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

Abstract

Background: Tick-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.

Results: We 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.

Conclusions: Our 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.

Figure 1

Number of reported human TBE infections in Germany from 2001-2008. Data source: Robert-Koch Institut: SurvStat, http://www3.rki.de/SurvStat. Accessed: 2/04/2009.

Figure 2

Spatial and temporal variation in reported numbers of human TBE infections from 2001-2008. The central map shows the distribution of TBE infection of entire Germany for the entire study period. Maps depicting the annual distribution of human TBE infections for the study area (States of Baden-Württemberg and Bavaria) are arranged clockwise around the central map. Data source: Robert-Koch Institut: SurvStat, http://www3.rki.de/SurvStat. Accessed: 02/04/2009.

Figure 3

Relative effects of significant variables of the final model explaining the variation of human TBE infections in southern Germany. The effect of single variables on the probability of TBE incidence was estimated by predicting the full model on the entire range of the target variable. Non-target variables entered the model prediction with their mean value and using the year 2001 as reference. For a definition of the variables shown in a) - j), see Table 1.

Figure 4

Histogram of the residuals of the final model explaining the variation of human TBE infections in southern Germany.

Figure 5

Spatial distribution of residuals derived from the negative binomial regression model on TBE infections in humans.

Figure 6

Distribution of reported TBE cases in Baden-Württemberg and Bavaria by age group and sex (m = male, f = female). Data source: Robert-Koch Institut: SurvStat, http://www3.rki.de/SurvStat. Accessed: 02/04/2009.