RELATIONSHIPS OF MALARIA MORBIDITY WITH EXPOSURE TO PLASMODIUM-FALCIPARUM IN YOUNG-CHILDREN IN A HIGHLY ENDEMIC AREA

To study incidence of clinical Plasmodium falciparum malaria in relati on to exposure to parasites, attendance of children less than eighteen months old at a village dispensary in a highly endemic area of Tanzan ia was recorded. Entomologic inoculation rates (EIRs), estimated as a function of time period and place of residence, exceeded one sporozoit e positive bite per adult per night in some village neighborhoods duri ng the wet season. Incidence of clinical P. falciparum malaria, define d either as fever with parasitemia or as fever with hyperparasitemia, increased with the EIR over the whole range of exposures. Each 10-fold increase in the EIR corresponded to a 1.6-fold increase in incidence of fever plus parasitemia (95% confidence interval = 1.4-2.0). Therefo re reduction of human-vector contacts will probably reduce morbidity i ncidence even at very high exposures. Incidence showed little relation ship to estimated cumulative numbers of inoculations since birth, but decreased steeply with estimated cu mulative time infected with tropho zoites. This suggests that clinical immunity depends mainly on the ext ent of exposure to blood-stage antigens, not on the diversity of inocu la seen, and thus temporary reductions in human-vector contacts are un likely to result in subsequent increases in morbidity.