A New Method to Test Molluscicides against the Philippine Schistosomiasis Snail Vectors

To expedite the discovery of novel molluscicides in the laboratory, this study aimed to evaluate the performance of a new molluscicidal assay. This assay is based on Oncomelania hupensis quadrasi snails and is called miniaturized plate test or mpt. To perform this assay, a 12-well plate, 3 snails per well, and 24-h exposure period were used. The performance of mpt was evaluated using niclosamide and Ardisia plant extract (tagpo extract) as test substances while WHO's guidelines for a conventional plate test (cpt) served as standard. One cpt and four mpt independent trials were performed for niclosamide and tagpo extract. Probit analysis of dose-response data was run in R to generate lethal concentrations (LC₅₀ and LC₉₀), while lethal ratio test was performed to detect significant difference between paired LC₅₀s (or LC₉₀s). Using niclosamide, the calculated LC₅₀ values were 0.104, 0.127, 0.136, 0.139, and 0.140 g/m² for cpt, mpt 1, mpt 2, mpt 3, and mpt 4, respectively, while the LC₉₀ values were 0.266, 0.268, 0.244, 0.251, and 0.261 g/m², using the same sequence, respectively. For tagpo extract, the LC₅₀ values were 1.467, 1.547, 1.659, 1.797, and 1.659 g/m², for cpt, mpt 1, mpt 2, mpt 3, and mpt 4, respectively, and the LC₉₀s were 2.188, 2.195, 2.501, 2.358, and 2.501 g/m², respectively. The lethal ratio test revealed that a significant difference exists between the LC₅₀s of cpt and mpt 1 when using niclosamide with a lethal ratio and confidence limits of 0.820 (0.663, 0.977, p < 0.05) and another significant difference between LC₅₀s of mpt 1 and mpt 3 using tagpo extract with computed lethal ratio and confidence limits of 0.861 (0.782, 0.939, p < 0.05). Taken together, the results point out that mpt generates accurate and reproducible lethal concentration values. Hence, mpt may be used as an alternative method to screen molluscicides that are active against schistosome snail vectors.