Experimental toxoplasmosis in mice and rabbits. Virulence and cyst formation of Toxoplasma gondii.

Avirulent Toxoplasma gondii were made virulent by passages in mice, and then attenuated by storage. In contrast to the avirulent parasites, attenuated organisms did not appear to form cysts in mice and rabbits, although parasites could be isolated from such animals as long as three months after their inoculation. Rabbits infected with parasites of attenuated virulence had high antibody levels, and survived challenge with virulent Toxoplasma gondii. Cysts were not detected in the brains of such rabbits following this second infection. By following the generation of Toxoplasma gondii in mouse peritoneal cavities, it was found that attenuated parasites resembled avirulent more than virulent organisms. Although the effect of attenuation was mainly on the parental generation of parasites, some genetic effect seemed to be involved as well.