Opisthorchis viverrini: finding and recognition of the fish host by the cercariae.

The cercaria of Opisthorchis viverrini finds and recognizes its fish host by using at least four steps of behavioral patterns. (1) Dispersal and selection of plant-free water microhabitat are achieved by intermittent swimming behavior with positive phototactic orientation. (2) Attachment to the host is stimulated by water currents and a hydrophilic component of fish skin surface which has a molecular weight of more than 30,000. This component is sensitive to digestion with hyaluronidase and seems to be a glycosaminoglycan other than hyaluronic acid and chondroitin sulfate. (3) Remaining on the host's surface is induced by an unknown chemical component of fish skin surface mucus. (4) Penetration into the host is triggered by a hydrophilic component of fish skin surface of a molecular weight of more than 30,000. This host signal has a proteinaceus character as it is sensitive to proteinase digestion but not to hyaluronidase and glycosidases. The requirement of O. viverrini cercariae for complete glycosaminoglycans and proteins as signals in host identification may have the advantage that the numerous small molecules in mud and decaying materials in the water cannot interfere with host-finding.