Chemical attractants of human skin for swimming Schistosoma mansoni cercariae

Cercariae of Schistosoma mansoni approach towards their host's skin with a chemokinesis. They respond to human skin extracts by inserting shifts between backward and forward swimming and therewith increase the rate of change of direction (RCD). We identified the host attractants by fractionating human skin surface extracts, exposing the fractions and chemicals to the cercariae and recording cercarial swimming by video tracking. The cercariae responded specifically to three skin surface compounds by increasing the frequency of shifts in swimming direction. (1) Free fatty acids. The effectiveness was limited to chain lengths between 10 and 14 carbon atoms in saturated chains and increased by the number of double bonds. (2) Free L-arginine at concentrations as low as 100 nM. (3) Small peptides with terminally located L-arginine at concentrations as low as 50 mu M. All chemokinesis stimuli are also involved in subsequent behavioural phases of host invasion: attachment and directed creeping on the skin surface, penetration and navigation within host tissues. The cercariae responded during swimming only to these small molecular water soluble cues and not to cues to which they respond in other behavioural phases of host invasion, but which are unsuitable as chemoattractants due to their chemical properties or distribution within the skin layers. This characterises the chemokinetic responses as adequate adaptation to advance transmission success.