Abstract: 【Objective】In order to provide a basis for study the pathogenic characteristic of Toxoplasma gondii (T. gondii), ME 49 strain of T. gondii tachyzoites were explored to understand its virulence.【Method】In this study, the tachyzoites of ME 49 strain T. gondii were diluted with the concentration of ＜10⁰, 10⁰-10⁶ and inoculated to Kunming mice by intraperitoneal injection. The tachyzoites in lung and mesentery lymph nodes of mice were checked, and the cysts of T. gondii in brain were numbered. Besides, the paraffin sections in the lungs were stained with HE and IHC. The results of smear and MAT were used to analyze the infection rate and survival rate of mice.【Result】0 DPI-60 DPI, the result of T. gondii IgG antibody by MAT showed that the seroprevalence of mice with ＜10⁰, 10⁰, 10¹ and ≥10² were 0, 20%, 60% and 100%, respectively. Three mice with 10⁴ tachyzoites died at 12 DPI, 16 DPI and 19 DPI, respectively. Two mice died at 11 DPI and one mouse died at 8 DPI with 10⁵ tachyzoites. Two mice with 10⁶ tachyzoites died at 9 DPI, three mice died at 7 DPI, 8 DPI and 10 DPI, respectively, and the survival rate of mice infected with T. gondii was 62.07% (18/29). The tachyzoites of lungs and mesentery lymph nodes of dead mice were found by microscopy, and the cysts and antigen of T. gondii were observed in the lung by HE and IHC. The cysts in brain of T. gondii infection-positive mice with ＜10⁰, and 10⁰ tachyzoites were not found, the cysts in brain of T. gondii positive mice with 10¹ tachyzoite were 60, 100 and 0, respectively, 10² tachyzoites of that were 20, 40, 60, and 0, respectively, 60 DPI-600 DPI, three mice died with 10³ tachyzoites at 184 DPI, 435 DPI and 569 DPI, respectively, but no cysts of T. gondii were detected in the brains of mice. One mouse died at 188 DPI with the 10⁵ tachyzoites and the cysts was 20, while the survived time of remaining mice was 590 DPI.【Conclusion】In this study, the pathogenicity of ME 49 strain of T. gondii tachyzoites in Kunming mice was explored. 100% infection rate for mice was ≥10² T. gondii tachyzoites, the 100% lethal dose was 10⁶ tachyzoites, the time of death in infected mice was 7 DPI-19 DPI, the brain cysts was 0-53 per mouse, and the longest survival time was 590 DPI. The T. gondii tachyzoites of ME 49 strain had low pathogenicity to Kunming mice, and the cysts could be formed in brain, and only cause death of mice at high concentration of tachyzoites.

Key words: Toxoplasma gondii, ME 49 strain, tachyzoites, Kunming mice, pathogenicity