Molluscicidal and antischistosomal activities of methanol extracts and isolated compounds from Eucalyptus globulus and Melaleuca styphelioides

Context: Schistosomiasis is a parasitic disease that results in severe organ damage. Snail control is the best measure to control schistosomiasis. Plant-derived molluscicides have gained increasing attention for the control of schistosomiasis because they have low toxicity towards non-target organisms. Tannins are particularly suitable for snail control because they are less toxic than saponins to non- target organisms. Objective: To identify the most toxic components of two plants belonging to the family Myrtaceae, namely Eucalyptus globulus Labill. and Melaleuca styphelioides Sm against the different developmental stages of Schistosoma mansoni and its snail host. Materials and methods: The 80% MeOH leaf extracts of the tested plants and their isolated compounds were screened for their molluscicidal activity (expressed as LC50 and LC90 after 24 h exposure) against the snail Biomphalaria alexandrina. The anti-schistosomal activity of the tested samples was determined at 20 ppm (after 1 or 2 h exposure) against the different developmental stages of S. mansoni, including the miracidia, cercariae and worms. Biochemical parameters were measured to determine the toxicity mechanisms of the treated snails. The structures of the isolated compounds were elucidated based on NMR, UV and HRESI-MS/MS data. Results: Potent molluscicidal activity was observed for the ellagitannin dimer eucalbanin B (12), with an LC50 value of 55 ppm. Treatment of the snails with the LC25 of eucalbanin B (30.8 ppm) resulted in a significant decrease in the protein level by 22.7% and 25.8% in the snail tissues and hemolymph, respectively. The decreased protein content was attributed to destruction of the snail tissue and impairment in protein synthesis under stress conditions of intoxication with eucalbanin B. Alterations in the activities of the transaminases and phosphatases in the treated snails indicated destruction and intoxication of the snail tissues. A significant increase in the levels of the transaminases alanine aminotransferase (ALT) (57.8%) and aspartate aminotransferase (AST) (113.2%) in the snail hemolymph and a significant decrease in their tissue levels to 7.4 and 48.6%, respectively, were attributed to their release from the damaged tissue into the hemolymph. Alkaline phosphatase (ALP) was significantly increased by 38.5 and 181.4% in the hemolymph and tissues, respectively. Acid phosphatase (ACP) was also significantly increased by 48.4 and 21.2% in the hemolymph and tissues, respectively. The 80% MeOH extract of E. globulus together with mallophenol B (3), 2,2,8-trimethyl-6-formyl-chrom-3-ene-7-O-beta-D- glucopyranoside (5) and benzyl alcohol 7-O-(30,40,6'-tri-O-galloyl)-beta-D-glucopyranoside ( 10) exhibited miracidicidal activity with almost 100% toxicity at 20 ppm for the three compounds and 80% toxicity for the extract. Moreover, E. globulus extract showed cercaricidal and schistosomicidal activity with 100 and 40% mortality, respectively. Conclusion: E. globulus is a potential source for biocidal compounds against S. mansoni and its snail host. This is the first study to test the biocidal activity of the isolated compounds.