Distribution of niclosamide spreading oil on water surface and its efficacy against cercariae of Schistosoma japonicum

Objective To investigate the distribution and spreading speed of niclosamide spreading oil,as well as its effect against cercariae of Schistosoma japonicum.Methods The foamed plastic with a diameter of 4 mm served as a buoyage,which was placed at the center of the still water surface.The niclosamide spreading oil was dropped at 0.5 cm from the buoyage,the floating distance of the buoyage was observed,and the spreading speed and area of the niclosamide spreading oil were measured.A cylindrical bucket(at a diameter of 40 cm and height of 50 cm) was filled with de-chlorinated water at a temperature of 25±1 ℃,and then 60 μl of the spreading oil was dropped at the center of the water surface.At 10 cm and 20 cm from the center,1 ml water was sampled at water depths of 10,20,30,40 cm and 50 cm,respectively,and the niclosamide concentrations were determined by using high-performance liquid chromatography in each sample.The niclosamide spreading oil was diluted into solutions at effective concentrations of 1.25 mg/L and 0.63 mg/L with ethanol,and then 10 μl of each solution was added to 24-well plates which contained S.japonicum cercariae to yield the niclosamide concentration of 6.25×10-3 mg/L and 3.13×10-3 mg/L per well,respectively.The survival of the cercariae was observed at different time.Results The spreading speeds and areas were 59,55,71,90,111,122 cm/s and 153 cm/s,and 5.31,5.89,7.07,10.06,12.56,15.20 m2 and 16.61 m2,respectively,while dropping 20,30,40,50,60,70 μl and 80 μl of the niclosamide spreading oil on water surface.The spreading showed an accelerating trend with the increasing dropping volume,and there was a good linear relationship observed between them.In addition,the spreading area also enlarged with the increase in the dropping volume.After dropping 60 μl of the niclosamide spreading oil on water surface,the peak concentration of niclosamide reached 1.27 mg/L on water surface,and remained more than 0.07 mg/L 2 h later.However,the concentration of niclosamide was all lower than 0.04 mg/L at 10 cm under surface or more.Following the treatment with 6.25×10-3 mg/L of niclosamide spreading oil for 1 min,all the cercariae were dead,while the mortality rates of the cercariae were 0,1.39%,13.89%,19.44%,43.06%,69.44% and 79.17% at 1,2,3,5,10,20 min and 30 min,respectively,after the treatment with 3.13×10-3 mg/L of the drug.Conclusions The niclosamide spreading oil is fast to spread and is kept retention for a long time on water surface,and exhibits high activity against S.japonicum cercariae,and it can be used for killing the cercariae on water surface and interrupting the transmission of schistosomiasis in the endemic field.