Swimming behaviour of Schistosoma mansoni cercariae: responses to irradiance changes and skin attractants.

The swimming behaviour of many cercarial species is governed by sensitive responses to light and dark stimuli. We studied the effect of irradiance changes on swimming behaviour of Schistosoma mansoni cercariae and found only insignificant responses. Decreasing light intensity results in a weak tendency of the cercariae to start swimming movements, and increasing light intensity tends to inhibit the start of swimming. These responses seem not suitable to increase the transmission success. Whether the cercariae show chemo-orientation towards human skin was studied by video-tracking their swimming movements around agar containing human-skin-surface extracts and when immersed into skin extracts. They showed no directed chemotactic orientation, as they did not correct their swimming paths in direction towards the skin-extract substrates, also not when shifting between forward and backward swimming. However, the cercariae shifted more between backward and forward swimming and therewith increased their rate of change of direction. This response may support an accumulation around the skin substrates and could guide the cercariae towards the host's skin surface when they are already in close proximity to it.