Objective To compare the susceptibility between the male and female cercariae of praziquantel-susceptible and praziquantel-resistant isolates of Schistosoma mansoni in order to explore the resistance mechanism of Schistosoma to praziquantel. Methods Two isolates of Schistosoma manaso-ni including a praziquantel-susceptible and a praziquantel-resistant isolate were established in the lab.The miracidia were produced by hatching eggs collected from faeces of the infected mice. The single- sex cercaria lines were made by infecting a single Biomphalaria glabrata snail with a single miracidi-urn The sex of single-sex cercariae lines was identified by a direct Wl-specific PCR technique. The single-sex cercariae of 2 isolates were exposed in the solutions of 10-4 ,10-5 ,6 X10-7mol/L and 4X 10-7 mol/L praziquantel respectively. The tail shedding of cercanae was observed un-
der a dissecting microscope for 0, 20, 40, 60, 80 and 100 minutes after adding praziquantel. The numbers of cercariae which had shed tails were countecL The differences of the tail shedding rates between the male and female cercariae of praziquantel-susceptible and praziquantel-resistant isolates were comparecL Results When the cercanae were exposed to the solutions of 10-4 ,10-5 , 6 X 10-7 mol/L and 4 X 10-7 mol/L praziquantel for 100 min, in the praziquantel-susceptible isolate of S. mamasoni, the tail shedding rates of male cercariae were 66. 7%, 75. 8% , 43. 5% and 21. i% , re-
spectively, while the tail shedding rates of female cercariae were 29.3%, 27.9%, 12. 1% and 7. 6%, the tail shedding rates of male cercanae were Hgher than those of the female (all P <0. 05) ; in the praziquantel-resistant isolate, the tail shedding rates of male cercariae were 43. 3% , 39. 4%,
25. 4% and 6. 9%, respectively, while the tail shedding rates of female cercariae were 47. o%, 38. 9%, 26. 3% and 6. 3%, respectively, and there were no differences of the tail shedding rates between the male and the female (all P>O. 05). When the cercariae were exposed to the solutions of
10-4 ,10-S , 6 X 10-7 mol/L and 4 X 10-'r moI/L praziquantel for 80 min, the tail shedding rates of male cercariae of the praziquantel-susceptible isolate were 54. 4% , 68. 6%, 42. 1% and 16. 1% , respectively, while those of male cercariae of the praziquantel-resistant isolate were 30. 2% , 34. 4%,
20. 1% and 2. 8%, raspectively, and the tail shedding rates in the praziquantel-susceptible isolate were higher than those in the praziquantel-resistant isolate (all P<O. 05). Conclusions There is asignificant difference of the susceptibility between the male and female cercariae of S. mansoni, andthe male cercariae of S. mansoni are more susceptible to praziquantel than the female cercariae. It is considered that the resistance of S. mamasoni to praziquantel is induced by the decrease of the susceptibility of male worms to praziquantel
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