Determinants of tick-borne encephalitis in counties of southern Germany, 2001-2008

Abstract

BackgroundTick-borne encephalitis (TBE) virus can cause severe symptoms in humans. The incidence of this vector-borne pathogen in humans is characterised by spatial and temporal heterogeneity. To explain the variation in reported human TBE cases per county in southern Germany, we designed a time-lagged, spatially-explicit model that incorporates ecological, environmental, and climatic factors.ResultsWe fitted a logistic regression model to the annual counts of reported human TBE cases in each of 140 counties over an eight year period. The model controlled for spatial autocorrelation and unexplained temporal variation. The occurrence of human TBE was found to be positively correlated with the proportions of broad-leafed, mixed and coniferous forest cover. An index of forest fragmentation was negatively correlated with TBE incidence, suggesting that infection risk is higher in fragmented landscapes. The results contradict previous evidence regarding the relevance of a specific spring-time temperature regime for TBE epidemiology. Hunting bag data of roe deer (Capreolus capreolus) in the previous year was positively correlated with human TBE incidence, and hunting bag density of red fox (Vulpes vulpes) and red deer (Cervus elaphus) in the previous year were negatively correlated with human TBE incidence.ConclusionsOur approach suggests that a combination of landscape and climatic variables as well as host-species dynamics influence TBE infection risk in humans. The model was unable to explain some of the temporal variation, specifically the high counts in 2005 and 2006. Factors such as the exposure of humans to infected ticks and forest rodent population dynamics, for which we have no data, are likely to be explanatory factors. Such information is required to identify the determinants of TBE more reliably. Having records of TBE infection sites at a finer scale would also be necessary.

Correlation of TBE Incidence with Red Deer and Roe Deer Abundance in Slovenia

“…Correspondingly, studies in Sweden, where tick abundance has been studied have found, that the increasing number of ticks can be explained by high availability of tick maintenance hosts, particularly roe deer, which has been increasing in the last three decades and the warmer climate, that influences the growing season, and consequently both the tick and deer [34]. Additionally, studies have confirmed a positive correlation between densities of roe deer and TBE infected ticks and TBE incidence [29], [31], [46], [47].…”

Ecosystem Services from Small Forest Patches in Agricultural Landscapes

“…In North America, landscape fragmentation seems to play a crucial role in explaining patterns of tick abundance and Lyme Borreliosis prevalence mediated by ungulate and small mammal dynamics [143]. The influence of the landscape context is not yet completely understood in Europe, but there are indications that landscape fragmentation is responsible for patterns of tick-borne encephalitis in European agricultural landscapes [144] and the prevalence of B. burgdorferi in forests and pastures [145]. This is presumably due to the greater amount of edges in fragmented landscapes where small vertebrates, ungulates and medium-sized mammals act as reservoir hosts.…”

Rodents as Sentinels for the Prevalence of Tick-Borne Encephalitis Virus

“…This method of classification might be misleading because of increasing vaccination rates in humans, particularly in risk areas, and a resulting decrease in the number of human TBE cases. Additionally, the exposure of humans to ticks might vary substantially in space and time and might thus also introduce bias to the current risk-mapping approach (Kiffner et al 2010). Further, humans are accidental hosts rarely becoming infected, and quite often an infection will not be recognized because of unspecific symptoms, mild courses of disease, or unawareness of physicians.…”