Before embarking on a review of male contraceptive techniques, it is worthwhile defining some of the terminology involved. In this review, the term ‘contraception’ in the male refers to any procedure that prevents spermatozoa achieving successful fertilization of the oocyte. In addition to immunological techniques, and procedures such as transection or occlusion of the ductus deferens, male contraception may involve inactivation of the gonads. Importantly, contraception commonly implies the possibility of reversibility (Munson, 2006). In contrast, castration is defined here as any process rendering a male incapable of reproduction through permanent removal, destruction or inactivation of the gonads. Castration may be achieved by surgical, chemical or possibly immunological means, but to be considered as castration in this discussion, the technique must be irreversible. Permanency is an important aspect of the definition that provides a universal understanding that castration is not a reversible procedure.

Male Contraception – Defining the Need

The application of male contraceptive techniques is justifiable only if there are benefits to one or more of the following:

1. The welfare or management of the treated animal.

2. The welfare or management of an animal population.

3. The welfare or management of another species (including humans) that may be adversely affected by the treated animals.

4. The sustainability of a habitat that might otherwise be adversely affected by the treated animals.

5. Disease control within animal and human populations.

With these factors in mind, male contraception is most commonly undertaken to control animal populations, modify undesirable behaviour, modify secondary sex characteristics to assist with animal management and assist with disease control. There are also very specific applications for male contraception, such as teaser animals (i.e. a sterile male used for the purposes of eliciting sexual behaviour in the female) for the preparation of sheep, cattle and horses to assist with the management of breeding programmes.

Population control

Population control may have different end points depending on the context. In many instances, it specifically refers to efforts to correct or to prevent excessive animal numbers. However, in animal husbandry, population control can relate to the close management of breeding in order to manipulate the timing of progeny production and/or the genetic make-up of the population.

Overpopulation can be problematic for both domestic and wild animal species. Domestic pet overpopulation is well recognized, with estimates of between 10 and 20 million unwanted dogs and cats being euthanized every year in the USA (Bowen, 2008), and similar trends noted throughout the world (RSPCA, 2012). Other undesirable consequences of pet overpopulation include the economic cost of animal control programmes to society, the potential for serious injuries being inflicted on other animals or humans, and sanitation problems in cities associated with animal faeces and urine. Depending on geographic location, there are significant overpopulation problems associated with many other animal species, including wild or feral horses, deer, elk, geese, pigs, possums, rabbits and elephants (Barfield et al., 2006).

There is also an annual loss of wild or feral animals due to starvation, being killed on roads, or death associated with fighting or hunting. Stray or wild animals on roads can pose a major risk to human safety. In countries where rabies is prevalent, stray dogs are important vectors of the virus, leading to the deaths of tens of thousands of people annually (Knobel et al., 2005).

Situations may arise in zoological collections where otherwise endangered animals become overpopulated, resulting in the need to restrict or prevent further reproduction (Bowen, 2008). Control of the genetic makeup of a zoological or production animal population, as well as the common managerial requirement to control when offspring are produced, are managed in many species by castration of excess males. This is usually complemented by the provision of suitable infrastructure to keep intact males and females separate until breeding is required. As castration is irreversible, the problem with this approach is that it permanently removes what may be genetically desirable individuals from the gene pool before they have had an opportunity to prove their value. These contexts provide the motivation to develop effective, but reversible, contraception and behaviour modification so that animals may be utilized safely, yet still retain breeding potential should they prove suitable.

Modification of behaviour

The sex steroids have profound effects on male behaviour (Senger, 2003), influencing social interactions, libido, aggression and territorial activity. Due to their anabolic, anti-inflammatory and behaviour-modifying effects, there is the possibility that sex steroids may also influence the feeling of wellbeing in animals, which could affect athletic performance in species such as horses and dogs. Therefore, when developing contraceptive techniques in any species, the influence of the technique on the sex steroids and behaviour of the animal needs to be considered in conjunction with the husbandry under which the animal will be maintained. With techniques such as orchidectomy, sex steroid concentrations are permanently reduced and consideration may be given as to whether it is appropriate or legal to supplement animals with exogenous steroids if the retention of male behaviour is required.

Modification of secondary sex characteristics

Secondary sex characteristics are not directly associated with the reproductive system, but distinguish gender. Male characteristics vary widely depending on the species, with manifestations as diverse as: tusks in boars, variations in vocal range, spines on the penis in cats, manes in lions, scent glands, meat taint, the presence and size of horns or antlers, pheromone production, increased muscular development and, in birds, varied plumage. When considering a method of contraception, it is necessary to determine whether or not the maintenance of secondary sex characteristics is desirable in a species. For example, boar taint in pork can adversely affect consumer acceptance, and requires contraceptive methods to reduce sex steroid production. In contrast, the loss of secondary sex characteristics in males of feral animal populations may leave them vulnerable to injury or traumatic death resulting from adverse social interactions with intact individuals, thus highlighting a need for contraception with retained production of sex steroids.

Influencing disease processes

Methods of contraception that reduce circulating concentrations of sex steroids can assist with disease control by preventing sexually transmissible disease, by reducing steroid hormone influence on specific tissues, or by modifying behaviour that may lead to traumatic injury. There are a number of disease conditions that may be positively influenced by reduced concentrations of sex steroids, such as prostatic disease in dogs (Howe, 2006), and a reduced prevalence of injuries associated with territorial behaviour in various mammals (Hart, 1974). In the domestic situation, this is particularly relevant to catfight wounds, but also has relevance in production animal and equine contexts. Examples of sexually transmitted diseases that can be reduced or prevented by contraceptive methods preventing copulation include: transmissible venereal tumour and brucellosis in dogs; campylobacteriosis and trichomoniasis in cattle; and contagious equine metritis and coital exanthema in horses.

From the aspect of human health, the control of feral animal populations can lead to a reduction in the prevalence of bite injuries and of many zoonotic diseases, such as rabies, leptospirosis, Q fever, toxoplasmosis and other parasitic infestations (Golden, 2009; Spay Neuter Task Force, 2011).

Contraception Based on Surgical Techniques

Surgical castration and contraceptive techniques in the male consist of: the destruction or removal of the gonads; occlusion or removal of a portion of the excurrent duct system (most commonly vasectomy, but also epididymectomy); methods of redirecting or restricting the penis to prevent intromission; or amputation of the penis. This section will focus on orchidectomy and vasectomy only.

Orchidectomy

Currently, the most reliable method of sterilization for male animals of most species is to physically disassociate the vascular supply from the testicles. This requires the testicles to either be removed or rendered atrophic, and can be achieved by open or closed techniques. The result is the generally desirable combination of infertility, behaviour modification and, where applicable, improvement in consumer acceptance of meat products. While these techniques are reliable and well tolerated, it should be noted that in a field situation, care should be taken with animals exhibiting conditions such as cryptorchidism or inguinal herniation. As with all surgical procedures, there may be complications associated with restraint, anaesthesia, infection, haemorrhage and cutaneous myiasis. It appears that fewer complications are encountered when the procedure is performed on younger animals, and this is reflected in the policy documents and legislature of many jurisdictions, where it is recommended that a higher level of skill and facilities is required for surgical castration as animals get older. For species in which the surgery is performed in the field, cool, dust-free environments are desirable in order to reduce wound contamination.

As a summary, there are four physical methods that are utilized to destroy or remove the testicles. These are:

• Orchidectomy via a scrotal or parascrotal incision, which is applicable to males of most species.

• Closed constriction of the vascular supply to the testicles utilizing elastic (Elastrator^®) rings, as are commonly used in young production animals.

• Closed crushing of the vascular supply to the testicles utilizing a Burdizzo^® device, as used in cattle.

Advantages

Surgical castration is currently perhaps the most common form of contraception used for male animals, regardless of animal species or animal management context. This is true in: the cattle industry (Coetzee et al., 2010); the pig industry, in which, with the exception of a few European countries and Australia, male pigs generally undergo orchidectomy at a very young age (Bonneau and Enright, 1995); dogs and cats, where orchidectomy is the most common method of male contraception (Bowen, 2008); and the horse industry (Reilly and Cimetti, 2005), where there is a requirement for both contraception and the behaviour modification of most males. The main advantage of this technique is that permanent contraception can be guaranteed when performed by an experienced operator.

Due to the widespread use of surgical castration techniques within each industry, there are generally adequate numbers of experienced operators to competently perform the procedure in a humane manner. With species variation, orchidectomy is also beneficial in reducing the prevalence of diseases such as prostatic hyperplasia, testicular tumours and sexually transmissible diseases.

Disadvantages

Surgical castration is labour intensive, can cause morbidity and mortality, is stressful to the animal (Von Waldmann et al., 1994), and there are ethical and animal welfare concerns (Bonneau and Enright, 1995; Coetzee et al., 2010). Importantly, as with all surgical procedures, there are inherent costs and risks associated with restraint, anaesthesia and surgery. In some production contexts, the removal of the source of sex steroids creates economic disadvantages, as intact males are usually more feed efficient and leaner than castrated animals (Bonneau and Enright, 1995; Oonk et al., 1998). Because intact males have betterfeed efficiency, avoiding the loss of sex steroids associated with castration may also significantly reduce the amount of biological pollutants excreted by production animals into the environment.

The adverse effects of castration on growth and efficiency can largely be reversed by the administration of anabolic steroids, although this is not an option in many countries, where their use is banned; it also does not solve the ethical and welfare concerns that are associated with surgical castration. So the challenge is to reduce management and meat quality concerns and, at the same time, maintain the advantages of intact animals (Bonneau and Enright, 1995).

In control programmes for the feral animal population, orchidectomy may not be practical and the loss of male behaviour associated with removal of the gonads may have an undesirable influence on social dynamics and territorial behaviour (Jewgenow et al., 2006). This may adversely affect the wellbeing of individual animals, as well as interfering with the effectiveness of population control methods that require sterile but sexually active males.

Vasectomy

In veterinary terminology, the duct leading from the tail of the epididymis to the ampulla or pelvic urethra is described as the deferent duct, or ductus deferens (Senger, 2003). However, in human terminology, it is described as the vas deferens, and remnants of this terminology associated with surgical or manipulative procedures have persisted within the veterinary literature. Thus, surgical vasectomy refers to bilateral removal of a portion of the ductus deferens, rendering the animal sterile by preventing sperm from being ejaculated during copulation (Johnston et al., 2001a). A technique resulting in a similar outcome, but not technically a vasectomy, is ductal occlusion.

Studies have reported azoospermia to occur from 2 to 21 days in the dog following bilateral vasectomy (Pineda et al., 1976; Schiff et al., 2003), and within 1 week in rams (Boundy and Cox, 1996). This is relatively quick compared with cats, in which live sperm have been identified for up to 49 days following pre-scrotal vasectomy (Pineda and Dooley, 1984). Variations in the duration from vasectomy to azoospermia is associated with sperm storage capacity and the presence or absence of accessory sex glands such as the seminal vesicles (Schiff et al., 2003), which may produce secretions to support sperm viability for a short duration.

Advantages

Vasectomy is a well-developed method for permanent contraception, which has been applied to pets in order to eliminate reproductive potential while retaining their testicular function. In humans, methods for successful reversal (vasovasostomy) are well documented (Bowen, 2008), and these techniques have been transferred into animal contexts. This provides the possibility of a return to fertility should genetic worthiness be identified subsequent to contraception. There is limited data from animals, but pregnancy rates following vasovasostomy in humans are estimated at 60% if the vasectomy was performed less than 5 years before reversal and 40% if performed more than 5 years from the original surgery (Barfield et al., 2006).

In the veterinary context, vasectomy is a relatively quick procedure, but still requires general anaesthesia in companion animals. Vasectomy of dominant males has been suggested as a method of feral cat population control (Howe, 2006), as vasectomized dominant tomcats can prevent submissive, intact toms from inseminating non-spayed females, and can reduce receptivity in queens by inducing periods of pseudopregnancy. In sheep, vasectomy is an ideal contraceptive choice when preparing teaser rams to assist with artificial breeding programmes, as it is a quick procedure and allows the required male behaviour to remain unaltered (Boundy and Cox, 1996). There is also application for vasectomy or ductal occlusion in captive animals, and the successful vasovasostomy of a zoo animal was confirmed when a bush dog at the St Louis Zoo in Missouri sired three healthy pups (Barfield et al., 2006).

Disadvantages

Despite the effectiveness of vasectomy as a contraceptive technique, most owners of domestic animals desire both contraception and behaviour modification for their pets. The persistence of male sex characteristics and behaviours after vasectomy may permit territorial fighting and androgen-dependent conditions such as prostatic disease to develop (Johnston et al., 2001a). These considerations, coupled with the fact that vasectomy in dogs and cats either costs the same as or, commonly, is more expensive than castration means that it is rarely practised in domestic species (Bowen, 2008). In a wild, or feral animal context there is a significant disadvantage in vasectomy owing to the need to capture, transport, anaesthetize and release animals.

Iatrogenic ‘high-flanker’ cryptorchidism

In cattle, there are anecdotal reports of placing rubber (Elastrator^™) rings around the scrotum, distal to the testicles, to push them firmly up into the inguinal region; thereby producing an iatrogenic, bilateral, ‘high-flanker’ cryptorchid. While there are no published studies reporting the effectiveness of this technique, placing the testicle close to the abdominal wall will ensure a sustained increase in testicular temperature compared with the normal scrotal position. A prolonged increase in temperature should adversely affect spermatogenesis but maintain testosterone production. Risks associated with this technique include wound dehiscence, clostridial infection and variable effectiveness.

Contraception Based on Non-surgical Castration and Ductal Obstruction

Non-surgical approaches to physical destruction of the testicles have been based on injection into the testes of a variety of materials that induce tissue destruction, orchitis and fibrosis. Other targets for blocking sperm flow include the epididymides and the ductus deferens. Numerous sclerosing agents have been injected into the tail of the epididymis to induce blockage of the tubules and subsequent azoospermia. Physical devices or chemical compounds have also been used to block the ductus deferens. Basic welfare requirements for any chemical injected into the testes, epididymides, or ductus deferens should be that they are non-mutagenic, non-carcinogenic, non-teratogenic and induce minimal pain.

Ductal obstruction

Methods of obstructing the ductus deferens can be divided into two broad categories: extravasal and intravasal techniques. For blocking the epididymides, sclerosing agents have most commonly been utilized.

Extravasal methods of obstructing the ductus deferens involve the placement of a device, such as clips or a suture, around the ductus deferens in order to cause occlusion. Extravasal techniques are not commonly used in humans owing to the difficulty in removing clips, which leads to inadequate restoration of fertility, where this is needed (Barfield et al., 2006). However, the requirement for reversibility may not be as relevant in animal species. Yet attempts to avoid surgery by placing clips across the scrotal skin to occlude the ductus deferens have proven ineffective and traumatic (Barfield et al., 2006).

Intravasal methods of ductal obstruction include the use of injectable silicone, cylindrical plugs, spherical and polypropylene beads, threads of silicone or suture material. Unfortunately, none of these techniques can sustain effective long-term sperm obstruction without generating fibrosis or perforation. A ductus deferens valve was developed, with the goal of turning sperm flow on or off, but unfortunately reliable occlusion and problems with ductus perforation could not be overcome (Barfield et al., 2006). Percutaneous injection of sclerosing chemicals into the lumen of the ductus using compounds such as ethanol, silver nitrate, acetic acid and formaldehyde has been used as a non-reversible method of ductal occlusion in rats and dogs (Freeman and Coffey, 1973). While generally effective, there is a possibility of retrograde flow of the chemical to the testis, causing testicular atrophy (Barfield et al., 2006). This may or may not be a concern, depending on the context in which it is used and any need for reversibility.

Zinc arginine (ZA) is one of the primary sclerosing agents that has been trialled for ductal obstruction of the canine epididymis, with the intra-epididymal injection of 50 mg of ZA (0.5 ml/testis) resulting in azoospermia within 90 days of injection (Fahim et al., 1993). ZA is used because it is considered to be non-mutagenic, non-carcinogenic, and nonteratogenic (Fahim et al., 1993). Contraception by injecting ZA into the epididymis initially seemed quite promising in both dogs and cats (Pineda and Dooley, 1984; Fahim et al., 1993). Unfortunately, when applied more widely in the context of animal shelters, it was associated with a high incidence of serious inflammatory responses, and is not currently considered suitable for contraception in domestic species (Bowen, 2008). Despite this, the ZA formulation has been found to be suitable for intra-testicular administration to induce testicular degeneration, and its use for this purpose will be discussed further in the next section.

Induced Testicular Degeneration

Chemical methods of inducing testicular degeneration

Testicular degeneration can be chemically induced by a number of methods. These include the direct injection of a chemical into the testicular parenchyma, the administration of a chemical by the oral or parenteral route, and, more recently, the conjugation of cytotoxic agents to gonadotrophins. This section will focus on the induction of testicular degeneration by the intra-testicular administration of chemicals.

One protocol that has generated significant interest involves the intra-testicular injection of solutions containing zinc gluconate neutralized to a pH of 7 with arginine. This formulation (commonly referred to as zinc arginine, or ZA) is considered to be efficacious and safe based on contraceptive efficacy and minimal adverse reactions, especially when applied to young dogs (Tepsumethanon et al., 2005). The product is quite expensive, but it was marketed for a brief time for canine contraception as Neutersol^®. It has been trialled in other species as well, but with equivocal results (Brito et al., 2011), possibly due to the dosing used being based on canine studies. Unfortunately, with canine use, there were also some undesirable inflammatory responses that warranted withdrawal of the product from the US market in 2011. With modifications to the formulation, the product may re-emerge commercially as Esterisol^®, Zeuterin^® or Testoblock^®, and with a claim to have use as a method of permanent contraception in dogs after a single administration. Despite this claim, it must be recognized that severe adverse reactions may still occur in close to 4% of cases, including necrotizing reactions requiring surgical orchidectomy and debridement (Levy et al., 2008). Such reactions can be minimized through careful dose placement and calculation of the dose for each testicle based on width measurements.

Physical methods of inducing testicular degeneration

Ultrasound has been utilized to interfere with sperm production or transport via the combined effect of heat and mechanical vibration (Fahim et al., 1977; Fried et al., 2002; Roberts et al., 2002; Leoci et al., 2009). Targeted structures include testicular parenchyma, the epididymides or the ductus deferentia. For the latter two structures, short-term (20 to 120 s), high-energy (3–19 W) ultrasound can be administered to induce coagulative necrosis, resulting in luminal occlusion within 2 weeks of treatment (Fried et al., 2002; Roberts et al., 2002). However, there is still a need for long-term studies to assess the contraceptive efficacy of this method. With current techniques and technology, there is an unacceptably high occurrence of adverse reactions such as skin burns (Fried et al., 2002), though studies to accurately determine the ideal therapeutic window for both power and duration of application for different species may help overcome this problem. The testicular parenchyma of dogs and toms were treated with a high-intensity, focused ultrasound consisting of 1–2 W/cm² for 10–15 min administered one to three times at 2–7 day intervals (Fahim et al., 1977; Leoci et al., 2009). Although it is apparent that further work is necessary to clearly define treatment intensity and duration, ultrasound administration was shown to suppress spermatogenesis without affecting testosterone concentrations.

Contraception Based on Immunological Approaches

The possibilities for immunocontraception in the male include stimulation of either the humoral or cell-mediated actions of the immune system, or a combination of these. The goals are to either stimulate antibody production against molecules (most commonly hormones) important for fertility, or to activate a cell-mediated response involving cytotoxic T-lymphocytes to specifically destroy cells required for fertility (Golden, 2009).

Secondly, immunocontraceptive targets include specific gonadal and extragonadal sites. Gonadal targets include the testicular germ cells and supporting somatic Sertoli and Leydig cells (McLaughlin and Aitken, 2011). Extragonadal sites for vaccine targeting include the epididymis and its luminal content of maturing spermatozoa. An example of this approach is immunization of male monkeys with human recombinant epididymal protease inhibitor (EPPIN) to suppress sperm maturation (Yan Cheng and Mruk, 2010). The development of vaccines against sperm membrane proteins such as glycosylphosphatidylinositol (GPI)-anchored sperm-specific protein (PH-20) is also being investigated (Sabeur et al., 2002), though these anti-sperm vaccines may be more suited to deployment within the female population.

Gonadal germ cells express unique antigens, some of which develop at the time of sexual maturation, long after the differentiation between self and non-self that occurs early in fetal development (Pöllänen and Cooper, 1994; Golden, 2009). Therefore, gonadal tissues contain non-self-antigens requiring protection from the body’s normal defence mechanisms. The traditional view has been that complete sequestration of testicular autoantigens behind the blood–testis barrier was the only protective mechanism preventing immune responses against them (Pöllänen and Cooper, 1994). Although the inter-Sertoli-cell tight junctions certainly protect the autoantigenic germ cells in the luminal compartment of the seminiferous tubules, there is evidence that there are also autoantigens on the surface of germ cells just about to enter meiosis, but still in the basal compartment of the seminiferous tubules (Yule et al., 1988; Saari et al., 1996). In addition, sperm autoantigens have been identified within the epididymis at concentrations exceeding that within the testis (Pöllänen and Cooper, 1994). It is now apparent that the regulation of immune responses to these antigens involves a system of interactions requiring a balance between activation and attenuation of responses (Golden, 2009) to ensure there are few or no immune responses mounted against most of these cells or tissues. In most cases, there is more than one protective mechanism in place to shield sperm autoantigens (Verajankorva et al., 1999).

Hence, the development of an immunocontraceptive targeting gonadal germ cells requires selection of a suitable target antigen within the gonad, production of an effective vaccine and development of a suitable mode of delivery to overcome whichever immunoprotective mechanism may be in place. Importantly, the development of immunity to autoantigens increases the risk of inducing autoimmune reactions. These can present complex challenges to address (Verajankorva et al., 1999).

However, perhaps, the greatest obstacle to the extensive use of immunocontraceptive technology is the variability of the duration and intensity of those immune responses that are attained (Bowen, 2008). Due to the difficulty in predicting the efficacy and duration of effect, there are problems in utilizing this form of contraception in light of the definitive contraceptive requirements for the reproductive management of most species. In general, hormonal vaccines targeting the HPG axis provide only short-term contraception from 3 to 12 months duration, but, with the use of suitable adjuvants, and the strategic administration of booster vaccinations, GnRH vaccines may be able to deliver useful fertility control in some species over extended periods (Dowsett et al., 1996; Miller et al., 2004; Walker et al., 2007; Bowen, 2008); this method would then have the potential advantage of reversibility.

GnRH and pituitary hormone vaccines

Since its initial discovery in 1971, GnRH has been considered to be the main hormone regulating and controlling gonadotrophins in the HPG axis (Schally et al., 1971), and it is only in the last decade that other peptide hormones, such as kisspeptin and gonadotrophin inhibitory hormone, have been identified to also be significant contributors to HPG control. Despite the recent identification and significance of these latter peptides, GnRH still plays a critical role in reproductive control, leading to a body of research into the development of GnRH vaccines as a means of contraception (Hoskinson et al., 1990; Bonneau and Enright, 1995; Dowsett et al., 1996; Levy et al., 2004; Golden, 2009).

In mammals, GnRH-producing neurons are generally found scattered within the hypothalamus extending from the median eminence (ME) through to the medial basal hypothalamus (MBH) and to the preoptic area (POA) (Jasoni and Porteous, 2009; Scott et al., 2010). Within this distribution, the GnRH neurons are grouped into nuclei, with species variation in GnRH activity within each nuclei. For example, GnRH neurons in rodents have been identified in the ME, POA and anterior hypothalamic areas (AHA); in horses, they are primarily located in the arcuate nucleus of the MBH (Scott et al., 2010); in primates and sheep, the distribution of GnRH neurons extends from the ME through to the arcuate nucleus (ARC) of the MBH (Herbison, 2006). It is only recently that the specific anatomical location and distribution of GnRH neurons in the canine hypothalamus has been described; the pattern of distribution is heavily concentrated in the MBH, including the ME and extending into the ARC (Buchholz et al., 2012).

GnRH is produced in cell bodies of the hypothalamic neurons and is transported by axonal flow to the terminal synapses. These synapses lie adjacent to the vessels of the capillary plexus within the ME (Herbison, 2006). Stimulation of the GnRH neurons causes release of stored peptide from their secretory granules into the extracellular space, followed by diffusion into the capillary blood of the ME. GnRH then travels via the hypophysial portal system to the capillary plexus within the anterior pituitary. Here, a portion of the GnRH leaves the capillaries, and becomes available for binding to the pituitary gonadotrophs, stimulating the release of FSH and LH (luteinizing hormone). It is during transport within the hypophysial portal blood that GnRH can be immunologically targeted.

GnRH is a small peptide that is identical in all mammals (Senger, 2003). These two factors (small and identical) result in it producing minimal immunogenicity. However, it can be made more immunogenic by coupling it to a carrier such as keyhole limpet haemocyanin (KLH) (Miller et al., 2004), or ovalbumin (Hoskinson et al., 1990). The coupled GnRH peptide is called a GnRH-conjugate; this is combined with an adjuvant to create a vaccine (Hoskinson et al., 1990).

In animals treated with GnRH vaccine, anti-GnRH antibody within the hypophysial portal blood binds to the newly released GnRH from the hypothalamus, thus preventing GnRH from binding to the pituitary gonadotroph cells (Miller et al., 2004). Provided that sufficient specific antibodies are present in the circulating blood entering the ME, virtually all of the GnRH peptide secreted into the hypophysial portal vessels will be bound and neutralized (Donald, 2000). The binding of the antibody to GnRH neutralizes it either by preventing it from diffusing through the capillary walls owing to the size of the GnRH-antibody complex, or by masking the receptor binding site on the GnRH molecule. This neutralization of endogenous GnRH results in profound suppressive effects on the pituitary gonadotroph cells. Further, physiological responses to GnRH and its efficacy are correlated with GnRH antibody titre; animals with high titres are sterile, whereas those with lower titres are not (Levy et al., 2004).

Advantages

Although large-scale trials are lacking, the induction of immunity to GnRH represents a promising approach to contraception for males. It can be administered with two injections, and possibly by a single injection; no surgery is required; there are minimal adverse reactions; it has the potential to stop both spermatogenesis and androgenrelated behaviour; and, in theory, it should be reversible. As the efficacy of treatment is closely correlated with antibody concentration, measurement of anti-GnRH antibody concentrations can be used as a means of assessing vaccination history and current function. Immunization with GnRH-conjugate vaccines has been reported to induce periods of sterility or infertility in dogs (Ladd et al., 1994; Jung et al., 2005), cats, rats (Levy et al., 2004), deer (Miller et al., 2004) and boars (Wagner et al., 2006). The vaccines also prevent androgen-driven aggression as effectively as surgical castration for bulls (Price et al., 2003), stallions, elephants (Barfield et al., 2006), piglets and lambs (Levy et al., 2004), though the influence on spermatogenesis has not been adequately studied in these latter species. With the development of suitable delivery methods, GnRH vaccines may be acceptable for the control of pest species (Barfield et al., 2006). Specific uses of the vaccine are outlined in the section on contextspecific applications of male contraception.

Disadvantages

Despite some successes, GnRH vaccine technology is still developing. For example, the same GnRH vaccine that was effective in dogs had no efficacy in cats (Ladd et al., 1994). More recently, it has been suggested that these anomalies may be overcome by modifications to the injection delivery method so that the antigen is protected from rapid destruction by the animal’s immune system (Miller et al., 2004).

Perhaps the biggest disadvantage of GnRH vaccine contraceptive technology is the difficulty in stimulating a prolonged immune response of predictable duration. Coupled with this is the requirement for administering one or two injections for the induction of immunity, with the possible requirement for booster vaccinations. This may be impractical in some animal management contexts. However, GnRH conjugation technology has been progressing since the 1990s; a commercial product has been released in the Australian market (Hoskinson et al., 1990), and further development of a single-injection product is available in the USA (Miller et al., 2004).

Within a given species, there is concern that a particular vaccine may be very effective in inducing infertility in some animals, but not effective in others. This variation in efficacy has led to concern that significant application of an immunocontraceptive may result in selection of a population of non-responders with altered or restricted immunogenetic make-up and responses (Cooper and Larsen, 2006). A corollary is that the duration of action of effective contraceptive vaccines is typically quite variable, making animal management less predictable than definitive methods such as surgical castration.

Immunocontraception targeting sperm membrane proteins

Immunocontraceptive technology targeting sperm membrane proteins is well suited to female contraception by the induction of sperm antibody formation in the female reproductive tract. When administered to females, such an immunocontraceptive can prevent fertilization by blocking sperm–oocyte interaction. In some contexts, there is also potential for its administration to be applicable to males (Sabeur et al., 2002). In general, successful immunocontraception in males targeting sperm-membrane proteins will prevent fertilization of the oocyte and, at the same time, preserve sexual behaviour and secondary sex characteristics.