Toxoplasma gondii: prototype immunization of lambs against formation of muscle and brain cysts.

Nine lambs were immunized with 10(6) oocysts of the relatively avirulent ME-49 strain of Toxoplasma gondii, and were challenged 45 days later with 4 x 10(6) oocysts of the M3 strain of this parasite. Less than 5 tissue cysts of the M3 strain formed per 8 g of muscle - if any - judging from a bioassay in mice in which all mice survived. In contrast, mortality occurred in mice fed muscle specimens from 4 lambs that had not been immunized with ME-49 prior to challenge with the M3 strain. This finding suggests the possibility of creating a vaccine for lambs that could diminish or abolish lamb and mutton as sources of human infection with T. gondii, and which could also prevent toxoplasmosis abortion in sheep.