Effects of copper sulfate toxicity on cercariae and metacercariae of Echinostoma caproni and Echinostoma trivolvis and on the survival of Biomphalaria glabrata snails

Copper in the form of copper sulfate (CuSO4) decreases the survival of Biomphalaria glabrata snails, but the effects of this molluscicide on Echinostoma caproni and Echinostoma trivolvis, 2 species of digeneans that use B. glabrata as intermediate hosts, are not known. Studies were done on the effects of various concentrations of CuSO4 in artificial spring water (ASW) on the survival and infectivity of E. caproni and E. trivolvis cercariae. Solutions containing 1.0, 0.1, and 0.01% CuSO4 were 100% lethal within 2 hr of exposure for both species. Time to 50% mortality in 0.001% CuSO4 was 8 hr for E. caproni and 16 hr for E. trivolvis; at 24 hr, the controls showed 50 and 65% mortality, respectively. Treatment of cercariae of both species for 0.5 hr in 0.001% CuSO4 had no effect on the ability of cercariae to form normal cysts in juvenile B. glabrata snails. However, treatment with 0.01% CuSO4 for 0.5 hr caused a significant reduction in the ability of cercariae of both species to encyst in snails. Treatment of encysted metacercariae of both species in 0.001% CuSO4 for I hr had no effect on subsequent excystation of these echinostomes in a trypsin-bile salts medium, whereas concentrations of 1.0, 0.1, and 0.01% CuSO4 and 1.0 and 0.1% CuSO4 decreased chemical excystation of E. caproni and E. trivolvis cysts, respectively. Survival studies on the effects of CuSO4 in Locke's solution on chemically excysted metacercariae of both species were also done. Excysted metacercariae of both species were killed by 2 hr in either 0.1 or 0.01% CuSO4 in Locke's solution. However, time to 50% mortality for both species of excysted metacercariae in 0.001% CuSO4 was approximately 5 hr. Time to 50% mortality for the controls was about 12 hr. Survival of juvenile B. glabrata snails was also examined. All B. glabrata snails were dead by 6 hr in 1 and 0.1% CuSO4 in ASW. Biomphalaria glabrata snails showed 50% mortality by about 6 hr in 0.01% CuSO4 and about 80% were still alive at 24 hr in 0.001% CuSO4. All controls were alive at 24 hr, at which time the experiment was terminated. Concentrations greater than 0.001% CuSO4 increased snail mortality, as well as that of the cercariae and excysted metacercariae of E. caproni and E. trivolvis. Our findings suggest that concentrations of copper sufficient to eliminate juvenile B. glabratta snails are also sufficient to kill the cercariae and excysted metacercariae of these digeneans but not the encysted metacercariae, which may be protected by their cyst walls.