To study the effect of high temperature 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 were sealed in plastic pouches, pressed to a uniform thickness of 2 mm, and subjected to water-bath temperatures of 49, 52, 55, 58, 61, 64, and 67 C for 0.01, 3, 6, 9, 12, 24, 48, and 96 min. Treated samples were digested in HCl-pepsin solution and bioassayed in mice. Toxoplasma gondii tissue cysts remained viable at 52 C for 9.5 min but not for 9.5 min at 58 C; tissue cysts were generally rendered nonviable by heating to 61 C or higher temperature for 3.6 min. Tissue cysts survived once at 64 C for 3 min. These data demonstrate that T. gondii tissue cysts are less heat resistant than encysted Trichinella spiralis larvae.

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.

Serum samples from 11,842 commercial pigs killed in 1983-1984 throughout the United States were tested for anti-Toxoplasma gondii antibodies by the agglutination test in dilutions of 1:25, 1:50, and 1:500. Anti-T. gondii antibodies were found in 23.9% of pigs. At dilutions of 1:25, 1:50, and 1:500, 13.5%, 6.9%, and 3.5% were serologically positive, respectively. The prevalence of anti-T. gondii antibodies was higher in breeder pigs (42%) than in market pigs (23%). These results indicate that anti-T. gondii antibodies are widespread in the national swine herd.