Vector-borne Disease Risk Assessment in Times of Climate Change: The Ecology of Vectors and Pathogens
Stephanie Thomas (03/2009-10/2014)
Support: Carl Beierkuhnlein, Konrad Dettner
Evidence suggests that European climate change in the 21st century will support a spread of disease vectors and vector-borne diseases. Ectothermic arthropods make up the largest group of vectors. They cannot self-regulate their body temperatures and are therefore considered to be very sensitive to changing climatic conditions. The duration of pathogen development inside the vector is also directly linked to the ambient temperature. This thesis addresses a multidisciplinary approach to improve risk assessment for vector species establishment and pathogen emergence based on ecological knowledge. The objective is to elucidate possible new hotspots of disease transmission.
In the first part of the thesis (articles 1 to 3), climatic factors for disease vectors are identified via literature analysis, statistical procedures in species distribution models, and vector life history trade experiments. Of particular interest here, is the mosquito Aedes albopictus is an invasive disease vector which originates in the tropics and subtropics, it is a competent vector of pathogens such as dengue and chikungunya, among others. By means of correlative species distribution models, suitable European regions are identified. The comparison of published results for various Aedes albopictus risk models, including the aforementioned, shows that beside the regions for which there are a consensus risk levels, there is a great deal of uncertainty in other regions about the future development. These uncertainties in risk model projections indicate that existing knowledge of mosquito ecology needs to be expanded and deepened, in particular with regard to the temperate European situation. The second step was therefore, the integration of detailed ecological knowledge on thresholds during vector life history to improve the correlative risk models. Here, of special interest is the ability of Aedes albopictus to survive winter conditions in Europe, as in its native range no frost temperatures occur. Here, the low-temperature threshold for egg survival was experimentally tested for post-diapause and non-diapause European eggs of Aedes albopictus and non-diapausing eggs of Aedes aegypti. Hatching success after the cold treatment was significantly increased in European eggs which have undergone a diapause compared to non-diapausing European eggs. The experiments help to detect potential regions capable of overwintering populations. Thresholds for survival can be derived by simulating extremes, which then can be related to climate change scenarios.
In the second part of the thesis (articles 4 to 6), climatic factors for pathogens are identified. Using the example of dengue, the temperature requirement for pathogen amplification is determined via statistical analysis of extrinsic incubation period experiments found in literature. The extrinsic incubation period is the time at a defined temperature needed to render the vector infective after a contaminated blood meal. Out of all described dengue extrinsic incubation period experiments a continuous temperature-time profile is provided which allows, via highly resolved spatio-temporal climate change projections, a detailed characterization of potential regions at risk in Europe. A second approach, demonstrated with the example of chikungunya is the analysis of temperature requirement for disease transmission during an outbreak. Once these climatic factors are identified, climate-derived risk maps are generated by combining vector and pathogen requirements. As a general tendency for Europe, it can be expected that the risk of Aedes albopictus establishment and vector-borne virus transmission will increase, especially for the latter decades of the 21st century. Concerning the evolving climatic suitability for Aedes albopictus, it can be inferred that Western Europe will provide especially favourable climatic conditions within the next decades. Furthermore, climatic suitability can be expected to increase in Central Europe and the southernmost parts of the United Kingdom. Climatic conditions will continue to be suitable in Southern France, as well as most parts of Italy and Mediterranean coastal regions in South-eastern Europe. Differences in results for scenarios become obvious, regarding the temporal scale in this century, but the spatial patterns remain the same. The climatic risk of chikungunya transmission will increase in Europe by the end of the century along the western coast of the Mediterranean Balkan States and Greece as well as in the Pannonian Basin and the Black Sea coast of Turkey. A persisting high suitability for Chikungunya transmission throughout the 21st century is projected for Northern Italy.
Finally, emerging tools and concepts are elucidated (article 7) by the means of specific examples in order to identify new multidisciplinary approaches in vector-borne disease risk assessment. However, this is difficult to achieve as scientists are part of specialised and mostly discrete scientific networks, it is necessary that results from other disciplines are understood and considered. To give a first impression of the current scientific cooperation, discipline specific citation behaviour for research on vector-borne diseases with respect to climate change is evaluated.
This thesis offers an ecologically focused evaluation of the spatially and temporally changing risk patterns of invasive vector establishment and emerging disease exposure for Europe during the 21st century. Disease surveillance and vector control measures can now be implemented effectively at locations and times to mitigate possible transmission.