Abstract:  AIM: To study the anti Toxoplasma effect of usnic acid in vitroand its mechanism . METHOD: After usnic acid (final concentration 5-50 μg/ml) was added into the ascites of experimentally infected mice for 30 min-4 h, the effect was observed under light microscope and transmission electron microscope. Spiramycin was used as control. In addition, mice were inoculated with the drug treated tachyzoites. RESULTS: Under the action of 10 μg/ml usnic acid for 30 min, the rhoptries lysed and the daughter cell membrane and membranate structures of tachyzo ites appeared broken. The tachyzoites treated by 50 μg/ml usnic acid for 4 h could not be recovered in the inoculated mice. The toxop lasmacidal effect of spiramycin was inferior to that of usnic acid at the same concentration. CONCLUSION: Usnic acid could kill Toxoplasma gondii tachyzoites in vitro. The penetration and multiplication of the tachyzoites could be inhibited by usnic acid. The toxoplasmacidal activity of the drug has positive correlation with the concentration and the time of action of usnic acid. The toxoplasmacidal effect of usnic acid was proved to be more remarkable than that of spiramycin.

Key words: Usnic acid, spiramycin, Toxoplasma gondii, tachyzoite, ultrastructure