Fertility Control to Reduce Disease Transmission

Fertility control has been advocated as a means to reduce contact rate and disease transmission between individuals in the context of activities for the control of wildlife diseases and zoonoses. Several theoretical and field studies have been carried out to determine the impact of fertility control on zoonoses in wildlife and in free-roaming cats and dogs. Examples for zoonoses such as bovine tuberculosis, brucellosis, leptospirosis, and rabies are provided in Table 13.1. Many studies indicated that culling can lead to immigration, social disruption, and actually result in increased contact rate as animals tend to move over long distances to fill the voids left by those that have been removed from the population, and to re-establish territories (e.g. Bolzoni et al., 2007; Woodroffe et al., 2009). Conversely, fertility control is less likely to affect social organization, as animal movements in particular are less affected by reproductive inhibition than by culling (e.g. Swinton et al., 1997; Tuyttens and MacDonald, 1998, Saunders et al., 2002). Contraception also offers an added benefit, as the elimination of reproductive behaviour results in a decreased contact rate between animals, and thus in a lower risk of disease transmission (Killian et al., 2007; Ramsey, 2007). The renewed interest in contraceptives to manage wildlife is also based on field and theoretical evidence suggesting that an effective, economically sustainable reduction of free-roaming animals can only be achieved by chemical sterilization, particularly in areas where veterinary care is not widely available nor affordable (Levy, 2011). The inability to sterilize a high proportion of free-roaming dogs through surgical sterilization was quoted as the main factor responsible for the failure of the Animal Birth Control (ABC) in India (Menezes, 2008). On the other hand, an intensive 8-year ABC in Jaipur (India), carried out on 24,986 dogs, resulted in sterilization of 64% of the females and in a decrease in rabies incidence (Reece and Chawla, 2006). In another Indian town, Jodhpur, a 3-year ABC programme led to 62–86% of free-roaming female and male dogs sterilized (Totton et al., 2010). Although these studies demonstrated that well-coordinated surgical sterilization can decrease the number of dogs, such programmes are likely to be significantly more expensive than those based on catch-injectand-release where dogs are caught, injected with a rabies vaccine and a contraceptive, and immediately released (Massei et al., 2010). Dog owners may be reluctant to consider surgery for other reasons than just cost. A study in Sao Paulo, Brazil, found that 56.5% of people who had adopted shelter dogs were against surgical sterilization, quoting compassion (58.1%), unnecessary procedure (11.4%), cost (9.5%), and behavioural changes (4.8%) as reasons against this method (Soto et al., 2005). Similarly, on Isabela Island (Galapagos), dog owners more frequently selected chemical castration over surgical castration to retain their dogs’ perceived protective and hunting behaviour, and to avoid anaesthesia (Levy et al., 2008). Recently developed mathematical models suggest that using contraceptives in conjunction with rabies vaccination considerably decreased the effort required for rabies elimination, by reducing both the proportion of dogs to be vaccinated against rabies and the time to achieve rabies elimination (Carrol et al., 2010). These findings confirm the predictions of previous models indicating that adding fertility control to vaccination could be more efficient than simple vaccination in eliminating diseases such as rabies in foxes, leptospirosis in brushtail possums, and tuberculosis in European badgers (e.g. White et al., 1997; Smith and Cheeseman, 2002; Ramsey, 2007) (Table 13.1).

Contraception can be achieved by preventing gamete formation, conception or implantation, or by disrupting pregnancy and causing resorption or abortion. The cascade of hormones leading to reproduction is regulated by the gonadotropin-releasing hormone (GnRH), which is produced in the hypothalamus at the base of the brain. GnRH controls the release of the pituitary gonadotropins LH (luteinizing hormone) and FSH (follicle-stimulating hormone). These gonadotropins regulate steroid hormones that drive sperm production, follicular development, and ovulation. In females, FSH stimulation of the follicles in the ovary results in secretion of oestrogen which in turn promotes the development of female secondary sexual characteristics, such as breasts, induces oestrus behaviour, vulvar swelling, and regulates vaginal secretions. When oestrogen production reaches a threshold, a surge of GnRH and LH is followed by ovulation. After ovulation, the production of oestrogen decreases; FSH and LH cause the remaining parts of the follicle to transform into the corpus luteum which produces significant amounts of hormones, particularly progesterone and to a lesser extent oestrogen. Progesterone is produced by both the corpora lutea and the placenta and is critical to support pregnancy. In males, FSH is responsible for the initiation of spermatogenesis at puberty, and at the beginning of each reproductive season for those species that are not sexually active all year round. LH causes the testes to produce testosterone which stimulates and maintains spermatogenesis. Testosterone plays a key role in the development of male reproductive tissues, such as the testis and prostate, and in promoting secondary sexual characteristics such as manes in lions, ornamental feathers or Steroid hormones such as progestins, oestrogens, and various combinations of oestrogens and progestins are frequently used as reproductive inhibitors in zoo species and in some wildlife species. The proposed mechanism of action of steroid hormones includes interference with folliculogenesis, ovulation, and egg implantation in females, and impairment of spermatogenesis in males. Higher doses are required to block ovulation than to achieve contraception: therefore it is possible that ovulation, physical, and behavioural signs of oestrus occur in animals that are otherwise effectively contracepted (Brache et al., 1990). Synthetic progestins are available in a variety of formulations and include megestrol acetate (MA), melengestrol acetate (MGA), levonorgestrel, and norgestrel. MA, used for dogs and cats over several decades in many countries under different brand names, was found to postpone oestrus in 92% of female dogs provided that it was administered orally for 8 days starting at a very specific time of the oestrus cycle (proestrus). The requirement for multiple, specific doses at a very precise time of a dog’s cycle makes this agent a classic example of a contraceptive that is very effective in confined companion animals but unsuitable for field applications to free-roaming dogs. Synthetic progestins are not recommended in pregnant animals because they might induce stillbirth or abortion in some species, or they might affect parturition by suppressing uterine contractions (Asa and Porton, 2005). Some progestins, such as MGA, have been widely used in zoo animals and are highly effective on many carnivore species, primates, and ungulates. In these species, an MGA implant can induce infertility for at least 2 years or longer, depending on species. However, MGA is associated with a variety of uterine pathologies and its use is not generally recommended for long-term contraception of canids and felids (Munson, 2006; Moresco et al., 2009).

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