Effect of Freezing on Infectivity of Toxoplasma Gondii Tissue Cysts in Pork

To study the effect of low temperatures on infectivity of Toxoplasma gondii tissue cysts, pork from infected pigs was mixed with infected mouse brains and homogenized thoroughly. Twenty-gram samples of infected homogenized meat which were sealed in plastic pouches, pressed to a uniform thickness resulting in samples having the dimensions of ~.2 × 16 × 18 cm, were subjected to temperatures of -1 to -171.1 °C for 1 s to 67.2 d. Treated samples were digested in HCl-pepsin solution and infectivity assayed in mice. A regression model from these data is described by the least squares linear regression: Square root of time for the inactivation of T. gondii (h) = 26.72 + 2.16 temperature (°C) with r = 0.77. T. gondii tissue cysts remained viable usually up to 22.4 d at -1 and -3.9°C and 11.2 d at -6.7°C but were usually rendered nonviable by freezing at -12.37°C. These data demonstrate that T. gondii tissue cysts are inactivated by freezing more readily than encysted Trichinella spiralis larvae.