Freshwater pollution and the links to the distribution of Schistosoma host snails in Western Kenya

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Schistosomiasis is the second most pressing tropical disease after malaria, with 218 million people infected worldwide. It is caused by trematodes of the genus Schistosoma sp. which parasitize humans as their definitive host and freshwater snails of the family planorbidae as their intermediate hosts. Hot spots of schistosomiasis infections in Sub-Saharan Africa are typically characterized by extensive agriculture and heavy rainfalls. In such conditions, there is a high risk of surface run-off that washes pesticides from agricultural fields into adjacent freshwaters. This agrochemical pollution affects the macro-invertebrate community structure such that this favours highly tolerant host snails of Schistosoma. Currently, the global control strategies generally rely on morbidity control through human treatment with drugs that kill adult worms harboured in human hosts, but it cannot prevent the re-infection. Accordingly, re-infection from freshwater biotopes needs to be restricted. This is where the SENTINEL project comes in. In the first SENTINEL project, we have already identified those environmental factors through field monitoring as well as laboratory experiment that increase the occurrence of the host snails – namely pesticide pollution and eutrophication. Now, in the follow-up project SENTINEL-ll, we aim to understand the environmental factors that are relevant for describing the infection rate of snails. For this, we will investigate the spatiotemporal population dynamics and recolonization of host snails, and their schistosomes throughout the year. Additionally, we aim to investigate spatiotemporal variation in pesticide pollution and its association with macroinvertebrate community structure, abundance of host snails and the temporal dynamics of schistosomes in the host snails. Finally, we also aim to investigate the effect of additional environmental stressors on the dynamics of parasite burden in host snails. We will address all of the research questions through field studies as well as laboratory investigations. We anticipate that the combined results of SENTINEL-1 and SENTINEL-2 will draw a complete picture of the ecotoxicological processes and interrelations driving the occurrence and infection rate of the parasites and their host based on the prevailing environmental conditions. This project will combine the expertise of three German and three Kenyan research institutes including the chemical analysis of complex environmental samples, the analysis of ecotoxicological effects across different biological organization levels, the ecology of tropical vector species and associated freshwater communities, and the molecular determination of the trematode species.