Circumstantial evidence for an increase in the total number and activity of borrelia-infected ixodes ricinus in the Netherlands

Abstract

BackgroundBetween 1994 and 2009, a threefold increase has been observed in consultations of general practitioners for tick bites and Lyme disease in The Netherlands. The objective of this study was to determine whether an increase in the number of questing ticks infected with B. burgdorferi sensu lato is a potential cause of the rise in Lyme disease incidence.MethodsHistoric data on land usage, temperature and wildlife populations were collected and analyzed together with data from two longitudinal field studies on density of questing ticks. Effective population sizes of Borrelia burgdorferi s.l. were calculated.ResultsLong-term trend analyses indicated that the length of the annual tick questing season increased as well as the surface area of tick-suitable habitats in The Netherlands. The overall abundances of feeding and reproductive hosts also increased. Mathematical analysis of the data from the field studies demonstrated an increase in mean densities/activities of questing ticks, particularly of larvae between 2006 and 2009. No increase in infection rate of ticks with Borrelia burgdorferi sensu lato was found. Population genetic analysis of the collected Borrelia species points to an increase in B. afzelii and B. garinii populations.ConclusionsTogether, these findings indicate an increase in the total number of Borrelia-infected ticks, providing circumstantial evidence for an increase in the risk of acquiring a bite of a tick infected with B. burgdorferi s.l. Due to the high spatiotemporal variation of tick densities/activities, long-term longitudinal studies on population dynamics of I. ricinus are necessary to observe significant trends.

The wild life of tick-borne pathogens

“…Our findings are in contrast to previous studies that showed a linear relationship 38,39 or a parabolic relationship 205 between deer density and I. ricinus density. This difference could be due to differences in host assemblage composition between different studies.…”

“…This, together with a large distribution in suitable habitat and climatic conditions, explains why I. ricinus has such a wide distribution, and why it occurs in high densities in many areas with a vertebrate assemblage existing of widespread species. It also supports the hypothesis that the increase in Lyme borreliosis may be due to an increase in I. ricinus distribution and abundance 39,61 , following increases in range and abundance of widespread host species such as Bank vole, Blackbird and Roe deer [147][148][149] .…”

“…Ixodes ricinus is sensitive to desiccation and is mostly found in forested areas throughout Europe 36 . Within forests, it is dependent on the availability of hosts for all stages of the life cycle, where the presence and abundance of deer, as hosts for the adult stage, is theorized to be an important determinant of I. ricinus density 38,39 . Ixodes ricinus encounters a host by sitting on the vegetation and waiting, called 'questing' 8 .…”

“…Van Buskirk and Ostfeld 27 , for example, modelled how nymph densities of I. scapularis responded to differences in densities of hosts for larvae and adults, and found that the density of hosts for adults was limiting nymph density only at very low host densities, where the availability of hosts for larvae then became limiting. Thus, nymph and adult density appear related to deer density according to a nonlinear threshold relationship, rather than the linear relationship used in most studies (e.g., Gilbert et al 38 and Sprong et al 39 ). Furthermore, Van Buskirk and Ostfeld 27 suggested that the threshold adult-host density is close to zero.…”

“…Lyme disease risk is determined by multiple biological, environmental and societal factors 8,62,63 . These can be split into two distinct groups, 1) factors determining the number of questing Ixodes ticks infected with B. burgdorferi, and 2) factors determining the level of human exposure to ticks 39 . In this review, we will focus on the first, with in particular the factors that determine the number of questing Ixodes ticks, and their infection with B. burgdorferi.…”