Abstract: 【Abstract】  Objective  To observe the ultrastructural alterations of adult Schistosoma japonicum induced by synthetic trioxolane OZ78.  Methods  Eight out of ten mice infected with 40-60 S. japonicum cercariae for 35 d were treated orally with OZ78 at a single dose of 400 mg/kg. Four groups of two mice were killed at 24 h, 3 d, 7 d, and 14 d post treatment, and schistosomes were recovered by perfusion technique, fixed, and examined by transmission electron microscopy. Schistosomes obtained from the remaining two untreated mice served as control.  Results  After infected mice were treated with OZ78 for 24 h, the prominent alterations in tegument of both male and female worms were observed, which revealed in flattened surface due to swelling of cytoplasmic processes, irregular expansion in distal end of cytoplasmic processes accompanied by decrease in rod-like and discoid-like secretory bodies, focal lysis of tegumental matrix; fusion of some cytoplasmic processes to form a large piece, disruption or disappearance of basal membrane, and destruction of internal structures in sensory organelles. In the subtegument, no or slight swelling and focal lysis of muscle bundles were seen, while the syncytium beneath the muscle showed enlargement of nucleus with indistinction of partial nuclear membrane, formation of small vacuoles due to focal lysis of chromatin, and emergence of degenerated mitochondria in perinuclear cytoplasm. As to parenchymal tissues, the major alterations included degeneration of mitochondria, formation of some small vacuoles and myelin-like structures. In gut epithelial cells, the prominent alterations were irregular enlargement of nucleus with light lysis of nucleoli and fusion of partial bi-layer nuclear membrane, degeneration of mitochondria in cytoplasm and collapse of microvilli. At this time point, in the vitelline cells of female worms, the most significant alteration was the collapse of many vitelline droplets, which led to release of the vitelline balls, followed by their lysis and fusion. Three to 7 d post treatment, the damage to the worms aggravated either in extent or in severity along with time. The significant damages to male and female worms were fusion of cytoplasmic processes, peeling or collapse of damaged cytoplasmic processes resulting in exposure of muscle bundles, severe destruction of sensory organelles and syncytium, focal or extensive swelling and lysis of muscle bundles, emergence of some larger piece of degenerated parenchymal tissues and severe damage to the gut epithelial cell. While in the vitelline cells of female worms, decrease in the number of vitelline droplet, focal lysis of nucleus and extensive lysis of parenchymal tissues among the vitelline cells were also observed. Fourteen days post OZ78 dosing, male and female worms which survived the treatment showed some renovation in damaged tegument and subtegument, while most gut epithelial cells and vitelline cells still revealed in prominent injury.  Conclusion  The results demonstrate that OZ78 possesses an extensive damage to the ultrastructure in tegument and subtegument tissues including syncytium, parenchymal tissues, gut epithelial cells, and vitelline cells of adult S. japonicum.

Key words: Synthetic trioxolane, OZ78, Schistosoma japonicum, Ultrastructural alteration, Transmis-sion electron microscopy