Objective To establish a test in vitro to monitor the susceptibility of field isolates of Schistosoma japonicum in stages of eggs, miracidia and cercariae to praziquantel. Methods Six isolates of S. japonicum were collected from the endemic regions in Hunan, Hubei, Jiangxi, Anhui, Jiangsu and Yunan provinces, Cluna and 3 isolates of S. mansoni established in the lab were as the control. The eggs were incubated in 5×10-6、10-6、5×10-7、l0-7mol／Lmol/L praziquantel for 24 hours, then the eggs were transferred to fresh water to hatch miracidia and the hatch rates were investigated and compared. The miracidia were exposed t05×10-6、 10-6、5×10-7、10-7 mol/L praziquantel for 0, 1 and 5 min and then the swimming behavior and morphological changes were observed. Thecercariae were exposed in 10-5、6×10-7，4×10-7、10-7mol/L praziquantel for 0, 20, 40, 60, 80,100 min and then the changes in the patterns of behavior, including swimming, contraction and tail shedding were observed under a dissecting microscope. The number of cercariae which had shed tails were counted. The differences between S. jaPonicum and S. mansoni were compared. Results Following the incubation in 10-6, 5X10-7, 10-7 mol/L praziquantel solutions for 24 h, the hatching rates of the eggs of S. japonicum were 0. 52%, 11. 90% and 49.15%, respectively, while thehatching rates of the eggs of S. mansoni which were 4.17%, 31.37% and 92.53%. When the miracidia were exposed t0 10-6 mol/L praziquante for l min ,100% of miracidia from S. japonicum changed their shape, while only 55. 73% of miracidia from S.mansoni isolates changed their shape. When the miracidia were exposed t0 5 Xl0-7, 10-7mollL praziquantel, respectively, for 5 min,96. 75% and 37. 57% of miracidia from S. japonicum changed their shapes while only 21. 80% and 0
of miracidia from S.mansoniisolates changed their shapes. When the cercariae were exposed t0 10-5
mol/L praziquantel over 40 min, 96. 75% of the cercariae from S. japonicum isolates shed their
tails, while only 28. 30% of the cercariae from S. mansoni isolates shed their tails. When the cer-
cariae were exposed t0 4X10-7 mol/L praziquantel over 100 min, 95. 82% of the cercariae from S.
japonicum isolates shed their tails, while only 11. 40% of the cercariae from S. mansoni isolates shed
their tails. When the cercariae were exposed t0 10-7 mollL praziquantel over 80 min, 29. 65% of
the cercariae from S. japonicum isolates shed their tails, while no cercariae from S. mansoni isolates
shed the tails. Conclusions There were no difference in responses to praziquantel at the egg,
miracidial and cercarial stages among S. japoniczcm isolates, but the in. vitro responses to praziquan-
tel of eggs, miracidia and cercariae of S. japonicum compared with S. mansoni demonstrate that
eggs. miracidia and cercariae of S. japonicum are more sensitive to praziquantel than those of S.
mansoni. The percentage of the changes in the shape of miracidia from S. japonicum isolates follow-
ing the exposure t0 5 Xl0-7mol/L praziquantel for 1 minute may be used to determine whether the
failed therapy in patients infected with S. japonicum is due to the presence of praziquantel unsuscep-
tible worms. The tail shedding rates of cercariae of following the exposure t0 4X10-7 mol/L prazi-
quantel for 80-100 minutes could be used to monitor for the presence of praziquantel-resistant
worms in infected snails collected from the field.
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