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Schistosoma mansoni: interference with cercarial host-finding by various aquatic organisms

ABSTRACT Schistosoma mansoni cercariae labelled with 75Se-methionine were used to study host-finding capacity by determining the radioactivity of exposed mouse “target” tails. Possible interfering effects of some aquatic vertebrates and invertebrates, plants and algae and secretions and excretions of aquatic organisms were examined. A marked interferent effect due to predatory behaviour was produced by Cyclops strenuus (Copepoda), Daphnia pulex and D. longispina (Cladocera), Notodromas monacha and Cypria ophthalmica (Ostracoda) and Lebistes reticulatus (guppy). Bufo bufo (Amphibia) and Planaria lugubris (Turbellaria) interfered significantly, presumably by secreting cercaricidal toxins. Several other organisms apparently produced no interference. The interferent effect of the plant species Ceratophyllum demersum, Elodea canadensis, Lemna minor, Ranunculus aquaticus, Holcus lanatus (grass) and Botryococcus braunii was particularly pronounced when they were confined to the surface layer of the water. Several other species did not interfere with host-finding.