Killing of different strains of Toxoplasma gondii tissue cysts by irradiation under defined conditions.

To study the effect of gamma-irradiation on the viability of Toxoplasma gondii tissue cysts, brains of mice inoculated with 95 newly isolated strains of T. gondii from pigs and 10 other laboratory isolates were pooled, flattened, packed in vacuum, and irradiated to absorbed doses of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, and 0.9 kGy at 5 C (+/- 0.5 C) with a 137Cs gamma-radiation source. Treated samples were bioassayed for viable T. gondii in mice, cats, or both. Tissue cysts of all strains were rendered nonviable at 0.4 kGy. To study the effect of temperature on inactivation by irradiation, tissue cysts were irradiated at -4, 0, 4, 8, 12, and 16 C (+/- 0.5 C) at 0.25 kGy. Temperature during irradiation had no marked effect on the viability of tissue cysts.