The lethal effect of soap on Schistosoma mansoni cercariae in water

Abstract

Background

Schistosomiasis is a parasitic disease which is spread through skin contact with water containing Schistosoma cercariae. Drug treatment has been the main control method, but it does not prevent reinfection. The use of soap can be a complementary measure to reduce transmission. Therefore, this study investigates the quantitative effect of different soaps on the mortality of Schistosoma mansoni cercariae.

Methodology

Four soaps including two powder soaps (Kleesoft and Omo) and two bar soaps (B29 and Rungu) which are used in a schistosomiasis-endemic Tanzanian village were studied. S. mansoni cercariae were exposed to powder soaps of 0 (control), 10, 50, 75, 100 and 1000 mg/L and to bar soaps of 0 (control), 100, 500 and 1000 mg/L. The highest concentration of 1000 mg/L was selected based on the laboratory-estimated average soap concentration during handwashing. Cercariae were observed under a microscope after 0, 5, 15, 30, 45 and 60 minutes of exposure to determine their survival.

Conclusions

All four soaps can kill S. mansoni cercariae and this lethal effect was related to soap concentration and exposure time. At the highest concentration of 1000 mg/L, all cercariae were dead at 5 minutes post-exposure with two powder soaps and Rungu, while 100% cercarial death was achieved between 5 minutes to 15 minutes for B29. Almost all cercariae survived after being exposed to 10 mg/L powder soaps and 100 mg/L bar soaps for 60 minutes. Powder soaps were more lethal than bar soaps. Considering the widely varying concentrations of soap during real-world hygiene activities and the necessity for a very high soap concentration to eliminate all cercariae in a short 5-minute exposure, providing the efficacy of soap in preventing schistosomiasis becomes challenging. Future studies should investigate whether soap can influence alternative mechanisms such as making cercariae unable to penetrate the skin, thereby providing protection.

Author summary

Schistosomiasis is a major public health problem in low-resource countries where there is insufficient access to safe water, sanitation and hygiene (WASH). Preventive chemotherapy with praziquantel has been the main control method, but it cannot protect against reinfection. With the increasing awareness of the importance of a multi-faceted control strategy, WASH has been recommended as one of the core strategic interventions for schistosomiasis control. The use of soap, one hygiene practice, might play a role in prevention by reducing the penetration of cercariae into human skin during water contact, thereby lowering the infection risk at transmission sites. However, to the best of our knowledge, there are no detailed guidelines on how to use soap to prevent schistosomiasis in endemic areas. Therefore, this study aimed to explore the efficacy of four commercial Tanzanian soaps against Schistosoma mansoni cercariae to understand soap lethality. The results provide scientific evidence of soap protection against schistosome cercariae and suggest that future studies should investigate other potential mechanisms by which soap may provide protection against cercariae (e.g. damaging them sufficiently to hinder penetration into the skin or protecting skin against cercarial penetration).