Finding and recognition of the snail intermediate hosts by 3 species of echinostome cercariae

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Abstract
SUMMARY: Finding and recognition of snail second intermediate hosts was studied in cercariae of 3 echinostome species. The cercariae of the 3 species accumulated in snail-conditioned water (SCW) with 2 types of orientation mechanisms and responded to different small molecular weight (< 500 Da) components of SCW. Pseudechinoparyphium echinatum and Echinostoma revolutum cercariae returned by swimming an arc, when swimming in decreasing concentration gradients of SCW (turnback swimming). The stimulating cues of SCW were identified as hydrophilic organic molecules, probably posessing amino groups. Amino acids contributed to the attractivity of SCW, at least in P. echinatum, but they could not account for the complete attractivity of SCW. Hypoderaeum conoideum were directed chemotactically and swam along increasing concentration gradients of small peptides within SCW, but in decreasing SCW gradients they showed no turn-back swimming. Chemotactic orientation in H. conoideum only started 1 h after emission, which may assist the cercariae to leave the immediate area of their first intermediate host snails and to disperse. Attachments occurred specifically to snail hosts in the 3 species and were stimulated by macromolecular mucus compounds, probably mainly by viscoelastic properties of the mucus. The results of this study show, that host-finding mechanisms and the stimulating host cues of snail invading echinostome cercariae differ considerably from those of schistosome miracidia.