Characterization of chemical stimuli for the penetration of Schistosoma mansoni cercariae. I. Effective substances, host specificity.

The stimulation of penetration of Schistosoma mansoni cercariae into agar was studied. This depends exclusively on chemical triggers whose specificity is analyzed in experiments with 230 chemicals. Only aliphatic hydrocarbon chains which have a polar as well as a nonpolar end group are effective. The effectiveness of saturated substances is limited, at pH 7.0, to chain lengths between 10 and 15 carbon atoms. Unsaturated substances are active at longer chain lengths. Their effectiveness is increased by the number of double bonds in the cis position, and these should not be placed too close to the nonpolar end of the chain. As well as double bonds, halogens, hydroxyl groups, and methyl side chains are equally effective. All penetration stimulating substances kill cercariae in free water, and the appropriate chemicals cause this lethality at very low concentrations. It is shown that the host recognition pattern at the stage of penetration is especially adapted to invasion of the human skin.