Estimating Disease Transmission in Wildlife, with Emphasis on Leptospirosis and Bovine Tuberculosis in Possums, and Effects of Fertility Control

Applied Ecology Group, University of Canberra, ACT 2601, Australia
Journal of Applied Ecology (Impact Factor: 4.56). 04/2002; 38(6):1362 - 1370. DOI: 10.1046/j.0021-8901.2001.00676.x


Summary • We present methods for estimating disease transmission coefficients in wildlife, using Leptospira interrogans infection (a bacterial disease transmitted predominantly during social contacts) in brushtail possums Trichosurus vulpecula as a model system. • Using data from a field experiment conducted on a naturally infected possum population, we estimated disease transmission coefficients assuming either ‘density-dependent’ or ‘frequency-dependent’ transmission. • A model-selection approach determined that density-dependent transmission was the most appropriate form of the transmission of L. interrogans infection in brushtail possums. • We used the chosen model of transmission to examine experimentally the effect of tubally ligating female brushtail possums on the epidemiology of L. interrogans. The estimated transmission coefficient was 28% higher (P = 0·16) in populations subject to tubal ligation, raising the possibility that fertility control of this type may increase disease transmission rates. • Altering mating behaviour through fertility control may have the potential to control diseases such as bovine tuberculosis in brushtail possums, although the potential of fertility control techniques to change disease transmission coefficients and disease epidemiology requires further investigation. This would require models that examine the combined effects of fertility control on population dynamics, social behaviour and disease transmission coefficients simultaneously.

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    • "Feedbacks amplifying the effect of fertility control can also occur. Disease transmission was 28% higher in populations of possums Trichosurus vulpecula with sterile females, illustrating the potential for decreased survival in fertility-controlled populations (Caley & Ramsey 2001). Phenological shifts in birth patterns among females that formerly received contraception may also have implications for survival of both females and their neonates if births occur asynchronously with forage resources (Ransom, Hobbs & Bruemmer 2013). "
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    ABSTRACT: Full article is Open Access at: Anthropogenic stress on natural systems, particularly the fragmentation of landscapes and the extirpation of predators from food webs, has intensified the need to regulate abundance of wildlife populations with management. Controlling population growth using fertility control has been considered for almost four decades, but nearly all research has focused on understanding effects of fertility control agents on individual animals. Questions about the efficacy of fertility control as a way to control populations remain largely unanswered. Collateral consequences of contraception can produce unexpected changes in birth rates, survival, immigration and emigration that may reduce the effectiveness of regulating animal abundance. The magnitude and frequency of such effects vary with species-specific social and reproductive systems, as well as connectivity of populations. Developing models that incorporate static demographic parameters from populations not controlled by contraception may bias predictions of fertility control efficacy. Many population-level studies demonstrate that changes in survival and immigration induced by fertility control can compensate for the reduction in births caused by contraception. The most successful cases of regulating populations using fertility control come from applications of contraceptives to small, closed populations of gregarious and easily accessed species. Fertility control can result in artificial selection pressures on the population and may lead to long-term unintentional genetic consequences. The magnitude of such selection is dependent on individual heritability and behavioural traits, as well as environmental variation. Synthesis and applications. Understanding species' life-history strategies, biology, behavioural ecology and ecological context is critical to developing realistic expectations of regulating populations using fertility control. Before time, effort and funding are invested in wildlife contraception, managers may need to consider the possibility that many species and populations can compensate for reduction in fecundity, and this could minimize any reduction in population growth rate.
    Full-text · Article · Oct 2013 · Journal of Applied Ecology
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    • "Habib et al. (2011) used a mechanistic model of deer contact rates to suggest that CWD transmission could be intermediate between densityand frequency-dependent because contact rates are a saturating function of density. Transmission functions are rarely estimated in wild populations (but see Caley and Ramsey 2001, Smith et al. 2009). Instead, they are inferred when disease and host dynamics are consistent with a particular transmission function (McCallum et al. 2009) depending on how social organization and behavior of the host (e.g., group size, rate of exchange between groups, between-group contact rate) are related to host density (Altizer et al. 2003, Cross et al. 2009, Habib et al. 2011), or based on the characteristics of the pathogen (e.g., transmission mode, duration of infectivity). "
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    ABSTRACT: Host-parasite dynamics and strategies for managing infectious diseases of wildlife depend on the functional relationship between disease transmission rates and host density. However, the disease transmission function is rarely known for free-living wildlife, leading to uncertainty regarding the impacts of diseases on host populations and effective control actions. We evaluated the influence of deer density, landscape features, and soil clay content on transmission of chronic wasting disease (CWD) in young (<2-year-old) white-tailed deer (Odocoileus virginianus) in south-central Wisconsin, USA. We evaluated how frequency-dependent, density-dependent, and intermediate transmission models predicted CWD incidence rates in harvested yearling deer. An intermediate transmission model, incorporating both disease prevalence and density of infected deer, performed better than simple density- and frequency-dependent models. Our results indicate a combination of social structure, non-linear relationships between infectious contact and deer density, and distribution of disease among groups are important factors driving CWD infection in young deer. The landscape covariates % deciduous forest cover and forest edge density also were positively associated with infection rates, but soil clay content had no measurable influences on CWD transmission. Lack of strong density-dependent transmission rates indicates that controlling CWD by reducing deer density will be difficult. The consequences of non-linear disease transmission and aggregation of disease on cervid populations deserves further consideration.
    Full-text · Article · Jan 2013 · Ecosphere
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    • "In addition, fertility control can induce behavioural changes, such as reproductionrelated long-distance movements, that reduce disease transmission rates (e.g. Caley and Ramsey 2001; Ramsey et al. 2002; Ramsey 2007). GonaConÔ is currently registered in the US as a contraceptive for white-tailed deer (Fagerstone et al. 2010 "
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    ABSTRACT: Context: Fertility control appears as a publicly acceptable alternative to lethal methods for limiting population growth in wildlife. Recently developed single-dose immunocontraceptive vaccines have induced infertility in several mammals. However, the potential side-effects and the long-term effectiveness of these contraceptives have been poorly investigated. Aims: We tested the long-term effectiveness and potential side-effects of the single-dose gonadotrophin-releasing hormone (GnRH) vaccine GonaCon™ on captive female wild boar. Methods: We carried out two sequential trials: Trial 1 (n = 6 GonaCon™-treated and 6 control wild boar) and Trial 2 which started two years later and replicated Trial 1. We assessed the effectiveness of GonaCon™ to cause infertility by measuring GnRH antibody titres, by monitoring the oestrous cycle through the concentration of faecal progesterone and by recording the sows’ reproductive output in the 4–6 years following treatment. We evaluated the potential side-effects by monitoring behaviour, bodyweight and haematological and biochemical variables. Key results: GnRH-antibody titres decreased with time but were still detectable in all females six years after vaccination with a single dose of GonaCon™. In Trial 1 none of the treated females gave birth in the six years after vaccination. In Trial 2, progesterone indicated that two of the six treated females were cycling. One of the cycling treated females gave birth one year after vaccination; the other five, including the second cycling sow, did not reproduce in the four years following vaccination. We found no differences in bodyweight, haematology, biochemistry and behaviour and no obvious sign of injection site reaction. Conclusions: GonaCon™ can suppress reproduction in wild boar with no long-term effects on behaviour and physiology. Therefore, GonaCon™ can be regarded as an effective and safe contraceptive for this species. Implications: The lack of evidence of adverse effects and the longevity of effect of GonaCon™ suggest that this contraceptive could be now tested in field trials and in contexts where culling of overabundant populations of wild boar is unfeasible, illegal or unacceptable. These instances include urban areas, parks, and management of diseases where culling might cause social perturbation and result in increased disease transmission rates.
    Full-text · Article · Jan 2012 · Wildlife Research
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