Plasmodium falciparum transmission pressure and malarial morbidity among young children in western Kenya.

Abstract

Anemia is an important complication resulting from Plasmodium falciparum infection in young African children. Exposure to elevated entomologic inoculation rate (EIR) is associated with higher parasitemia densities at cross-sectional time points. It is not well understood how the dynamics of parasitemia influence hemoglobin (Hb) concentration. It is also unclear whether exposure to higher EIRs may account for increased malarial anemia. Data from two large epidemiologic investigations in western Kenya were used to investigate relationships among transmission intensity, parasitemia, and Hb. In the first analysis, data collected from 6 contiguous villages in 1987-88 were analyzed to investigate whether EIR predicted incidence or degree of P. falciparum parasitemia. Once per month a cohort of 50 children was enrolled, cured of malaria parasites, and monitored over 84 days for recurrent parasitemia (n = 20 cohorts). Outcomes included time to parasitemia, time to parasitemia $\ge$5,000/$\mu$l, and parasitemia density. EIR, analyzed as a time-dependent covariate, was associated with a 2.8 times higher rate of parasitemia $\ge$5,000/$\mu$l in children exposed to $\ge$1 infective bite per day as compared to the referent (RR = 2.82, 95% CI 2.24-3.56). Cumulative number of infective bites, exposure duration, and age were significantly associated with recurrent parasitemia density in multiple linear regression models. The second analysis included longitudinal data contributed by children between birth and 48 months of age during 1992-96. Monthly Hb and parasitologic measures, and weekly household level entomologic data were obtained from 15 contiguous villages. A mixed random effects model for correlated data was used to describe the temporal pattern of 14,317 Hb measures contributed by 942 children. Higher geometric mean parasitemias over the previous 90-day period had a significant effect on Hb concentration in the first 18 months of life. The results support the contention that reductions in P. falciparum transmission intensity, in the absence of complete elimination, will reduce parasitemia levels and possibly reduce the risk of severe malarial anemia.