The protective mechanisms of soap against schistosome cercariae in water

Schistosomiasis is a parasitic water-based disease which affects almost 240 million people worldwide currently. This disease occurs because of skin contact with water containing the parasite larvae, schistosome cercariae. The use of soap is central to many hygiene practices and might play a role in schistosomiasis prevention by reducing the penetration of cercariae into human skin during water-contact activities. This PhD thesis determined the efficacy of soap against cercariae via potential protective mechanisms. Firstly, a systematic review revealed that soap might protect individuals by directly harming cercariae or by protecting skin against cercarial infectivity. However, the knowledge was insufficiently detailed to inform soap use practices that would prevent infection. A novel method of using tails of deceased mice was then developed to study cercarial infectivity. Laboratory experiments were performed to investigate three potential protective mechanisms of soap: (1) killing cercariae, (2) impairing cercarial infectivity, and (3) protecting skin against cercarial infectivity. The latter two mechanisms, related to cercaria infectivity, were tested using the mouse tail method that was established. Powder and bar soaps that are used by people living in a schistosomiasis-endemic village in Tanzania were included in the research. For the first mechanism of soap lethality, all soaps were able to kill cercariae, with their efficacy related to the soap concentration and exposure time. Among the second and third mechanisms, only the third mechanism, protection of tails after soap treatment, reduced cercarial infectivity, but this result was observed only with powder soap. This research provides scientific evidence and new understanding of the protection provided by soap against schistosomiasis infection, via direct killing of cercariae and protection of skin. However, due to the limited protection that soap provides, it can play a complementary role, e.g. alongside preventive chemotherapy and/or mollusciciding, rather than a primary one in overall schistosomiasis prevention strategies.