PhD Thesis

Applying regional climate change projections for spatio-temporal risk analyses of vector-borne diseases

Dominik Fischer (08/2008-08/2011)

Support: Carl Beierkuhnlein, Thomas Nauß, Konrad Dettner, Björn Reineking

This thesis addresses the topic of climate change effects on vector-borne diseases. Disease vectors gather the pathogen from an infected host (usually mammals), become infected but usually do not fall sick. They are capable to transmit the pathogens between hosts. Most of the vector-borne diseases are considered to be climate-sensitive. This has several reasons: First of all, the majority of the vectors belong to ecto-thermic arthropods which cannot regulate their body temperature. In addition, pathogen development within the vector is dependent on temperature constraints. Data concerning expected climate change during the 21st century are provided by climate models in different spatial and temporal resolutions. Projections of future climate include emission scenarios of greenhouse gases. In this thesis the use of regional climate projections with special respect to their capacity for being implemented in spatio-temporal risk analyses of vector-borne diseases is presented. Regional climate models are recom-mended for risk analyses of vector-borne disease on smaller spatial scales, due to their remarkably better performance in comparison to global climate models. Possible applications and opportunities of regional climate projections are introduced (article 1). Within this thesis, data obtained by the regional climate models REMO and COSMO-CLM are used for risk analyses. Both models were derived by a dynamical downscaling procedure. They are nested within the well-established global model ECHAM5, in the latest versions. In the first methodological part (articles 2 and 3) the transfer of already understood vector and/or pathogen temperature constraints to expected future conditions is addressed. By using this approach, the required temperatures for dengue virus amplification within the primary vector Aedes aegypti are compared with expected future European conditions. Results indicate a growing threat for Europe in a twofold manner. First of all, a spatial range expansion of regions, which offer the window of opportunity for dengue-virus amplification, is projected. Starting from southern parts of Europe in the upcoming years, also Central Europe will provide suitable temperature conditions from mid-century onwards. Furthermore, the inter-annual season of possible virus amplification in Europe will increase remarkable (article 2). On a further example, the threat of autochthonous transmission of visceral leish-maniasis, the most severe sandfly-borne disease is determined. Therefore the temperature constraints of vector (genus: Phlebotomus) and pathogen (Leishmania infantum complex) are projected to future conditions on the regional example of Germany. The results for projected vector and pathogen requirements are finally overlaid with the aim to generate risk classes of potential establishment of visceral leishmaniasis. Western and southern German regions will be at risk, expectedly from mid-century onwards. At the end of the 21st century, the establishment of this disease can no longer be excluded for large parts of Germany (article 3). In the second part (articles 4 to 6), regional climate change projections are implemented within species distribution models for vector species. For this approach, statistical analyses are applied in order to determine species´ preferred bioclimatic niches. Once the specific climatic niches of species are identified, these can be transferred to future climatic conditions. Based on this approach it is indicated that the climatic suitability will in-crease for the invasive mosquito Aedes albopictus (vector of several human-pathogenic viruses) in most European regions. Especially in Western Europe (e.g. in France) suitability will increase. With temporal delay also central and eastern parts of Europe will provide favourable conditions, while the suitability will decrease in some Mediterranean areas (article 4). On the example of sandfly species (vectors of the Leishmania complex), the results of the species specific niche modelling show that climatic suitability can be expected to increase in Central Europe within this century and especially for Germany. Potential future dispersal pathways for the species in a rapidly changing environment are identified via least-cost analysis. This indicates that the studied sandfly species will hardly be able to occupy all of their potentially suitable future areas due to their limited dispersal ability (articles 5 and 6). In different case studies it is already shown that European climate change within the 21st century will probably support a spread or at least range expansions of the mentioned disease vectors and vector-borne diseases. As a general tendency for Central Europe it can be expected that the risk will increase especially for the outgoing 21st century. This may be due to the fact that the applied regional climate models indicate a more rapid warming in the second half of the 21st century, regardless the chosen scenario. To conclude, it is worth mentioning that further factors beyond climate change addit-ionally facilitate the potential spread and new-establishment of vector-borne diseases. The influence of these factor (“drivers”) on vector-borne diseases, however, depends on the con-sidered spatial and temporal scale. Therefore, a scale-dependent risk assessment for vector-borne diseases is proposed. Starting at broader scales, climatic risk assessments can be ini-tiated to identify risk areas. Then, the relevant factors must be identified on smaller scales for the detected risk areas and integrated in follow-up studies (article 7). The results can then support more efficiently the development and implementation of surveillance strategies for vector-borne diseases. This enables to initiate counteractions against spreading vector-borne diseases - or supports societal adaptations to this novel threat.