During infection of mice with a virulent Y strain of Trypanosoma cruzi, macrophages

By sequentially and histologically studying the evolution of a virulent Y strain infection in young mice, two striking features were observed: a) the progressive increase of macrophage parasitism in the spleen followed by a massive destruction of both macrophages and parasites at the height of infection, which coincided with a sudden drop in parasitemia and which often preceded animal death; b) when mounting splenic parasitism was abrogated by chemotherapy, infected animals survived for prolonged periods of time, splenic parasitism completely disappearing, while chronic infection followed its course. Thus, the dramatic and massive parasite destruction represents a transition from susceptibility to immunity. Although several well-conducted sequential histological studies of the spleen in acute T. cruzi infection with the Y strain have been made, most of the characteristics of this transitional phase here considered have been overlooked 5 9 .