Cross-species regulation of Plasmodium parasitemia in semi-immune children from Papua New Guinea.

Malariologists have long been fascinated by the question of whether Plasmodium spp. interact in the human host. The first genetic study of the longitudinal dynamics of multiple Plasmodium spp. and genotypes in humans has been completed in Papua New Guinea, where all four Plasmodium spp. that infect humans are present. The broad implications of the data from this study are covered here and they show that the total parasite density of Plasmodium species oscillates around a threshold and that peaks of infection with each species do not coincide. It is proposed that malaria parasitemia is controlled in a density-dependent manner in these semi-immune children and that a cross-species mechanism of parasite regulation exists. A model of how multiple immune responses could act in concert to explain these within-host dynamics are discussed here in relation to observed epidemiological patterns of mixed-species infections.