Historical Perspective: Snail Control to Prevent Schistosomiasis

Effective interruption of the Schistosoma life cycle is essential to blocking the parasite’s transmission, and thus truly preventing human schistosomiasis over the long term. Our current mass treatment campaigns were expected to limit transmission by reducing environmental contamination with parasite eggs. However, the process of transmission has proven to be very focal and highly efficient, such that, in practice, mass drug treatment by itself has not been effective in lowering transmission in many affected areas. Alternative means for interruption of transmission are now being considered, including means to reduce or eliminate intermediate snail hosts from local habitats, and means to prevent water contamination through sanitation. Before the introduction of safe oral drug therapy (e.g., praziquantel and oxamniquine), snail control for prevention of Schistosoma transmission was an important component of many regional schistosomiasis control programs (Fig 1). Now, following the 2012 London Declaration on Neglected Tropical Diseases and the recent recommendation by the World Health Assembly favoring local schistosomiasis elimination “where feasible” (WHA Resolution 65.21 [1]), interest has revived in supplementary non-drug means to achieve local elimination of Schistosoma transmission. This scoping historical review highlights some of the perceived strengths and weakness of transmission-interruption methods based on snail control, reexamining the results of programs of the 1950–1980s that aimed at control of Bulinus and Biomphalaria host snail species. In particular, we focus on the required inputs for effective control using the chemical molluscicide niclosamide, which is the 2-amino ethanol salt of 2', 5'-dichloro-4'nitro salicylanilide, and which is sold as Bayluscide, Mollutox, and other names. Application of chemical molluscicides, either by blanket or focal treatments, was the most common approach to snail control at Schistosoma transmission sites in Africa and the Americas during the 1950s–1970s. Although copper sulfate and sodium pentachlorophenate were in use in the 1950s, niclosamide became the compound most frequently studied in published reports after 1961 [2]. The perceived advantages of niclosamide, a newer compound, were its very low toxicity for humans and livestock and its ability to kill snails, their eggs, and cercariae at concentrations of<1 part per million (ppm, equivalent to 1 mg per liter concentration) [3]. Niclosamide was also relatively stable to ultraviolet radiation, with persistent lethal effects for up to 24 hours after application. Typical values for 90% snail mortality (LC90) are 0.25 ppm for Biomphalaria pfeifferi, 0.5 ppm for Bulinus globosus, and 0.25 ppm for Bulinus tropicus. Perceived strengths and weaknesses of mollusciciding are listed in Tables 1 and 2, and a list of materials needed for a typical implementation of molluscicide application is provided in Table 3.