EVIDENCE FOR AN AGE-DEPENDENT PYROGENIC THRESHOLD OF PLASMODIUM-FALCIPARUM PARASITEMIA IN HIGHLY ENDEMIC POPULATIONS

The high prevalence of asymptomatic malaria infections and the nonspec ific signs and symptoms of the disease make the individual diagnosis o f clinical malaria uncertain in highly endemic areas. Longitudinal dat a obtained during a four-month period from a daily survey of 200 perma nent inhabitants (one month-83 years old) living in a holoendemic area were analyzed in a random-effects logistic regression model to invest igate the relationship between the level of Plasmodium falciparum para sitemia and risk of fever. It was not possible to build a model that d escribed/summarized correctly this relationship by a continuous functi on. Findings provide evidence for an age-dependent threshold effect of parasitemia on the occurrence of fever. The level of this threshold v aried by 2.45 trophozoites per leukocyte, maximum at one year of age, to 0.5 trophozoites per leukocyte, minimum at 60 years of age. When th e parasite density of a person crossed the threshold level correspondi ng to his or her age, the individual's risk of fryer was multiplied by 44 (95% confidence interval = 13.6-144.8). The existence of this thre shold effect allows parasite density to be used to distinguish malaria attacks from other causes of fever within an individual and should fa cilitate the accurate evaluation of the incidence of clinical malaria in highly endemic areas.