Resolution of acute malaria (Plasmodium berghei in the rat): Reversibility and spleen dependence

Six-week-old rats infected with Plasmodium berghei developed a peak parasitemia of 55.2 ± 3.1% by day 15 of infection, followed by spontaneous resolution of the infection during a process referred to as crisis. Crisis was accompanied by the appearance in circulation of infected erythrocytes in which the parasites appeared abnormal ('crisis forms'). Rats splenectomized at different times during the crisis period experienced a sudden increase in parasitemia, with a marked decrease in the number of circulating crisis forms. Splenectomy resulted in a 100% mortality, whereas all control and sham-operated rats survived their infection. Although P. berghei is restricted to developing within young erythrocytes, the observations could not be explained by the effects of splenectomy on the number of circulating reticulocytes. Indeed, the reticulocytosis which accompanies crisis was unaffected by splenectomy. The observations therefore suggest that crisis is a reversible process and, specifically, that the spleen is necessary for its maintenance.