony.
New Products: Contraceptive Drugs and Vaccines
Zinc gluconate (Neutersol®/ EsterilSolTM/Infertile®) is the first permanent, non-surgical method of sterilization for companion animals. It is currently licensed for use in the U.S. for chemical castration of puppies 3-10 months of age, although it has been shown to be effective in adult dogs and cats as well. It is delivered via an intratesticular injection, which results in sclerosis of the testes and permanent sterility. It is 99% effective and very safe. The precise mechanism of action is unknown; the testicles atrophy over weeks to months following injection, resulting in a 70-90% reduction in testicular size in very young puppies and 50% in older dogs (atrophy may not be symmetrical). Sterility may take up to 60 days in postpubescent males. In most cases, zinc gluconate can be administered without sedation. FDA studies showed that zinc gluconate reduces but does not abolish testosterone production, and its effects on hormone dependent diseases and behaviors have not been established. However, studies have revealed a significant decrease in prostate size in zinc gluconate-injected dogs versus controls. The only significant safety concern is the development of scrotal ulcers at the injection site in a small percentage of dogs. This appears to be most commonly observed in large adult dogs (an off-label use) and may be related to poor injection technique allowing some of the chemical to contact the scrotal tissues. Injection site reaction rates are similar to rates of wound complications in surgically castrated dogs, but use of zinc gluconate avoids adverse events or deaths associated with the use of anesthesia. This product is a useful option in veterinary practice as well as animal shelters. The obvious advantage is that it eliminates the need for anesthesia and surgery and saves substantial time. While available previously for a short time from Addison Laboratories, Neutersol was used successfully by programs in the U.S. (including one event in which 200 dogs were sterilized in one day) and abroad (including a 10,000 dog study conducted by the head of the Mexican VMA which demonstrated safety and efficacy in dogs over 10 months of age). A new product with a slightly different formulation of zinc gluconate with DMSO has recently been released in Brazil under the brand name Infertile®).
Other Approaches Under Development
GonaCon®: Dr. Lowell Miller and Kathy Fagerstone of the National Wildlife Research Center of the USDA presented data on the GnRH vaccine GonaCon, developed and tested for use in several wildlife species. Data are being submitted to the EPA initially for approval for use in deer and other cervids. Pilot studies in dogs by Drs. Brenda Griffin and Henry Baker at Auburn University revealed severe injection site reactions and inconsistent suppression of fertility in male beagles. Evaluation in cats by Dr. Julie Levy at UF has yielded more promising results, and a majority of female cats responded to a single injection with more than three years of infertility. Injection site reactions consisting of a small subcutaneous lump occurred after two years in a few cats, several of these resolved spontaneously.
CURRENT RESEARCH: NEW MODES OF CONTRACEPTION
These include reports of several promising approaches intended to interfere with GnRH action, with function of pituitary gonadotrope cells responsible for secretion of LH and FSH, or with the action of LH on the ovary. Such approaches include presentation of GnRH or GnRH-like peptide multimers and/or protein conjugates as vaccines in protocols using new and more readily acceptable, less irritating adjuvants; delivery of a GnRH-vaccine expressed by modified virus; administrations of GnRH-peptides with GnRH or GnRH-agonist linked to a cytotoxin and/or membrane-active lytic peptide, intended to result in gonadotrope cell uptake of the cytotoxin and cell destruction; and use of a vaccine for immunization against bovine LH-receptor presented via silastic implants and serial booster injections, intended to prevent ovarian cells from responding properly to endogenous LH. Additional vaccine approaches include myriad proposed methods for immunization against the peri-oocyte zona pellucida proteins, especially ZP3, using as immunogen either a purified ZP protein or crude ZP preparation with one of many proposed acceptable but potentially sufficiently effective adjuvant or carrier system,
CONCLUSION
With the worldwide overpopulation of dogs and cats, a variety of options are needed for population control. Nonsurgical methods of contraception are one such option. Gonazon, Suprelorin, Neutersol, Esterilsol, and Infertile are the only approved products for dogs or cats, but they are limited to certain countries. Extensive research and development are under way on Gn-RH agonists and antagonists, VCD, immunocontraception, and chemical castration. None of the products meets all of the criteria set forth by the Alliance for Contraception in Cats and Dogs. However, it is likely that new products will be available in the future that offer hope for controlling the free-roaming dog and cat populations.
DR VINOD KUMAR SRIVASTAVA,DUVASU
REFERENCE-ON REQUEST
