Estimating the temporal and spatial risk of bluetongue related to the incursion of infected vectors into Switzerland

Abstract

Background: The design of veterinary and public health surveillance systems has been improved by the ability to combine Geographical Information Systems (GIS), mathematical models and up to date epidemiological knowledge. In Switzerland, an early warning system was developed for detecting the incursion of the bluetongue disease virus (BT) and to monitor the frequency of its vectors. Based on data generated by this surveillance system, GIS and transmission models were used in order to determine suitable seasonal vector habitat locations and risk periods for a larger and more targeted surveillance program.

Results: Combined thematic maps of temperature, humidity and altitude were created to visualize the association with Culicoides vector habitat locations. Additional monthly maps of estimated basic reproduction number transmission rates (R0) were created in order to highlight areas of Switzerland prone to higher BT outbreaks in relation to both vector activity and transmission levels. The maps revealed several foci of higher risk areas, especially in northern parts of Switzerland, suitable for both vector presence and vector activity for 2006.Results showed a variation of R0 values comparing 2005 and 2006 yet suggested that Switzerland was at risk of an outbreak of BT, especially if the incursion arrived in a suitable vector activity period. Since the time of conducting these analyses, this suitability has proved to be the case with the recent outbreaks of BT in northern Switzerland.

Conclusion: Our results stress the importance of environmental factors and their effect on the dynamics of a vector-borne disease. In this case, results of this model were used as input parameters for creating a national targeted surveillance program tailored to both the spatial and the temporal aspect of the disease and its vectors. In this manner, financial and logistic resources can be used in an optimal way through seasonally and geographically adjusted surveillance efforts. This model can serve as a tool for other vector-borne diseases including human zoonotic vectors which are likely to spread into Europe.

Figure 1

Location of sentinel herds for BT early detection from 2004–2007 in Switzerland. Background colour scheme represents risk zones in terms of general Culicoides habitat suitability for 2006. Black stars are location of sentinel herds involved in serological sampling, red stars indicate location of sentinel herds undergoing both serological and entomological sampling. Circles include herds which contributed data for this study.

Figure 2

R₀ equation. Equation for calculating the Basic Reproductive number, circled I, II, and III to represent the different components.

Figure 3

a) Monthly R₀ values plotted versus monthly temperature for Switzerland, 2005. Calculated R₀ values for 2005 in Switzerland (secondary y-axis) along with monthly mean, minimum and maximum temperatures (primary y-axis). b) Monthly R₀ values plotted versus monthly temperature for Switzerland, 2006. Calculated R₀ values for 2006 in Switzerland (secondary y-axis) along with monthly mean, minimum and maximum temperatures (primary y-axis)

Figure 4

Suitability maps of BT occurrence in Switzerland, 2006. Suitability maps (left margin) which were added to R₀ maps (middle margin) to create final combination maps (right margin) Selected months of January, July, September and October are shown.