Navigation within host tissues: cercariae orientate towards dark after penetration.

The survival of skin penetrating cercariae depends on information on the direction to move toward deeper layers in the epidermis (the direction of further migration) and toward the surface (direction which must be avoided when migrating). We tested the hypothesis that parasites can use their photo-sensitivity for orientation away from the light-exposed skin surface towards darker locations. Cercariae of species invading humans ( Schistosoma mansoni), birds ( Trichobilharzia ocellata) and fish ( Diplostomum spathaceum) oriented towards light sources when free swimming in cuvettes. However, they shifted to a negative photo-orientation when migrating in agar substrates after penetration and transformation to schistosomula. This is a first hint that parasites may use photo-orientation when they navigate in host tissues.