Resistance to avirulent Toxoplasma gondii in normal and vaccinated rats.

Investigation of the innate resistance to infection with Toxoplasma gondii showed that the Toxoplasma-hostile factor present in rats has an important lytic effect on the parasites but is unable to prevent an infection alone. Vaccination experiments showed that innate resistance was also related to specific immunity, which appeared faster in rats than in rodents with less innate resistance. Complete resistance to toxoplasmosis was only present in rats vaccinated with live parasites. In rats vaccinated with killed parasites, avirulent parasites were able to encyst themselves almost as readily as in unvaccinated rats.