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

doi:10.1186/s13071-018-3057-4

Abstract

BackgroundTick-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.ResultsWith 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.ConclusionsThe 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).

Highlights

Tick-borne encephalitis (TBE) is one tick-transmitted disease where the human incidence has increased in some European regions during the last two decades
Is the increase in TBE incidence due to wildlife abundance? When a time lag of one year was applied, the results showed that, on average, the red deer, roe deer, mountain hare and European hare exhibited positive, significant covariance; the Eurasian elk and fallow deer negative, significant covariance, whereas the wild boar, lynx and red fox showed no covariance with the incidence of human TBE (Fig. 4)
The de-trended standardised values for roe deer abundance versus incidence of TBE are shown in Fig. 5 for each of the six regions included in this study

Summary

Introduction

Tick-borne encephalitis (TBE) is one tick-transmitted disease where the human incidence has increased in some European regions during the last two decades. The great medical importance of Ixodes ricinus In Europe Ixodes ricinus is the most important vector of viruses, bacteria and protozoa, which potentially can cause disease in humans, companion animals or livestock. In several European countries the numbers of diagnosed cases of tick-borne diseases transmitted by I. ricinus to humans have increased during the last two decades [10,11,12,13,14,15,16,17,18,19], but in some regions disease incidence appears not to have increased [20] or has even declined [21].

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