Transport of Schistosoma japonicum cercariae and the feasibility of niclosamide for cercariae control.

Field experiments were conducted in southwestern China to estimate the distances over which cercariae of Schistosoma japonicum are transported and remain infective in village irrigation ditches and to determine the feasibility of interrupting this transport using niclosamide, the molluscicide commonly used for snail control in this environment. The number of worms recovered from bioassay mice downstream from experimentally introduced cercariae, but without niclosamide, decreased about 90% over the first 100 m of ditch and then remained essentially constant for the remaining 300 m of the ditch that was monitored. The niclosamide experiments resulted first in the finding that it was possible to achieve ditch average concentrations close to the target values of 0.25 ppm, but that concentrations at any particular location varied by a factor of up to 3. There was no correlation between water turbidity and niclosamide concentration. An average water concentration of 0.29 ppm resulted in no viable cercariae after a contact time of about 1 min and a travel distance of only 10 m. We regard these results as proof of the feasibility of chemical interruption of transport of S. japonicum cercariae although the toxicity of niclosamide to fish at the levels used here make it a questionable candidate for routine use.