The importance of wildlife in the ecology and epidemiology of the TBE virus in Sweden: Incidence of human TBE correlates with abundance of deer and hares

Thomas G.T. Jaenson*, Erik H. Petersson, David G.E. Jaenson, Jonas Kindberg, John H.O. Pettersson, Marika Hjertqvist, Jolyon Medlock, Hans Bengtsson

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

19 Citations (Scopus)

Abstract

Background: Tick-borne encephalitis (TBE) is one tick-transmitted disease where the human incidence has increased in some European regions during the last two decades. We aim to find the most important factors causing the increasing incidence of human TBE in Sweden. Based on a review of published data we presume that certain temperature-related variables and the population densities of transmission hosts, i.e. small mammals, and of primary tick maintenance hosts, i.e. cervids and lagomorphs, of the TBE virus vector Ixodes ricinus, are among the potentially most important factors affecting the TBE incidence. Therefore, we compare hunting data of the major tick maintenance hosts and two of their important predators, and four climatic variables with the annual numbers of human cases of neuroinvasive TBE. Data for six Swedish regions where human TBE incidence is high or has recently increased are examined by a time-series analysis. Results from the six regions are combined using a meta-analytical method. Results: With a one-year time lag, the roe deer (Capreolus capreolus), red deer (Cervus elaphus), mountain hare (Lepus timidus) and European hare (Lepus europaeus) showed positive covariance; the Eurasian elk (moose, Alces alces) and fallow deer (Dama dama) negative covariance; whereas the wild boar (Sus scrofa), lynx (Lynx lynx), red fox (Vulpes vulpes) and the four climate parameters showed no significant covariance with TBE incidence. All game species combined showed positive covariance. Conclusions: The epidemiology of TBE varies with time and geography and depends on numerous factors, i.a. climate, virus genotypes, and densities of vectors, tick maintenance hosts and transmission hosts. This study suggests that the increased availability of deer to I. ricinus over large areas of potential tick habitats in southern Sweden increased the density and range of I. ricinus and created new TBEV foci, which resulted in increased incidence of human TBE. New foci may be established by TBE virus-infected birds, or by birds or migrating mammals infested with TBEV-infected ticks. Generally, persistence of TBE virus foci appears to require presence of transmission-competent small mammals, especially mice (Apodemus spp.) or bank voles (Myodes glareolus).

Original languageEnglish
Article number477
JournalParasites and Vectors
Volume11
Issue number1
DOIs
Publication statusPublished - 29 Aug 2018

Bibliographical note

Funding Information:
TJ’s research on the ecology of ticks and tick-borne infections is funded by Carl Tryggers stiftelse, Helge Ax:son Johnsons stiftelse, Längmanska kulturfon-den and Magnus Bergvalls stiftelse, all in Stockholm, Sweden. JP is supported by the Swedish Research Council FORMAS (grant agreement 2015-710). This work was carried out under the auspices of ESGBOR (the European Study Group on Lyme Borrelioses) and Avia-GIS/VectorNet, which is funded by EFSA and ECDC. None of the funding bodies played any role in the planning and design of the study, data collection, analyses, data interpretations, writing of the manuscript, and in the decision about when and where to submit the manuscript for publication.

Publisher Copyright:
© 2018 The Author(s).

Keywords

  • Capreolus capreolus
  • Cervus elaphus
  • Climate change
  • Hare
  • Ixodes ricinus
  • Lepus
  • Red deer
  • Roe deer
  • TBE virus
  • Tick-borne encephalitis

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