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

Subject Area Parasitology and Biology of Tropical Infectious Disease Pathogens
Term from 2016 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 299273352
 
Final Report Year 2025

Final Report Abstract

Schistosomiasis, a snail-borne parasitic disease is ranked as the second most important infectious disease after malaria, with 218 million infected people. In the project SENTINEL, we focussed on the association between chemical pollution and the distribution of Schistosoma host snails. The primary objective of this project was to characterize chemical pollution in waterbodies of Western Kenia and to identify the potential effects of this freshwater pollution by agricultural toxicants and other compounds on the abundance of hosts for human-pathogenic trematodes that cause schistosomiasis. We further aimed to understand the ecological mechanisms behind these effects and the consequences for the population development and the vector capacity of the host snails. In the first phase, we investigated 48 freshwater systems including small and medium streams, irrigation channels, oxbow lakes, reservoirs and flooded rice fields of western Kenya which are part of the Lake Victoria South Basin. For characterization of the chemical contamination, water, sediment and snail samples were collected from selected sites and analysed in the laboratory. Risk assessment was also performed to check for potential risk of toxicity from the chemicals found in water to fish, crustaceans and algae. The toxic pressure of pesticides on the macroinvertebrate community structure was identified by applying the bio-indicator “SPEARpesticides”. Our findings revealed significant water contamination in the study area. Pesticide pollution indirectly favoured vector snails (e.g., Biomphalaria pfeifferi), primarily by negatively affecting their competitors, such as sensitive macroinvertebrate species, rather than their predators. Further, SPEARpesticides bioindicator was effectively calibrated for East African streams, proving to be a cost-effective means of monitoring pesticide impacts on macroinvertebrate communities. In the second phase, we investigated seasonal pollution patterns and the spatiotemporal dynamics of host snails and their Schistosoma parasites, focusing on recolonization, pesticide pollution, and additional environmental stressors. We assessed how pesticide variation influenced macroinvertebrate community structure, snail abundance, and parasite dynamics throughout the year. Comprehensive assessments revealed the widespread presence of organic micropollutants (OMPs), including pesticides, pharmaceuticals, hormones, and industrial chemicals, across water, sediments, and biota. In western Kenya, pesticides such as diazinon and pirimiphos-methyl emerged as major ecological stressors, significantly impacting crustaceans and algae. Steroid hormones, even at low concentrations, disrupted endocrine systems. Many pollutants exceeded acute and chronic risk thresholds, underscoring the need for continuous monitoring and mitigation. Biological data, including macroinvertebrate assemblages assessed using SASS5 and SPEAR biomonitoring methodologies, were combined with data on chemical pollution physicochemical parameters to analyse factors influencing snail presence over two dry-wet cycles. The findings reinforced SENTINEL I results, confirming pesticide pollution as a key determinant of snail abundance, particularly by reducing predator and competitor populations during peak runoff periods. Laboratory experiments further explored the effects of multiple stressors – pesticides and heat – on snail survival and Schistosoma infection. Temperature had the most significant impact, with higher temperatures increasing snail mortality but also accelerating parasite development, shortening the incubation period for human-infective cercariae.

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