Active immunization of rats with a cell-free extract of the erythrocytic parasites of Plasmodium berghei

Weanling rats were vaccinated with a nonliving product prepared by grinding erythrocytic forms of Plasmodium berghei, liberated from their red-cell hosts by treatment with saponin. Vaccinated and control rats were then exposed to viable P. berghei. Infections in vaccinated rats were milder than in controls in that the prepatent period was lengthened, the patent period was curtailed, the peak parasitemias and cumulative parasite counts were reduced, and the mortality rates fell. The relation of these results to the vaccination of other hosts to their homologous plasmodia is discussed.