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.
1Zoonotic Diseases Laboratory.