Schistosomiasis, also known as bilharzia, is the second most pressing tropical disease after malaria (WHO 2012), with 218 million people infected worldwide. It is caused by parasitic flatworms 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.
The balance between efficient host snails and antagonistic species is governed by additional ecological stressors (Barbosa 1987). The loss of biodiversity and the ecological degradation of freshwater habitats are known to support schistosomiasis and other water-borne diseases (WHO 2001, Johnson & Thieltges 2009). Freshwater pollution and spread of infectious diseases is highly relevant. Agricultural runoff presents a crucial source of freshwater pollution (Liess & Schulz 1999). However, information on pesticide concentrations and other pollutants in tropical freshwater streams and their effects on the macroinvertebrate community are often fragmentary and inadequate (London et al. 2005, Musa et al. 2011). Thus, the project SENTINEL was designed to investigate the freshwater pollution and the links to the distribution of Schistosoma host snails in Western Kenya.
In the project SENTINEL-I, 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-I and SENTINEL-II 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.
Musa S, Gichuki WW, Raburu PO, Mulanda Aura CM (2011). Organochlorine and organophosphorus pesticide residues in water and sediment from Yala/Nzoia River within Lake Victoria Basin, Kenya. Journal of Ecology and the Natural Environment 3, 392-399.