Impact of the experimental removal of lizards on Lyme disease risk

Department of Integrative Biology, University of California, 3060 Valley Life Sciences Building, Berkeley, CA 94720-3140, USA.
Proceedings of the Royal Society B: Biological Sciences (Impact Factor: 5.05). 02/2011; 278(1720):2970-8. DOI: 10.1098/rspb.2010.2402
Source: PubMed


The distribution of vector meals in the host community is an important element of understanding and predicting vector-borne disease risk. Lizards (such as the western fence lizard; Sceloporus occidentalis) play a unique role in Lyme disease ecology in the far-western United States. Lizards rather than mammals serve as the blood meal hosts for a large fraction of larval and nymphal western black-legged ticks (Ixodes pacificus--the vector for Lyme disease in that region) but are not competent reservoirs for the pathogen, Borrelia burgdorferi. Prior studies have suggested that the net effect of lizards is to reduce risk of human exposure to Lyme disease, a hypothesis that we tested experimentally. Following experimental removal of lizards, we documented incomplete host switching by larval ticks (5.19%) from lizards to other hosts. Larval tick burdens increased on woodrats, a competent reservoir, but not on deer mice, a less competent pathogen reservoir. However, most larvae failed to find an alternate host. This resulted in significantly lower densities of nymphal ticks the following year. Unexpectedly, the removal of reservoir-incompetent lizards did not cause an increase in nymphal tick infection prevalence. The net result of lizard removal was a decrease in the density of infected nymphal ticks, and therefore a decreased risk to humans of Lyme disease. Our results indicate that an incompetent reservoir for a pathogen may, in fact, increase disease risk through the maintenance of higher vector density and therefore, higher density of infected vectors.

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    • "Juvenile ticks carrying Borrelia that feed on these lizards are cleansed of infection through complement-mediated innate immunity (Kuo et al. 2000). Recent attention has therefore focused on the role of S. occidentalis in Lyme disease ecology, and studies have shown that removal of S. occidentalis from oak woodland communities can have dramatic and complex effects on the prevalence of the bacteria (Swei et al. 2011). Studying the fine details of the Ixodes feeding mechanisms and the responses of the host may help elucidate the complexities inherent in this host–parasite relationship. "
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    ABSTRACT: The western black-legged tick (Ixodes pacificus) is a common tick species throughout the western USA and is the major vector for Borrelia burgdorferi, the Lyme disease causing bacterium. Western fence lizards (Sceloporus occidentalis) are a major host for juvenile I. pacificus, but are incompetent hosts for B. burgdorferi, which makes this host-parasite relationship of particular interest. In order to shed further light on this complex host-parasite relationship, we investigated the effects of temperature on feeding duration (number of days to repletion), success (number feeding to repletion), and efficiency (replete tick mass) of larval I. pacificus. Western fence lizards were experimentally infested with larval ticks and exposed to three constant temperatures (21, 27, 33 °C). Larvae feeding at 21 °C took approximately twice as long as larvae at 27 and 33 °C. Effects of temperature on feeding duration are likely mediated through effects on host blood circulation and functionality of tick salivary proteins. Our results here suggest temperature is another important factor influencing the feeding dynamics of I. pacificus, and likely other tick species. Future research is needed to clarify the exact mechanisms behind temperature effects on tick feeding.
    Full-text · Article · Jul 2015 · Experimental and Applied Acarology
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    • "This effect has been demonstrated empirically for western fence lizards (Sceloporus occidentalis ), which are non-competent hosts for Borrelia burgdorferi in California. Because lizards provide meals for ticks, experimental removal of lizards reduces infected tick density (Swei et al. 2011), which suggests that this non-competent host increases disease risk for humans. "
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    ABSTRACT: Control of human infectious disease has been promoted as a valuable ecosystem service arising from the conservation of biodiversity. There are two commonly discussed mechanisms by which biodiversity loss could increase rates of infectious disease in a landscape. First, loss of competitors or predators could facilitate an increase in the abundance of competent reservoir hosts. Second, biodiversity loss could disproportionately affect non-competent, or less competent reservoir hosts, which would otherwise interfere with pathogen transmission to human populations by, for example, wasting the bites of infected vectors. A negative association between biodiversity and disease risk, sometimes called the "dilution effect hypothesis," has been supported for a few disease agents, suggests an exciting win-win outcome for the environment and society, and has become a pervasive topic in the disease ecology literature. Case studies have been assembled to argue that the dilution effect is general across disease agents. Less touted are examples in which elevated biodiversity does not affect or increases infectious disease risk for pathogens of public health concern. In order to assess the likely generality of the dilution effect, we review the association between biodiversity and public health across a broad variety of human disease agents. Overall, we hypothesize that conditions for the dilution effect are unlikely to be met for most important diseases of humans. Biodiversity probably has little net effect on most human infectious diseases but, when it does have an effect, observation and basic logic suggest that biodiversity will be more likely to increase than to decrease infectious disease risk.
    Full-text · Article · Apr 2014 · Ecology
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    • "The density of infected nymphs (DIN) is considered a useful predictor of LD risk9101112131415. Factors that influence DIN (including tick survivorship[16], host composition[17,18], and abiotic variables[16]) are thought to influence the magnitude of risk[11,192021. DIN is calculated by multiplying the density of nymphs collected in a given area by the B. burgdorferi infection prevalence of those nymphs[9,14,15,22]. "
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    ABSTRACT: In the northeastern United States, the main vector of Lyme disease is the nymphal life-stage of the blacklegged tick (Ixodes scapularis). For example, the majority of Borrelia burgdorferi-infected ticks removed from humans in northeastern states by the Department of Defense (DoD) Tick Test program are nymphs. Consequently, in these states there is a very strong association between the late spring to early summer peak in nymphal host-seeking and the timing of human disease onset (e.g., Falco and Fish 1999). A very different pattern is evident, however, in southeastern states. Despite robust populations of I. scapularis in coastal habitats there, a very small proportion of the ticks biting humans are I. scapularis, and those few that do attach are almost entirely the adult life-stage. We discuss several lines of evidence -- including DoD Tick Test program data, adult:nymph ratios in field ‘flagging’ surveys, and behavioral observations on captive ticks -- that support our conclusion that a dramatic change in nymphal I. scapularis questing behavior helps explain why human Lyme disease case reports from southeastern states are two orders of magnitude lower than in the Northeast.
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