Abstract
BackgroundRodents are considered to contribute strongly to the risk of tick-borne diseases by feeding Ixodes ricinus larvae and by acting as amplifying hosts for pathogens. Here, we tested to what extent these two processes depend on rodent density, and for which pathogen species rodents synergistically contribute to the local disease risk, i.e. the density of infected nymphs (DIN).MethodsIn a natural woodland, we manipulated rodent densities in plots of 2500 m2 by either supplementing a critical food source (acorns) or by removing rodents during two years. Untreated plots were used as controls. Collected nymphs and rodent ear biopsies were tested for the presence of seven tick-borne microorganisms. Linear models were used to capture associations between rodents, nymphs, and pathogens.ResultsInvestigation of data from all plots, irrespective of the treatment, revealed a strong positive association between rodent density and nymphal density, nymphal infection prevalence (NIP) with Borrelia afzelii and Neoehrlichia mikurensis, and hence DIN’s of these pathogens in the following year. The NIP, but not the DIN, of the bird-associated Borrelia garinii, decreased with increasing rodent density. The NIPs of Borrelia miyamotoi and Rickettsia helvetica were independent of rodent density, and increasing rodent density moderately increased the DINs. In addition, NIPs of Babesia microti and Spiroplasma ixodetis decreased with increasing rodent density, which had a non-linear association with DINs of these microorganisms.ConclusionsA positive density dependence for all rodent- and tick-associated tick-borne pathogens was found, despite the observation that some of them decreased in prevalence. The effects on the DINs were variable among microorganisms, more than likely due to contrasts in their biology (including transmission modes, host specificity and transmission efficiency). The strongest associations were found in rodent-associated pathogens that most heavily rely on horizontal transmission. Our results draw attention to the importance of considering transmission mode of a pathogen while developing preventative measures to successfully reduce the burden of disease.
Highlights
Rodents are considered to contribute strongly to the risk of tick-borne diseases by feeding Ixodes ricinus larvae and by acting as amplifying hosts for pathogens
Effect of treatment on rodent density, density of nymphs (DON), D INB. afzelii, and DINN. mikurensis Rodent density was affected by treatment (Fig. 1)
The removal of rodents led to a lower (P = 0.0031) rodent density and the addition of acorns led to a higher (P = 0.042) rodent density than in the control plots in years 2013 and 2014 (Fig. 1)
Summary
Rodents are considered to contribute strongly to the risk of tick-borne diseases by feeding Ixodes ricinus larvae and by acting as amplifying hosts for pathogens. The density of infected questing ticks is a product of the density of questing ticks and infection prevalence of a pathogen, which both express high temporal variations, presumably attributed to changes in weather conditions and fluctuations in the abundance of vertebrate hosts [6,7,8] The mechanisms underlying these variations are complex, as climatic conditions, vertebrate hosts and their food source, ticks, and tick-borne microorganisms form biological networks with multiple direct and indirect interactions [9]. Quantifying these interactions will help us to understand changes in the distribution and incidence of Lyme borreliosis and other tick-borne diseases
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