title: Circumstantial evidence for an increase in the total number and activity of Borrelia-infected Ixodes ricinus in the Netherlands.
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.
authors.affiliation: Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands. hsprong@gmail.com
authors.forename: Hein
authors.initials: H
authors.lastname: Sprong
authors.forename: Agnetha
authors.initials: A
authors.lastname: Hofhuis
authors.forename: Fedor
authors.initials: F
authors.lastname: Gassner
authors.forename: Willem
authors.initials: W
authors.lastname: Takken
authors.forename: Frans
authors.initials: F
authors.lastname: Jacobs
authors.forename: Arnold J H
authors.initials: AJ
authors.lastname: van Vliet
authors.forename: Marijn
authors.initials: M
authors.lastname: van Ballegooijen
authors.forename: Joke
authors.initials: J
authors.lastname: van der Giessen
authors.forename: Katsuhisa
authors.initials: K
authors.lastname: Takumi
mesh_terms: D000818:Animals; D001096:Arachnid Vectors / Q000382:microbiology* / Q000502:physiology*; D001717:Birds / Q000502:physiology; D025065:Borrelia burgdorferi / Q000302:isolation & purification*; D017753:Ecosystem; D005247:Feeding Behavior; D006801:Humans; D018884:Ixodes / Q000382:microbiology* / Q000502:physiology*; D007814:Larva / Q000502:physiology; D008193:Lyme Disease / Q000453:epidemiology / Q000382:microbiology* / Q000635:transmission; D009426:Netherlands / Q000453:epidemiology; D011157:Population Dynamics; D012377:Rodentia / Q000502:physiology