Thymol as Molluscicidal Agent Against Biomphalaria alexandrina, Intermediate Host of Schistosoma mansoni

Biomphalaria alexandrina snails have received much concern due to their great medical importance as vectors for transmitting Schistosoma mansoni infection to humans. Sustainable control of B. alexandrina snails represents a logical approach to achieve its elimination targets. This work aimed to evaluate the efficacy of the monoterpenoid thymol as a plant molluscicide in the control of this schistosomiasis vector snail. The molluscicidal activity of thymol was determined according to the WHO method. The results showed that the lethal concentration LC50 and LC90 values of thymol were 21.90 and 34.06 ppm, respectively. Also, it was observed that the accumulative effect of sublethal concentration LC25 (15.94 ppm) of thymol for 4 successive weeks had destructive effects on the survival rate, egg-laying capacity, reproduction rate, and biochemical parameters of exposed snails. Our results demonstrated that thymol showed an oviposition inhibitory activity against B. alexandrina snails. Furthermore, the sublethal toxicity of thymol induced a significant increase in levels of total proteins and glucose as well as in enzyme activities of alkaline phosphatase and acid phosphatase of hemolymph of exposed snails, whereas a significant inhibition in aspartate aminotransferase and alanine aminotransferase activities was recorded. These significant biochemical disorders in turn explained noticeable changes in reproduction and other biological activities of treated snails. It was concluded that thymol can be used as a safe promising plant molluscicide for B. alexandrina snails' control as it interferes with the snails' biology and physiology. So thymol can be effectively used in the National S. mansoni control programs.