Prevalence, diversity, and load of Borrelia species in ticks that have fed on humans in regions of Sweden and Åland Islands, Finland with different Lyme borreliosis incidences

PLoS One. 2013 Nov 21;8(11):e81433. doi: 10.1371/journal.pone.0081433. eCollection 2013.


The incidence of Lyme borreliosis (LB) in a region may reflect the prevalence of Borrelia in the tick population. Our aim was to investigate if regions with different LB incidences can be distinguished by studying the prevalence and diversity of Borrelia species in their respective tick populations. The Borrelia load in a feeding tick increases with the duration of feeding, which may facilitate a transmission of Borrelia Spirochetes from tick to host. Therefore, we also wanted to investigate how the Borrelia load in ticks that have fed on humans varies with the duration of tick feeding. During 2008 and 2009, ticks that had bitten humans were collected from four regions of Sweden and Finland, regions with expected differences in LB incidence. The duration of tick feeding was estimated and Borrelia were detected and quantified by a quantitative PCR assay followed by species determination. Out of the 2,154 Ixodes ricinus ticks analyzed, 26% were infected with Borrelia and seven species were identified. B. spielmanii was detected for the first time in the regions. The tick populations collected from the four regions exhibited only minor differences in both prevalence and diversity of Borrelia species, indicating that these variables alone cannot explain the regions' different LB incidences. The number of Borrelia cells in the infected ticks ranged from fewer than ten to more than a million. We also found a lower number of Borrelia cells in adult female ticks that had fed for more than 36 hours, compared to the number of Borrelia cells found in adult female ticks that had fed for less than 36 hours.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Bacterial Load
  • Borrelia / classification*
  • Borrelia / genetics
  • Female
  • Finland / epidemiology
  • Geography
  • Humans
  • Incidence
  • Lyme Disease / epidemiology
  • Lyme Disease / transmission
  • Male
  • Polymerase Chain Reaction
  • Prevalence
  • RNA, Bacterial / genetics
  • RNA, Ribosomal, 16S / genetics
  • Sweden / epidemiology
  • Ticks / microbiology*


  • RNA, Bacterial
  • RNA, Ribosomal, 16S

Grant support

The Medical Research Council of South-East Sweden (grant no. FORSS-8967, FORSS-12573, FORSS-29021, FORSS-86911) The Swedish Research Council (Medicine) (grant no. 2011-345) ALF Grants from the County Council of Östergötland ( Wilhelm and Else Stockmanns Foundation ( Ålands Kuturstiftelse ( Ålands Självstyrelses 75-års Jubileumsfond ( An EU Interreg IV A supported ScandTick (grant no. 167226). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.