Abstract: Objective To investigate the killing effect of hypericin on tachyzoites of Toxoplasma gondii RH strain in vitro. Methods Normal saline （group A） and different concentrations of hypericin （5 μg/ml, group B; 50 μg/ml, group C; 500 μg/ml, group D） were added to T. gondii tachyzoites in 24-well plate（1×106/well）. The tachyzoites were harvested after 2, 4 and 6 h, and underwent the following treatment: trypan blue staining to calculate the dyeing rate, Giemsa staining to observe the morphological and structural alterations of tachyzoites, and transmission electron microscopy to observe the ultrastructure of tachyzoites. In addition, flow cytometry was performed to calculate the survival rate of YFP-carrying Toxoplasma with the same treatment. Results The trypan blue dyeing rate at 2 h after treatment in groups B, C and D was（11.0±3.6）%, （25.0±6.3）% and（40.0±2.7）% respectively, with a significant difference of group D versus B and C （P<0.01）, and groups C and D versus group A ［（6.0±3.0）%）］. The dyeing rate at 4 h and 6 h in group D was（97.0±2.0）% and （98.0±1.7）%, respectively, both significantly higher than that of groups C ［（30.0±7.2）%, （42.7±5.5）%］, B ［（20.0±3.0）%, （34.0±6.6）%］ and A ［（10.0±1.0）%, （19.3±4.9）%］（P<0.01）. Giemsa staining showed gradual end swelling and necrosis of tachyzoites with increased treatment duration and dosage. Transmission electron microscopy showed swelling of worm body, gap between cell membrane and matrix, increase and enlargement of vacuoles inside worm body, disruption of cell membrane, and dissolving of inner structures, with increased treatment duration. Flow cytometry showed significant difference of tachyzoite survival rate at 2, 4 and 6 h after hypericin treatment with that of the control group（P<0.01）. The survival rate of group C at 2 h after hypericin treatment was（7.9±1.9）%, significantly lower than that of groups B ［（38.1±5.5）%］ and A ［（81.8±6.0）%］ （P<0.01）. No tachyzoite was found to survive in group D at 2 h and in group C at 4 h. The survival rate of group B at 4 and 6 h after hypericin treatment was（14.3±7.9）% and （1.4±1.8）%, respectively, both significantly lower than that of group A［（73.8±11.3）% and（64.1±14.4）%, respectively］ （P<0.01）. Conclusion Hypericin has a remarkable killing effect on T. gondii tachyzoites, and the efficacy positively correlates with the dose and treatment duration.

Key words: Hypericin, Toxoplasma gondii, Tachyzoites, In vitro, Effect