Fading of malaria immunity in mice.

Fading of immunity in Swiss and C3H/StZ mice was progressive, and related to both, the interval elapsed since the last challenge infection and survival of parasites. In both mouse strains the proportion of mice that remained immune to challenge decreased with increasing fading periods. Moreover, a shift from delayed mortality to a normal course of infection as seen in non-immune controls was observed in C3H/StZ mice. Early parameters of fading were increasing peak parasitaemias after challenge in an increasing proportion of mice, and lethal infections instead of transient parasitaemias. Fading was also reflected by changes in the host cells preferentially infected early after reinfection: with increasing fading periods host cell preference shifted from predominantly polychromatophilic erythrocytes to mixed infections and finally to predominantly oxyphilic cells, especially in Swiss mice. The results of isodiagnosis indicated a positive correlation between persistence of parasites and immunity; whereas the absence of parasites was related to various phases of a fading immune response.