Apparent tolerance of Plasmodium falciparum in infants in a highly endemic area.

The incidence of fever among infants in the village of Idete in the Morogoro region of Tanzania was analyzed in relation to densities of Plasmodium falciparum parasites in the peripheral blood. Parasite densities in both fever cases and in asymptomatic infants, were compared and a Bayesian non-parametric mixture decomposition algorithm was used to estimate the proportion of fevers attributable to malaria and hence the incidence of clinical malaria. Age group-specific densities of peripheral parasitaemia showed little seasonality, but the clinical malaria incidence showed a clear peak in the wet season in children aged less than 9 months. Estimates of the parasitaemia-specific incidence of clinical malaria were used to quantify apparent tolerance of parasites, and indicated that clinical episodes occurred on average at lower parasite densities during the wet season than in the dry season. These patterns could reflect differences in levels of anti-toxic immunity, but the nature of the seasonal differences supports the alternative explanation that the variation in apparent tolerance may be an effect of changes in the ratio of peripheral parasite densities to the sequestered mass.