Chemoattraction and penetration of Echinostoma trivolvis and E-caproni cercariae in the presence of Biomphalaria glabrata, Helisoma trivolvis, and Lymnaea elodes dialysate

A petri-dish bioassay was used to study the chemoattraction and penetration of the cercariae of Echinostoma trivolvis and E. caproni in the presence of snail dialysates from Helisoma trivolvis (Pennsylvania and Colorado strains), Biomphalaria glabrata, and Lymnaea elodes. Significant chemoattraction was seen with E. trivolvis cercariae in the presence of all snail dialysates released from nonperforated dialysis sacs with a molecular-weight exclusion of 12,000. Under the same conditions, E. caproni was significantly attracted to B. glabrata and H. trivolvis (CO strain) but not to L. elodes or H. trivolvis (PA strain). Dialysis sacs were perforated with needles to allow the release of snail substances of all molecular weights into the bioassay. Cercariae of both species were significantly attracted to all snail dialysates released from perforated sacs. Moreover, cercariae entered these sacs and penetrated the snails, and 24 h later the percentage of cysts per snail species ranged from 70% to 83% for E. trivolvis and from 73% to 93% for E. caproni. Dialysates released from intact sacs were extracted in choloroform-methanol (2:1) to obtain hydrophilic and lipophilic fractions. When these extracts were placed on agar plugs in the bioassay, the lipophilic fraction, but not the hydrophilic fraction, was mainly chemoattractive.