Abstract
BackgroundControl of snail intermediate hosts has been proved to be a fast and efficient approach for interrupting the transmission of schistosomiasis. Some plant extracts have shown obvious molluscicidal activity, and a new compound Luo-Wei, also named tea-seed distilled saponin (TDS), was developed based on the saponins extracted from Camellia oleifera seeds. We aimed to test the molluscicidal activity of 4% TDS against the intermediate host snails in China and Egypt, and evaluate its environmental safety to non-target organisms.MethodsIn the laboratory, Oncomelania hupensis, Biomphalaria alexandrina and Bulinus truncatus were exposed to 4% TDS, and the median lethal concentration (LC50) was estimated at 24, 48 and 72 h. In the field, snail mortalities were assessed 1, 2, 3 and 7 d post-immersion with 2.5 g/m3 4% TDS and 1, 3, 7 and 15 d post-spraying with 5 g/m2 4% TDS. In addition, the acute toxicity of 4% TDS to Japanese quail (Coturnix japonica), zebrafish (Brachydanio rerio) and freshwater shrimp (Macrobrachium nipponense) was assessed by estimations of LC50 or median lethal dose (LD50).ResultsIn the laboratory, the LC50 values of 4% TDS for O. hupensis were 0.701, 0.371 and 0.33 mg/L at 24, 48 and 72 h, respectively, and 4% TDS showed a 1.975 mg/L 24 h LC50 against B. alexandrina, and a 1.396 mg/L 24 h LC50 against B. truncatus. Across all study regions, the pooled mortalities of O. hupensis were 72, 86, 94 and 98% at 1, 2, 3 and 7 d, following field immersion of 4% TDS at a dose of 2.5 g/m3, and were 69, 77, 85 and 88% at 1, 3, 7 and 15 d, following field spraying at 5 g/m2, respectively. 4% TDS had moderate toxicity to Japanese quail (7 d LD50 > 60 mg/kg) and to shrimp (96 h LC50 = 6.28 mg/L; 95% CI: 3.53–11.2 mg/L), whereas its toxicity to zebrafish was high (96 h LC50 = 0.15 mg/L; 95% CI: 0.14–0.17 mg/L).Conclusions4% TDS is active against O. hupensis, B. alexandrina and B. truncatus under laboratory and field conditions, and it may be a candidate molluscicide of plant origin.
Highlights
Control of snail intermediate hosts has been proved to be a fast and efficient approach for interrupting the transmission of schistosomiasis
Healthy non-infected snails were maintained at the Department of Medical Malacology, Theodor Bilharz Research Institute (TBRI), Egypt, in plastic aquaria provided with dechlorinated tap water (10 snails/L, 25 ± 1 °C) for at least 3 weeks before tests, and 4% tea-seed distilled saponin (TDS) were formulated to concentrations of 0.75, 1, 1.25, 1.5, 1.75, 2, 2.25, 2.5, 2.75 mg/L in dechlorinated tap water
The lowest lethal concentration of snail populations at 24 h exposure was observed for O. hupensis, for which Median lethal concentration (LC50) was nearly three-fold lower than that of B. alexandrina (1.975 mg/L) or B. truncatus (1.396 mg/L), with observed LCR50 (O. hupensis/B. alexandrina/B. truncatus) of 0.355 and 0.502, respectively (Table 1)
Summary
Control of snail intermediate hosts has been proved to be a fast and efficient approach for interrupting the transmission of schistosomiasis. We aimed to test the molluscicidal activity of 4% TDS against the intermediate host snails in China and Egypt, and evaluate its environmental safety to non-target organisms. Parasitic Schistosoma infections in humans depend absolutely on the local presence of their intermediate freshwater snail hosts [1]. Molluscicide-based control of snail intermediate hosts is a fast and efficient approach for interrupting the transmission of this parasite [2, 3]. The synthetic chemical molluscicides typically used to control these snails are expensive and can be toxic to other living organisms in the snail environmental habitat. Recent phytochemical screening has indicated that many plants are endowed with pesticidal properties that can be harnessed cheaply for vector control [5], and plant extracts have been studied as alternatives to chemical molluscicides [6, 7]
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