Experimental Toxoplasma Infection in Mice with Strains Producing Oocysts

Abstract

The pathogenicity and infectivity of oocysts, cysts, and trophozoites of the M-7741 strain of toxoplasma were studied in mice after oral and parenteral inoculations. The intraperitoneal and the subcutaneous routes were most sensitive for the demonstration of oocysts in the feces or of cysts and trophozoites in tissues of infected animals. After parenteral inoculation, cysts and oocysts were more pathogenic than a comparable dose of trophozoites. Oocysts were more pathogenic after oral than after parenteral administration. Oocysts of the 4 strains studied varied considerably in their pathogenicity. One to 10 oocysts of the M-7741 and the Aldrin strains killed orally inoculated mice, whereas up to 1,000 oocysts of the BWM and the S-1 strains gave rise to inapparent infections. About 10% of sporulated oocysts fed to mice passed through the gut without excysting and were infectious to cagemates. Sporulated oocysts remained infective for 4 days in dry bedding, for at least 7 days in moist bedding, for 7 months in 2% sulfuric acid at room temperature, and up to 18 months in 2% sulfuric acid at 4 C. Precautionary methods to avoid human infections while working with oocysts are discussed. Toxoplasmosis could be precisely diagnosed by seroconversion 16 days after inoculation of mice. The biology of cystand trophozoite-induced toxoplasma infections in mice has been studied for 30 years, but little is known about oocystinduced infections. We will present comparative data on the infectivity and pathogenicity of cysts, trophozoites, and oocysts of the M-7741 strain introduced by the oral, subcutaneous, and intraperitoneal routes. Strains of toxoplasma vary markedly in their virulence to mice. Most of the strains isolated from nature are of low virulence and give rise to subclinical infections; to make a diagnosis, the mice (or other hosts) must be tested serologically. It is important, therefore, to determine how quickly diagnostic antibodies develop in mice inoculated with different strains of toxoplasma. We will present data on the development of antibody in mice infected with oocysts of four recently isolated strains of toxoplasma, to compare serological results with morphologic diagnosis based on presence of