Environmental determinants ofIxodes ricinusticks and the incidence ofBorrelia burgdorferisensu lato, the agent of Lyme borreliosis, in Scotland

Abstract

Lyme borreliosis (LB) is the most common arthropod-borne disease of humans in the Northern hemisphere. In Europe, the causative agent, Borrelia burgdorferi sensu lato complex, is principally vectored by Ixodes ricinus ticks. The aim of this study was to identify environmental factors influencing questing I. ricinus nymph abundance and B. burgdorferi s.l. infection in questing nymphs using a largescale survey across Scotland. Ticks, host dung and vegetation were surveyed at 25 woodland sites, and climatic variables from a Geographical Information System (GIS) were extracted for each site. A total of 2397 10 m2 transect surveys were conducted and 13 250 I. ricinus nymphs counted. Questing nymphs were assayed for B. burgdorferi s.l. and the average infection prevalence was 5·6% (range 0·8-13·9%). More questing nymphs and higher incidence of B. burgdorferi s.l. infection were found in areas with higher deer abundance and in mixed/deciduous compared to coniferous forests, as well as weaker correlations with season, altitude, rainfall and ground vegetation. No correlation was found between nymph abundance and infection prevalence within the ranges encountered. An understanding of the environmental conditions associated with tick abundance and pathogen prevalence may be used to reduce risk of exposure and to predict future pathogen prevalence and distributions under environmental changes.

The wild life of tick-borne pathogens

“…This relationship is straightforward as more deer can feed more adult ticks, resulting in a larger number of larvae in the environment 32,38 (Chapter 4), and was reflected in our study by a higher larval burden on rodents in areas with high deer abundance ( Figure 6.2). Our results are also consistent with previous studies that hypothesized that deer could indirectly increase disease risk by increasing larval densities and subsequently the number of larvae feeding on reservoircompetent host species 208,239 . Some studies have suggested a possible dilution effect of high deer densities on disease risk, where deer, which are incompetent as hosts for several pathogens (e.g.…”

“…Although deer abundance was not directly correlated to nymphal density (Chapter 4), the indirect correlation might explain the increase of I. ricinus densities with deer abundance found in previous studies 38,55,208 . Especially in locations or habitats where small mammals are scarce, the abundance of deer or thrushes might be an important determinant of nymphal density 141,209 .…”

“…Small mammals are considered the most important hosts for larval I. ricinus 190 (Chapter 2), however, when small mammals are absent, deer are able to feed a substantial number of larvae. This might be the explanation for a linear correlation of I. ricinus density and estimates of deer density in several studies in Scotland 38,[206][207][208] where small mammals occur in low abundance and carry few ticks in some habitats 209 . Therefore, deer are important hosts for I. ricinus, but the other hosts in the vertebrate assemblage need to be considered as well when studying relationships between host abundance and I. ricinus density.…”

“…We first tested if temperature and humidity influenced our estimates of nymphal density 208 , which was not the case (methods and results in Appendix S6.2).…”

“…Air temperature and humidity can influence the efficiency of drag sampling, resulting in biased tick density estimates 208 . We therefore measured air temperature and relative humidity at the start and the end of each sampling session within the vegetation using a hygro-thermometer (TH-1; Amprobe, Everett, WA, U.S.A.) and averaged these values per drag session.…”