Susceptibility of Biomphalaria alexandrina to the plant Azolla pinnata and some herbicides in relation to infection with Schistosoma mansoni miracidia.
The molluscicidal activity against Biomphalaria alexandrina (laboratory-bred) of the herbicides 2,4-D and Graminol [paraquat + monolinuron], and extracts and dry powder of the plant Azolla pinnata (collected from irrigation canals in Qaluobiya Governorate, Egypt), was evaluated. 30 snails were used to test each concentration of each substance. 2,4-D was the most toxic compound, with a LC90 of 52 ppm after 24 h of exposure (Graminol had a slightly lower LC90 but a considerably higher LC50). The most effective Azolla derivative was ethanol extract (LC90=3300 ppm). Ethanol extract was also tested for its activity against Schistosoma mansoni larvae; 3 h of exposure to 6600 ppm killed 100 and 19.4% of miracidia and cercariae, respectively. The molluscicidal activity of 2,4-D was not significantly influenced by the presence of Azolla (900 plants per litre) for 7 days prior to addition of snails to the mixture; the effect of Graminol was significantly reduced. The infectivity of S. mansoni miracidia to B. alexandrina appeared to be increased rather than reduced by the presence of Azolla.