Impact of contraceptive hormones on the reproductive potential of male and female commensal black rats (Rattus rattus)

The black rat is considered one of the world's top pests. With increased restrictions on rodenticides, new alternatives to manage rats are urgently needed. Research on the use of contraceptive hormones, levonorgestrel (LE), and quinestrol (QU), have been evaluated against some rodent species, and this research is the first study to assess these on black rats. Hormones were incorporated into rodent bait at 10 and 50 ppm concentrations singly and in combination (EP-1). Groups of 10 animals of each sex were fed the baits over 7 days. Lower bait consumption was observed with slight body mass reductions. On dissection, it was observed that the uterus was in a state of edema and male reproductive organs weighed less with reduced sperm counts/motility. The 2 most promising baits, 50 ppm QU and EP-1, were used to assess impact on pregnancy and litter size. Pregnancy was reduced from 70% success when both males and females consumed untreated bait, down to 30% when males had consumed contraceptive bait but females had not, and down to 0% when females had consumed contraceptive bait, regardless of whether they had paired with a treated or untreated male. Litter size in the untreated pairs was 8 pups, but only 4 pups in those cases where the male only had consumed the contraceptive. Further studies should investigate how long the effect lasts and its reversibility. Field studies at the population level may also shed light on the practicality of using contraceptive baits for black rats in different habitats.

Effect of synthetic hormones on reproduction in Mastomys natalensis

Rodent pest management traditionally relies on some form of lethal control. Developing effective fertility control for pest rodent species could be a major breakthrough particularly in the context of managing rodent population outbreaks. This laboratory-based study is the first to report on the effects of using fertility compounds on an outbreaking rodent pest species found throughout sub-Saharan Africa. Mastomys natalensis were fed bait containing the synthetic steroid hormones quinestrol and levonorgestrel, both singly and in combination, at three concentrations (10, 50, 100 ppm) for 7 days. Consumption of the bait and animal body mass was mostly the same between treatments when analysed by sex, day and treatment. However, a repeated measures ANOVA indicated that quinestrol and quinestrol + levonorgestrel treatments reduced consumption by up to 45%, particularly at the higher concentrations of 50 and 100 ppm. Although there was no clear concentration effect on animal body mass, quinestrol and quinestrol + levonorgestrel lowered body mass by up to 20% compared to the untreated and levonorgestrel treatments. Quinestrol and quinestrol + levonorgestrel reduced the weight of male rat testes, epididymis and seminal vesicles by 60-80%, and sperm concentration and motility were reduced by more than 95%. No weight changes were observed to uterine and ovarian tissue; however, high uterine oedema was observed among all female rats consuming treated bait at 8 and 40 days from trial start. Trials with mate pairing showed there were significant differences in the pregnancy rate with all treatments when compared to the untreated control group of rodents.

Potential effects of incorporating fertility control into typical culling regimes in wild pig populations

Effective management of widespread invasive species such as wild pigs (Sus scrofa) is limited by resources available to devote to the effort. Better insight of the effectiveness of different management strategies on population dynamics is important for guiding decisions of resource allocation over space and time. Using a dynamic population model, we quantified effects of culling intensities and time between culling events on population dynamics of wild pigs in the USA using empirical culling patterns and data-based demographic parameters. In simulated populations closed to immigration, substantial population declines (50-100%) occurred within 4 years when 20-60% of the population was culled annually, but when immigration from surrounding areas occurred, there was a maximum of 50% reduction, even with the maximum culling intensity of 60%. Incorporating hypothetical levels of fertility control with realistic culling intensities was most effective in reducing populations when they were closed to immigration and when intrinsic population growth rate was too high (> = 1.78) to be controlled by culling alone. However, substantial benefits from fertility control used in conjunction with culling may only occur over a narrow range of net population growth rates (i.e., where net is the result of intrinsic growth rates and culling) that varies depending on intrinsic population growth rate. The management implications are that the decision to use fertility control in conjunction with culling should rely on concurrent consideration of achievable culling intensity, underlying demographic parameters, and costs of culling and fertility control. The addition of fertility control reduced abundance substantially more than culling alone, however the effects of fertility control were weaker than in populations without immigration. Because these populations were not being reduced substantially by culling alone, fertility control could be an especially helpful enhancement to culling for reducing abundance to target levels in areas where immigration can't be prevented.

Fertility control to mitigate human–wildlife conflicts: a review

As human populations grow, conflicts with wildlife increase. Concurrently, concerns about the welfare, safety and environmental impacts of conventional lethal methods of wildlife management restrict the options available for conflict mitigation. In parallel, there is increasing interest in using fertility control to manage wildlife. The present review aimed at analysing trends in research on fertility control for wildlife, illustrating developments in fertility-control technologies and delivery methods of fertility-control agents, summarising the conclusions of empirical and theoretical studies of fertility control applied at the population level and offering criteria to guide decisions regarding the suitability of fertility control to mitigate human–wildlife conflicts. The review highlighted a growing interest in fertility control for wildlife, underpinned by increasing numbers of scientific studies. Most current practical applications of fertility control for wild mammals use injectable single-dose immunocontraceptive vaccines mainly aimed at sterilising females, although many of these vaccines are not yet commercially available. One oral avian contraceptive, nicarbazin, is commercially available in some countries. Potential new methods of remote contraceptive delivery include bacterial ghosts, virus-like particles and genetically modified transmissible and non-transmissible organisms, although none of these have yet progressed to field testing. In parallel, new species-specific delivery systems have been developed. The results of population-level studies of fertility control indicated that this approach may increase survival and affect social and spatial behaviour of treated animals, although the effects are species- and context-specific. The present studies suggested that a substantial initial effort is generally required to reduce population growth if fertility control is the sole wildlife management method. However, several empirical and field studies have demonstrated that fertility control, particularly of isolated populations, can be successfully used to limit population growth and reduce human–wildlife conflicts. In parallel, there is growing recognition of the possible synergy between fertility control and disease vaccination to optimise the maintenance of herd immunity in the management of wildlife diseases. The review provides a decision tree that can be used to determine whether fertility control should be employed to resolve specific human–wildlife conflicts. These criteria encompass public consultation, considerations about animal welfare and feasibility, evaluation of population responses, costs and sustainability.

The use of immunocontraception to improve rabies eradication in urban dog populations

ContextRabies causes ~55 000 human deaths each year, primarily as a result of bites from dogs, which are the major rabies reservoir in the developing world. Current rabies control strategies include vaccination, culling and surgical sterilisation of dogs. However, recently developed immunocontraceptives could be used alongside vaccination to apply fertility control to more animals. AimsWe used a modelling approach to explore (1) whether adding single-dose contraceptives to rabies vaccination would improve effectiveness of rabies eradication, (2) how sensitive control methods are to variation in population parameters and (3) the effects of applying control continuously or in pulses on rabies eradication. MethodsA continuous time, compartmental model was created to describe canine rabies epidemiology. Parameters were derived from the literature. The following three control methods were applied at varying rates and durations: vaccination, vaccination plus fertility control (v + fc) and culling. Outcomes were classified into the following three categories: rabies persistence, rabies eradication and population extinction. Key resultsWhen control was applied continuously for up to 24 months, vaccination was least effective; the effort required to eradicate rabies was about twice that required with culling or v + fc. At realistic control rates, only v + fc consistently resulted in rabies eradication. Increasing population growth rate and city size made rabies eradication harder; for vaccination, considerably greater control rates and durations were required, whereas culling and v + fc showed only minor decreases in effectiveness. When control was applied for 1 or 2 months (for one month every 12 months or every 6 months) per year for up to 20 years, vaccination became less effective because of population turnover between control periods; v + fc lost little effectiveness, as decreased birth rates reduced the input of susceptible animals. ConclusionsUsing immunocontraception alongside vaccination could improve rabies control campaigns by reducing the proportion of the population that must be treated, or reducing the necessary duration of the campaign. It could also make control effective under larger population growths, in larger cities and when control is pulsed. ImplicationsImmunocontraceptives could become a useful tool in canine rabies control by allowing fertility control to be applied on a large scale. Further work is required to improve understanding of dog ecology and parameterise location-specific models, which could be used to inform management plans.

Contraceptive potential of porcine and feline zona pellucida A, B and C subunits in domestic cats

Feral cat populations are a major problem in many urban regions throughout the world, threatening biodiversity. Immunocontraception is considered as an alternative and a more humane means to control overpopulation of pest animals than current methods including trapping, poisoning and shooting. In this study, we evaluate porcine zona pellucida (ZP) polypeptide (55 kDa) and feline ZP A, B and C subunits expressed by plasmid vectors as candidate vaccines against fertility in the female domestic cat. Cats were injected subcutaneously with three doses of the ZP vaccines. Vaccinated cats were compared with naïve cats for ZP-antibody response, ovarian histology and fertility after mating. Vaccination with native porcine ZP 55 kDa polypeptide induced anti-porcine ZP antibodies detected by ELISA. However, these antibodies did not cross-react with feline ZP as assessed by immunohistochemistry and no effect on fertilityin vivowas observed after mating. However, vaccination of cats with feline ZPA or feline ZPB+C DNA vectors elicited circulating antibodies specific for feline ZP as assessed by ELISA, with reactivity to native feline ZP in ovarian folliclesin situ. Vaccination with feline ZPA and ZPB+C DNA did not elicit changes in ovarian histology. Although sample sizes were small, conception rates in mated females were 25 and 20% in the ZPA and ZPB+C vaccinated groups respectively, compared with 83% in the control group. We conclude that feline ZPA and ZPB+C subunits are potential candidate antigens for immunocontraceptive vaccines in the domestic cat.

The future biological control of pest populations of European rabbits, Oryctolagus cuniculus

European rabbits are exotic pests in Australia, New Zealand, parts of South America and Europe, and on many islands. Their abundance, and the damage they cause, might be reduced by the release of naturally occurring or genetically modified organisms (GMOs) that act as biological control agents (BCAs). Some promising pathogens and parasites of European rabbits and other lagomorphs are discussed, with special reference to those absent from Australia as an example of the range of necessary considerations in any given case. The possibility of introducing these already-known BCAs into areas where rabbits are pests warrants further investigation. The most cost-effective method for finding potentially useful but as-yet undiscovered BCAs would be to maintain a global watch on new diseases and pathologies in domestic rabbits. The absence of wild European rabbits from climatically suitable parts of North and South America and southern Africa may indicate the presence there of useful BCAs, although other explanations for their absence are possible. Until the non-target risks of deploying disseminating GMOs to control rabbits have been satisfactorily minimised, efforts to introduce BCAs into exotic rabbit populations should focus on naturally occurring organisms. The development of safe disseminating GMOs remains an important long-term goal, with the possible use of homing endonuclease genes warranting further investigation.

Effect of the GnRH vaccine GonaCon on the fertility, physiology and behaviour of wild boar

Fertility control has the potential to be used as an attractive alternative to lethal methods for limiting population growth in overabundant species. This study tested the effectiveness and potential side effects of the single-dose gonadotrophin-releasing hormone (GnRH) vaccine GonaCon on the physiology and behaviour of two groups of captive female wild boar in two sequential trials (Trial 1 and Trial 2). Following vaccination with GonaCon, data on contraceptive effectiveness were recorded as well as data on time budget, social rank, bodyweight, haematology and biochemistry. The concentration of GnRH-antibody titres peaked 2–6 weeks after vaccination and remained relatively high 12 weeks after vaccination. In Trial 1, all control females and none of the treated females gave birth. In Trial 2, faecal progesterone of treated females decreased to basal levels within a month of vaccination. No differences in time budget, social rank and blood parameters were observed between treated and control females. Bodyweight increased more in treated females than in controls. These results indicated that GonaCon can suppress reproduction of wild boar with no significant short-term effects on behaviour and physiology. GonaCon can be regarded as an effective, humane and safe contraceptive for managing wild boar populations.

Pest or prized possession? Genetically modified biocontrol from an international perspective

This article provides an overview of current research, regulations and international issues concerning genetically modified (GM) organisms for use as biological controls of vertebrates. There is increasing interest in using biotechnology to solve vertebrate pest problems around the world. A major issue lies in the fact that individual countries focusing on internal problems of pest management may overlook the potential of transborder entry. Animals considered a pest in one country may well be prized possessions in another, and research and management strategies should consider the adverse effects of biocontrol agents entering the ‘wrong’ country. There is a wealth of guidance in the form of national and international regulations and ethics guidelines. However, current legislation and agreements may not be adequate to ensure that all risks of GM biocontrols, particularly disseminating agents, have been considered from an international perspective. Major issues include concerns of transboundary movement, non-target effects and the need for an international body to consult with and regulate the use of GM biocontrols. We live in a finite and interconnected world: it is vital that impacts of potential control strategies are assessed at a local and international level, and from social, environmental and economic perspectives.

Immunocontraception of mammalian wildlife: ecological and immunogenetic issues

Immunocontraception involves stimulating immune responses against gametes or reproductive hormones thus preventing conception. The method is being developed for the humane control of pest and overabundant populations of mammalian wildlife. This paper examines three fundamental issues associated with its use: (1) the difficulties of obtaining responses to self-antigens, (2) the likely evolution of genetically based non-response to immunocontraceptive agents, and (3) the possible changes in the array of pathogens possessed by the target species after generations of immunocontraception. Our review of the literature demonstrates that the barriers to an effective immunocontraceptive are at present very basic. Should they be overcome, the effects of immunocontraception on the immunogenetic constitution of wildlife populations through the selection for non-responders must be examined. We suggest that the attempt to use the animal’s own immune system to modulate reproduction may be incompatible with the basic biological function of protection against infectious disease. Research programs on mammalian immunocontraception should involve measurement of the heritability of non-response and an assessment of the likely change in the response of the contracepted population to possible pathogens.

The inadvertent introduction into Australia of Trypanosoma nabiasi, the trypanosome of the European rabbit (Oryctolagus cuniculus), and its potential for biocontrol

Wild rabbits (Oryctolagus cuniculus) in Australia are the descendents of 24 animals from England released in 1859. We surveyed rabbits and rabbit fleas (Spilopsyllus cuniculi) in Australia for the presence of trypanosomes using parasitological and PCR-based methods. Trypanosomes were detected in blood from the European rabbits by microscopy, and PCR using trypanosome-specific small subunit ribosomal RNA (SSU rRNA) gene primers and those in rabbit fleas by PCR. This is the first record of trypanosomes from rabbits in Australia. We identified these Australian rabbit trypanosomes as Trypanosoma nabiasi, the trypanosome of the European rabbit, by comparison of morphology and SSU rRNA gene sequences of Australian and European rabbit trypanosomes. Phylogenetic analysis places T. nabiasi in a clade with rodent trypanosomes in the subgenus Herpetosoma and their common link appears to be transmission by fleas. Despite the strict host specificity of trypanosomes in this clade, phylogenies presented here suggest that they have not strictly cospeciated with their vertebrate hosts. We suggest that T. nabiasi was inadvertently introduced into Australia in the 1960s in its flea vector Spilopsyllus cuniculi, which was deliberately introduced as a potential vector of the myxoma virus. In view of the environmental and economic damage caused by rabbits in Australia and other islands, the development of a virulent or genetically modified T. nabiasi should be considered to control rabbits.