Parasite virulence and disease patterns in Plasmodium falciparum malaria.

Heterogeneity in parasite virulence is one of several factors that have been proposed to contribute to the wide spectrum of disease severity in Plasmodium falciparum malaria. We used observed age-structured patterns of disease to define a population structure of P. falciparum, where the latter contains several independently transmitted antigenic types or "strains" that each induce some degree of strain-specific antidisease immunity upon infection. Patterns of incidence of severe and mild disease may be explained by assuming that a majority of these strains are associated with mild disease and that although severe malarial anemia is a complication occurring in a certain proportion of early infections with "mild" parasites, cerebral malaria is caused by a few distinct highly virulent strains. Considerable variation in parasite virulence, as a major factor of disease severity in malaria, is made possible by the absence of competition between the various parasite strains, arising from weak shared immune responses. The theoretical framework presented in this paper can explain other epidemiological observations, such as the results of interventions with insecticide-impregnated bednets.