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. 2012 Dec 17:5:294.
doi: 10.1186/1756-3305-5-294.

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

Hein Sprong  1 , Agnetha Hofhuis, Fedor Gassner, Willem Takken, Frans Jacobs, Arnold J H van Vliet, Marijn van Ballegooijen, Joke van der Giessen, Katsuhisa Takumi
Affiliations

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

Hein Sprong et al. Parasit Vectors. .

Abstract

Background: Between 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.

Methods: Historic 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.

Results: Long-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.

Conclusions: Together, 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.

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Figures

Figure 1
Figure 1
Bird population densities 1980–2008. Dots represent population index values of each bird species per year. The bird population index was calculated by dividing the number of a bird species each year by the number of that bird species in the baseline year in 1990 and multiplying the result by 100. Lines represent the best-fit negative binomial distribution with the mean (solid) and the 95% confidence interval (dashed).
Figure 2
Figure 2
Change in the number of Barn owl (Tyto alba) 1990 – 2006. The change in the number of Barn owl was calculated by dividing the number of barn owls each year by the number in the baseline year 1990 and multiplying the result by 100. The solid line is the best fit exponential function.
Figure 3
Figure 3
Roe deer population densities and questing tick abundance. Horizontal axis is the local population density of Roe deer at all fourteen study sites in 2008. Vertical axis is the peak number of questing ticks at the corresponding study sites in 2009. The solid line is the best generalized-linear-model fit.
Figure 4
Figure 4
Numbers of questing ticks at Duin and Kruidberg 2000 – 2009. The population of larvae is shown in panel A. The population of nymphs plus adults is shown in panel B. A dot represents the number of ticks collected per drag (100 m2). Shaded area represents the best-fit for negative binomial distributions with the 95% confidence interval.
Figure 5
Figure 5
Skyline plot of B. afzelii and B. garinii populations. Changes in coalescent population sizes over years are shown in solid lines indicating B. afzelii (panel A) and B. garinii (panel B). Dashed lines enclose 95% credible interval.
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