Live and killed vaccines against toxoplasmosis in mice

Mice were immunized with live organisms of the different stages (i.e., tachyzoites, bradyzoites, or sporozoites) of Toxoplasma gondii, or with killed tachyzoites with or without adjuvants. The adjuvants used were liposomes, anhydrides of myristic or lauric acid, levamisole and Freund's complete or incomplete adjuvant. The following strains of T. gondii were used: RH, M-7741, the nonpersisting, temperature-sensitive mutants ts-1, ts-4, or ts-5, and the "back mutant" of ts-1 (Pfefferkorn and Pfefferkorn, 1976). The protection afforded was measured by challenge with the pathogenic M-7741 strain. Killed tachyzoites alone, or with adjuvants, offered only slight protection against challenge with M-7741 and no protection against challenge doses that were lethal to all control mice. Chronic infection and live nonpersisting vaccines conveyed a strong immunity to challenge, except strain ts-1. Because it was less pathogenic and did not require chemoprophylaxis, strain ts-4 best fulfilled the requirements for a good vaccine; its effect in hosts other than the mouse remains to be determined. The immunity induced by tachyzoites, bradyzoites, or sporozoites appeared equally strong when challenged with sporozoites.