Lloyd-Smith, J. O. et al. Should we expect population thresholds for wildlife disease? Trends Ecol. Evol. 20, 511-519

Department of Environmental Science, Policy and Management, University of California at Berkeley, Berkeley, CA 94720-3114, USA.
Trends in Ecology & Evolution (Impact Factor: 16.2). 10/2005; 20(9):511-9. DOI: 10.1016/j.tree.2005.07.004
Source: PubMed


Host population thresholds for the invasion or persistence of infectious disease are core concepts of disease ecology and underlie disease control policies based on culling and vaccination. However, empirical evidence for these thresholds in wildlife populations has been sparse, although recent studies have begun to address this gap. Here, we review the theoretical bases and empirical evidence for disease thresholds in wildlife. We see that, by their nature, these thresholds are rarely abrupt and always difficult to measure, and important facets of wildlife ecology are neglected by current theories. Empirical studies seeking to identify disease thresholds in wildlife encounter recurring obstacles of small sample sizes and confounding factors. Disease control policies based solely on threshold targets are rarely warranted, but management to reduce abundance of susceptible hosts can be effective.

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    • "Understanding the mechanisms underpinning the spread of infectious diseases in populations is critical for disease control (Anderson & May 1991; Grenfell & Dobson 1995; Lloyd-Smith et al. 2005; Keeling & Rohani 2008). The contact structure of a population can significantly affect infectious disease transmission, and therefore, knowledge of host contact patterns can be crucial for predicting and controlling disease outbreaks (Keeling 1999; Newman 2002; Keeling & Eames 2005; Craft 2015). "
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    ABSTRACT: Infectious disease transmission often depends on the contact structure of host populations. Although it is often challenging to capture the contact structure in wild animals, new technology has enabled biologists to obtain detailed temporal information on wildlife social contacts. In this study, we investigated the effects of raccoon contact patterns on rabies spread using network modeling. Raccoons (Procyon lotor) play an important role in the maintenance of rabies in the US. It is crucial to understand how contact patterns influence the spread of rabies in raccoon populations in order to design effective control measures and to prevent transmission to human populations and other animals. We constructed a dynamic system of contact networks based on empirical data from proximity logging collars on a wild suburban raccoon population, and then simulated rabies spread across these networks. Our contact networks incorporated the number and duration of raccoon interactions. We included differences in contacts according to sex and season, and both short-term acquaintances and long-term associations. Raccoons may display different behaviors when infectious, including aggression (furious behavior) and impaired mobility (dumb behavior); the network model was used to assess the impact of potential behavioral changes of rabid raccoons. We also tested the effectiveness of different vaccination coverage levels on rabies spread. Our results demonstrate that when rabies enters a suburban raccoon population, the likelihood of a disease outbreak affecting the majority of the population is high. Both the magnitude of rabies outbreaks and the speed of rabies spread depend strongly on the time of year that rabies is introduced into the population. When there is a combination of dumb and furious behaviors in the rabid raccoon population, there are similar outbreak sizes and speed of spread to when there are no behavioral changes due to rabies infection. By incorporating detailed data describing the variation in raccoon contact rates into a network modeling approach, we were able to show that suburban raccoon populations are highly susceptible to rabies outbreaks, that the risk of large outbreaks varies seasonally, and that current vaccination target levels may be inadequate to prevent the spread of rabies within these populations. Our findings thus provide new insights into rabies dynamics in raccoon populations and have important implications for disease control. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.
    Journal of Animal Ecology 07/2015; 84(6). DOI:10.1111/1365-2656.12422 · 4.50 Impact Factor
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    • "E-mail: s.j.tollington@kent.ac.uk Dobson 1995; Hudson et al. 2002; De Castro & Bolker 2005; Lloyd-Smith et al. 2005). Congruent with these regulatory effects predicted by ecological theory, the success of many high-profile species reintroduction programmes has been challenged by the threat of novel or endemic infectious diseases. "
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    ABSTRACT: Infectious diseases are widely recognized to have substantial impact on wildlife populations. These impacts are sometimes exacerbated in small endangered populations, and therefore, the success of conservation reintroductions to aid the recovery of such species can be seriously threatened by outbreaks of infectious disease. Intensive management strategies associated with conservation reintroductions can further compound these negative effects in such populations. Exploring the sublethal effects of disease outbreaks among natural populations is challenging and requires longitudinal, individual life-history data on patterns of reproductive success and other indicators of individual fitness. Long-term monitoring data concerning detailed reproductive information of the reintroduced Mauritius parakeet (Psittacula echo) population collected before, during and after a disease outbreak was investigated. Deleterious effects of an outbreak of beak and feather disease virus (BFDV) were revealed on hatch success, but these effects were remarkably short-lived and disproportionately associated with breeding pairs which took supplemental food. Individual BFDV infection status was not predicted by any genetic, environmental or conservation management factors and was not associated with any of our measures of immune function, perhaps suggesting immunological impairment. Experimental immunostimulation using the PHA (phytohaemagglutinin assay) challenge technique did, however, provoke a significant cellular immune response. We illustrate the resilience of this bottlenecked and once critically endangered, island-endemic species to an epidemic outbreak of BFDV and highlight the value of systematic monitoring in revealing inconspicuous but nonetheless substantial ecological interactions. Our study demonstrates that the emergence of such an infectious disease in a population ordinarily associated with increased susceptibility does not necessarily lead to deleterious impacts on population growth and that negative effects on reproductive fitness can be short-lived. © 2015 The Authors. Journal of Animal Ecology published by John Wiley & Sons Ltd on behalf of British Ecological Society.
    Journal of Animal Ecology 03/2015; 84(4). DOI:10.1111/1365-2656.12348 · 4.50 Impact Factor
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    • "To demonstrate this variability, we repeated 20 simulations of the model, using a set of baseline parameter values (Table S1) obtained from a combination of estimates in the literature and tuning the results of our simulations to include the current epidemic as a possible realization of our model. From the results of these 20 simulations (Table 1), we see that the process failed to break out 20% (4 simulations) of the time: these are the " fadeout " rates that occur even when í µí±… 0 > 1 [13] [14]. The number of total cases throughout the course of the simulated epidemic exceeded 1,000 in half of the remaining runs (8 simulations) but reached a cumulative total of less than 1,000 in the other half. "
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    ABSTRACT: We present a stochastic transmission chain simulation model for Ebola viral disease (EVD) in West Africa, with the salutary result that the virus may be more controllable than previously suspected. The ongoing tactics to detect cases as rapidly as possible and isolate individuals as safely as practicable is essential to saving lives in the current outbreaks in Guinea, Liberia, and Sierra Leone. Equally important are educational campaigns that reduce contact rates between susceptible and infectious individuals in the community once an outbreak occurs. However, due to the relatively low R0 of Ebola (around 1.5 to 2.5 next generation cases are produced per current generation case in naïve populations), rapid isolation of infectious individuals proves to be highly efficacious in containing outbreaks in new areas, while vaccination programs, even with low efficacy vaccines, can be decisive in curbing future outbreaks in areas where the Ebola virus is maintained in reservoir populations.
    Computational and Mathematical Methods in Medicine 01/2015; 2015. DOI:10.1155/2015/736507 · 0.77 Impact Factor
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