Attachment, the first phase of host identification by Schistosoma japonicum cercariae, can occur in 2 different ways. Cercariae clinging to the water surface simply swing around and transfer to the host skin. Free-swimming cercariae behave like S. mansoni: upon touching a substrate, they switch from tailward to forward movement, swim in an arc, and attach to it with the penetration organ. Neither type of attachment is influenced by chemical, thermal, or specific mechanical stimuli from the host. The second phase, remaining on the host, requires a solid hydrophobic surface and seems to depend only on the cercaria's ability to cling to it. This phase is not influenced by chemical or thermal stimuli. The third phase, creeping across the host surface, is independent of chemical and mechanical stimuli. Cercariae migrate in thermal gradients to a preferred temperature of 37 +/- 3 C and then attempt to penetrate. Penetration, the fourth phase, was evoked by human skin surface lipids. The free fatty acid (FA) fraction was identified as exclusively stimulating components. Saturated FA's were effective at chain lengths between 10 and 14 carbon atoms (pH 7.0), and unsaturated FA's were effective at longer chains and their activity increased with increasing number of double bonds. Dog skin surface components did not stimulate cercarial penetration, which can be attributed to the lack of free FA's. A temperature of 32-40 C also stimulated penetration responses, which might be the main stimulus in animal hosts, whose skin surfaces contain no or only a few free FA's. FA's and heat evoked a transformation of cercarial tegument simultaneous with penetration behavior, making the organisms osmotically sensitive. The host identification of S. japonicum cercariae is very nonspecific compared with the differentiated host recognition of S. mansoni.
The cercaria of Schistosoma spindale finds and identifies its bovine host with at least five behavioral phases. (1) Dispersal in and selection of midwater and water surface as the microhabitat are achieved by an intermittent swimming behavior with a weak geonegative but not photopositive orientation. (2) Attachments are stimulated by host-specific higher temperatures of the substrate but not by chemical host signals. (3) Remaining of the attached cercariae on the substrate is stimulated by host-specific higher temperatures of the substrate; chemical host signals have no effect. (4) The creeping of the cercariae is directed to the higher temperature in thermal gradients as weak as 0.07 degrees C/mm. Chemical gradients had no effect on the creeping direction. This behavior may enable the cercariae to migrate along hairs to the host's skin surface. (5) Penetrations are stimulated by the free fatty acid fraction of bovine skin-surface lipids. The characteristics of the stimulating fatty acids are the same as those identified for other schistosome species. Higher temperatures of the substrate alone do not stimulate penetrations. S. spindale cercariae do not use as many chemical host cues as stimuli for the identification of their host as do S. mansoni cercariae. S. spindale seems to be adapted to hairy hosts that are infected in shallow, muddy waters. The low host specificity of the cercarial host-finding behavior is compensated by an intimate parasite-snail intermediate host relationship, resulting in a high cercarial production of up to greater than 7,000 cercariae per snail per day.
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