Toxoplasma gondii: partial cross-protection among several strains of the parasite against congenital transmission in a rat model

Rats were immunized with cysts of two Toxoplasma strains or with RH strain tachyzoites prior to pregnancy. The litters of the 13 rats that received homologous challenges with cysts during pregnancy, were all protected, whereas of 173 rats that received heterologous challenges with cysts or oocysts, only 21 protected their litters. 38.3 and 17% of rats immunized with the RH and with complete strains respectively, and 57% of control rats challenged with cysts, transmitted the infection congenitally. The percentages when similar groups were challenged with oocysts, were 33.3, 48.2, and 56.2%, respectively. Immunization with cysts did not completely protect against challenge with oocysts, even if the same strain was used. The divergence of these results from the complete protection against congenital toxoplasmosis observed in immune women and ewes, might be due to the use of excessive challenge doses in the model.