Objective To study the effects of Omphalia lapidescens and praziquantel on the infectivity and ultrastructure of Spironetra erinacei plerocercoids. Methods The plerocercoids were taken from frogs （Rana nigromaculata）. A total of 168 mice were divided into 21 groups（8 mice per group）， each of them was orally infected with 5 plerocercoids. The mice in group 1-9 were inoculated with plerocercoids cultured in media respectively containing different concentrations of O. lapidescens suspension （20， 40 or 80 mg/ml） for 4， 12 or 24 h， respectively. The mice in group 10-18 were inoculated with plerocercoids cultured in media respectively containing different concentrations of praziquantel（20， 80 or 320 μg/ml） for 4， 12 or 24 h， respectively. The mice in group 19-21 were inoculated with plerocercoids cultured in normal culture fluid for 4， 12 or 24 h， respectively， and served as controls. One week after infection， the mice were sacrificed to collect the plerocercoids. Worm reduction rate was calculated. The ultrastructure changes of plerocercoids were observed under transmission electron microscope（TEM） and scanning electron microscope（SEM）， respectively. Results The average number of plerocercoids detected from mice infected by pleroceroids treated with 40， 80 mg/ml O. lapidescens suspension for 4， 12 or 24 h were 1.6， 1.0， and 0.3； 0.3， 0， and 0， respectively， and significantly lower than that of the infected controls（4.1， 3.5 and 3.3）（P<0.05）； the worm reduction rates were 60.0%， 71.4%， and 90.1%； 92.7%， 100%， and 100%， respectively. The average number of pleroceroids detected from mice infected with pleroceroids treated with 320 μg/ml praziquantel for 4， 12， or 24 h were 1.9， 1.3， and 0.4， and significantly lower than that of the infected controls （P<0.05）； the worm reduction rates were 53.7%， 62.9%， and 87.9%， and lower than that of 20 μg/ml praziquantel group（14.6%， 2.9%， and 6.1%） and 80 μg/ml praziquantel group（24.4%， 17.1%， and 24.2%）（P<0.05）. The ultrastructure of plerocercoids cultured in 20 mg/ml O. lapidescens suspension， 20 or 80 μg/ml praziquantel for 4， 12 or 24 h had no significant difference compared with control groups. The plerocercoids cultured in 40 mg/ml O. lapidescens for 4 h or 320 μg/ml praziquantel for 4 or 12 h， showed mild contracture. The pleroceroids cultured in 40 mg/ml O. lapidescens for 12-24 h showed： agglutinate， fusion， fracture or abscission of microtriches, breakdown of plasma membrane, excretion of calcareous corpuscles, and tegument tissue damages. After cultured in 80 mg/ml of O. lapidescens for 24 h， the tissues of plerocercoid were damaged seriously. After cultured in 320 μg/ml praziquantel for 24 h， the plerocercoids showed： obvious contracture in the anterior end of plerocercoid， edema and bulge of plasma membrane， morphological changes of calcareous corpuscles， increase of secretory granules， glycogen depletion， and chromatin compaction in flame cells. Conclusion The infectivity of Spironetra erinacei plerocercoids decreases along with the time of culture and the increase of drug concentration. Omphalia lapidescens and praziquantel can cause extensive tissue damage to the plerocercoids in vitro， and the effect of O. lapidescens on the infectivity and ultrastructure of plerocercoid is more considerable than that of praziquantel.
