Predicting Outcome in Malaria: Correlation between Rate of Exposure to Infected Mosquitoes and Level of Plasmodium falciparum Parasitemia

The level of Plasmodium falciparum parasitemia at clinical presentation has repeatedly been shown to correlate with severity of disease. Using data collected in western Kenya over 21 months, we examined associations between exposure variables, especially exposure to infective mosquitoes, and prevalence and density of P. falciparum parasitemia among 1,007 children six months to six years of age. The prevalence of P falciparum infection was similar at all exposure levels, but there was a correlation between exposure to sporozoite-infected mosquitoes over the previous 28-day period, and geometric mean parasite density of each cohort (Spearman rank coefficient = 0.724, P = 0.002). The relative odds of having a parasite density > or = 5,000/microliters was increased almost two-fold among individuals exposed to more than 10 infective bites during the prior 28-day period. Children enrolled during the highest incidence period were 80% more likely to have a density > or = 5,000/microliters relative to individuals enrolled during periods of lower incidence. The data suggest that measures, such as malaria vaccines, that reduce parasite densities by limiting numbers of sporozoites reaching the liver, or merozoites released from the liver, will reduce malaria-associated morbidity and mortality, even when they do not prevent all infections.