SummaryMalaria infection crisis, at which the parasitemia drops precipitously and the parasite loses infectivity to the mosquito vector, occurs in many natural malaria systems, and has not been explained. We demonstrate that in a simian malaria parasite (Plasmodium cynomolgi in its natural host, the toque monkey), 'the loss of infectivity during crisis is due to the death of circulating intraerythrocytic gametocytes mediated by crisis serum . These parasite-killing effects in crisis serum are due to the presence in the serum of cytokines tumor necrosis factor and interferon y, which are produced by the host as a result of the malaria infection . The killing activity of each cytokine is absolutely dependent upon the presence of additional, as yet unidentified factor(s) in the crisis serum . nfectivity of malarial infections to mosquitoes is due to the presence of circulating gametocytes, the sexual stages that fertilize in the midgut o£ a blood-fed mosquito. In several malaria host-parasite systems, it has been noted that peaks of parasitemia are associated with "crisis" in the infection (1-3) and the simultaneous reduction or loss of infectivity of the parasite to mosquitoes (4, 5); neither of these phenomena has been explained . During a blood infection of Plasmodium cynomolgi in its natural host, the toque monkey, Macaca sinica, peak parasitemia is often accompanied by a crisis in the infection; this is most pronounced in splenectomized animals and is characterized by the appearance of morphologically abnormal intraerythrocytic parasites. At crisis, there is a sudden loss of infectivity of the parasites to mosquitoes, which persists for 5 to 7 days. We show here that the loss of infectivity at crisis is due to death of circulating intraerythrocytic gametocytes mediated by crisis serum . These killing effects in the crisis serum are due to the presence of the cytokines TNF and IFN-y ; the killing activity of each cytokine is absolutely dependent upon the presence of additional, as yet unidentified, factor(s) in the crisis serum . Materials and MethodsAnimals. The toque monkey, Macaca sinica, the natural host ofP. cynomolgi ceylonensis, was used in this study. Wild-caught adult animals of either sex, weighing between 1 and 4 kg, which were free of malaria infections as determined by the indirect immunofluorescence test, were used for experiments. Splenectomies were performed using standard sterile techniques as previously described (6) .
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