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The effectiveness of water treatment processes against schistosome cercariae

Schistosomiasis is a water-based tropical disease that affects millions of people in marginalised communities. It is acquired through contact with freshwater that contains cercariae, the parasite larvae that lead to schistosomiasis. Whilst treatment with the drug praziquantel is effective, it does not prevent reinfection which is common among communities living in endemic regions. These often have no choice but to rely on cercaria-contaminated freshwater for their daily water activities. Water treatment for schistosomiasis control seeks to eliminate viable cercariae, thereby providing safe alternative water supplies for recreational and domestic activities. This research explored what is known about the effectiveness of water treatment processes against schistosome cercariae. A systematic review concluded that currently available information was insufficient for providing evidence-based water treatment design recommendations, and that standard protocols for assessing cercaria viability did not exist. Therefore, a novel fluorescence assay was developed that can determine the viability of cercariae in water with an accuracy of at least 92.2% and be used to quantify the effectiveness of water treatment processes. Furthermore, the effectiveness of chlorine against S. mansoni cercariae was tested in laboratory and field settings. The results have unveiled that cercariae are sensitive to chlorine (CT2-log of 30 mg·min/l) and that their sensitivity to chlorine increases with age, i.e. the time since being shed from the host snail. The research has documented the physical effect of chlorine on cercariae, shedding light on how these parasite larvae are inactivated. The recommended CT value can be used to disinfect cercaria-infested water in communities that lack safe alternative water sources, potentially after application of safety factors where needed. The design, implementation and sustainable use of safe water infrastructure will require strong interdisciplinary collaborations in the future, but if successful, has the potential for contributing towards the elimination of this disease as a public health concern.

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