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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    • "For instance, we assumed that ND transmission was density dependent in white-winged parakeets. Density-dependent transmission is commonly assumed for wildlife diseases[103,157]. In most cases of wildlife diseases, empirical data are difficult to obtain to confirm transmission, but Hochachka & Dhondt[140]used pre-and post-enzootic data to conclude that mycoplasma conjunctivitis transmission in house finches was density dependent. "
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    ABSTRACT: Illegal wildlife-pet trade can threaten wildlife populations directly from overharvest, but also indirectly as a pathway for introduction of infectious diseases. This study evaluated consequences of a hypothetical introduction of Newcastle disease (ND) into a wild population of Peru’s most trafficked psittacine, the white-winged parakeet (Brotogeris versicolurus), through release of infected confiscated individuals. We developed two mathematical models that describe ND transmission and the influence of illegal harvest in a homogeneous (model 1) and age-structured population of parakeets (model 2). Infection transmission dynamics and harvest were consistent for all individuals in model 1, which rendered it mathematically more tractable compared to the more complex, age-structured model 2 that separated the host population into juveniles and adults. We evaluated the interaction of ND transmission and harvest through changes in the basic reproduction number (R0) and short-term host pop-ulation dynamics. Our findings demonstrated that ND introduction would likely provoke considerable disease-related mortality, up to 24% population decline in two years, but high harvest rates would dampen the magnitude of the outbreak. Model 2 produced moderate differences in disease dynamics compared to model 1 (R0 = 3.63 and 2.66, respectively), but highlighted the importance of adult disease dynamics in diminishing the epidemic potential. Therefore, we suggest that future studies should use a more realistic, age-structured model. Finally, for the presumptive risk that illegal trade of white-winged parakeets could introduce ND into wild populations, our results suggest that while high harvest rates may have a protective effect on the population by reducing virus transmission, the combined effects of high harvest and disease-induced mortality may threaten population survival. These results capture the complexity and consequences of the interaction between ND transmission and harvest in a wild parrot population and highlight the importance of preventing illegal trade.
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    • "A fundamental principle of epidemiological theory is that there is a minimum population threshold for pathogen invasion and a critical community size required for disease persistence for diseases with density dependent transmission[40]. While exact thresholds in wildlife populations are difficult to determine, identifying host distribution ranges around these thresholds, as we did here, can improve our understanding of disease dynamics[41]and therefore ability to forecast future outbreak events. For non-infectious diseases, where pathogens are endemic in the population and infection results from an imbalance in the host–pathogen relationship, variables besides host density may be important determinants of disease prevalence[42]. "

    Full-text · Article · Jan 2016 · Remote Sensing
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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.
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