Novel strategy for control of schistosomiasis : Decoy snails that can reduce the number of miracidia reaching the intermediate snails

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Decoy snails could be used as a new tool for the control of schistosomiasis in the developing countries. The present study was attempted to find the decoy snail candidate in main species of non-susceptible snails at the endemic area of schistosomiasis hematobia, in Kwale, Kenya. We studied 1) miracidial chemotaxis generated by excretion and secretion from snails (snail-conditioned water : SCW), 2) miracidial penetration in snails, 3) assessment of decoy effect in non-susceptible snails in laboratory, and 4) field application of the decoy snail candidate at the transmission sites. The snails used were susceptible snail Bulinus globosus and non-susceptible snails Cleopatra ferruginea, Bellamya unicolor, Lanistes ovum, Melanoides sp. The miracidia used were from the eggs obtained from the school children in Kwale.Results : 1) Miracidial chemotaxis generated by SCW. SCW from B.globosus elicited the highest miracidial response. The response of miracidia to SCW from L.ovum mimicked those gen … More erated by B.globosus. Miracidia showed slight positive chemotaxis to SCW from C.ferruginea. Other two species did not elicit the response. 2) Miracidial penetration in snails. PCR confirmed miracidial penetration in B. globosus, but did not show the presence of miracidia in other snails. 3) Assessment of decoy effect in candidate snails in laboratory. We attempted to find whether interposed non-susceptible snails could protect the susceptible snails from infection when susceptible and non-susceptible snails both occupied the same space. In other experiment, non-susceptible snails were placed at the center of the channels. Then B.globosus was placed at one end and miracidia were introduced at the other end. In any experiments, we failed to find that interposed non-susceptible snails protected B.globosus. 4) Field application of decoy snail candidate at the transmission sites. After one year snail survey at 3 study sites, the population of non-susceptible snail, C.ferruginea, was raised to the density 3 times bigger than the original by introduction of huge number of C. ferruginea collected from other breeding sites. Then the follow-up snail survey was conducted for one year more. The results showed a slight reduction in the infection rate of B.globosus at all sites, but there was no statistical significance.Discussion : The present study showed that some snails breeding in our study area elicited miracidial response. However, the present study suggests that frequency of infection in a natural environment might not be influenced by the presence and density of non-susceptible snails. Less