Immunization of NMRI mice against virulent Toxoplasma gondii. Differing efficacy of eleven cyst-forming Toxoplasma strains.

Mice were infected with eleven cyst-forming Toxoplasma strains of varying virulence and challenged 1 month later with highly virulent BK strain parasites. The early cellular reaction in vivo was estimated by collecting the peritoneal exudates 24 h after challenge. This consisted of 54.34--77.61% lymphocytes and 0-8.88% infected macrophages in the eleven immunized groups in contrast to 27.17% lymphocytes and 18.64% infected macrophages in the control group. The peritoneal exudate 72 h after challenge comprised of 55.88-73.86% lymphocytes and 0-6.97% infected macrophages compared to 25.88% lymphocytes and 92% infected macrophages in the control group. Following the virulent challenge, deaths occurred in seven of the eleven groups immunized with live strains and ranged 5-42% at the end of a 6-week observation period. The most virulent of the Toxoplasma strains used - Alt and Gail - gave higher mortality while the least virulent ones - 558, 1070, K8 and KSU - were solidly resistant. The significantly different mortality obtained with strains Alt, Gail and Witting in a similar experiment performed at an interval of 7 months is discussed.