Experimental evidence for opposing effects of high deer density on tick-borne disease pathogen prevalence and hazard

Gandy, S. , Kilbride, E., Biek, R. , Millins, C. and Gilbert, L. (2021) Experimental evidence for opposing effects of high deer density on tick-borne disease pathogen prevalence and hazard. Parasites and Vectors, 14(1), 509. (doi: 10.1186/s13071-021-05000-0) (PMID:34593023) (PMCID:PMC8485466)

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Abstract

Background: Identifying the mechanisms driving disease risk is challenging for multi-host pathogens, such as Borrelia burgdorferi sensu lato (s.l.), the tick-borne bacteria causing Lyme disease. Deer are tick reproduction hosts but do not transmit B. burgdorferi s.l., whereas rodents and birds are competent transmission hosts. Here, we use a long-term deer exclosure experiment to test three mechanisms for how high deer density might shape B. burgdorferi s.l. prevalence in ticks: increased prevalence due to higher larval tick densities facilitating high transmission on rodents (M1); alternatively, reduced B. burgdorferi s.l. prevalence because more larval ticks feed on deer rather than transmission-competent rodents (dilution effect) (M2), potentially due to ecological cascades, whereby higher deer grazing pressure shortens vegetation which decreases rodent abundance thus reducing transmission (M3). Methods: In a large enclosure where red deer stags were kept at high density (35.5 deer km−2), we used an experimental design consisting of eight plots of 0.23 ha, four of which were fenced to simulate the absence of deer and four that were accessible to deer. In each plot we measured the density of questing nymphs and nymphal infection prevalence in spring, summer and autumn, and quantified vegetation height and density, and small mammal abundance. Results: Prevalence tended to be lower, though not conclusively so, in high deer density plots compared to exclosures (predicted prevalence of 1.0% vs 2.2%), suggesting that the dilution and cascade mechanisms might outweigh the increased opportunities for transmission mechanism. Presence of deer at high density led to shorter vegetation and fewer rodents, consistent with an ecological cascade. However, Lyme disease hazard (density of infected I. ricinus nymphs) was five times higher in high deer density plots due to tick density being 18 times higher. Conclusions: High densities of tick reproduction hosts such as deer can drive up vector-borne disease hazard, despite the potential to simultaneously reduce pathogen prevalence. This has implications for environmental pathogen management and for deer management, although the impact of intermediate deer densities now needs testing.

Item Type:Articles
Additional Information:SG was supported by a PhD studentship jointly funded by the University of Glasgow and the Macaulay Development Fund. LG was supported by the Scottish Government’s Rural and Environmental Science and Analytical Services Division.
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Biek, Professor Roman and GANDY, SARA and Millins, Dr Caroline and Kilbride, Mrs Elizabeth and Gilbert, Dr Lucy
Authors: Gandy, S., Kilbride, E., Biek, R., Millins, C., and Gilbert, L.
College/School:College of Medical Veterinary and Life Sciences
College of Medical Veterinary and Life Sciences > Institute of Biodiversity Animal Health and Comparative Medicine
Journal Name:Parasites and Vectors
Publisher:BMC
ISSN:1756-3305
ISSN (Online):1756-3305
Copyright Holders:Copyright © 2021 The Authors
First Published:First published in Parasites and Vectors 14(1):509
Publisher Policy:Reproduced under a Creative Commons License

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