PARASITE VIRULENCE AND DISEASE PATTERNS IN PLASMODIUM-FALCIPARUM MALARIA

Heterogeneity in parasite virulence is one of several factors that hav e been proposed to contribute to the wide spectrum of disease severity in Plasmodium falciparum malaria. We use observed age-structured patt erns of disease to define a population structure of P. falciparum, whe re the latter contains several independently transmitted antigenic typ es or ''strains'' that each induce some degree of strain-specific anti disease immunity upon infection. Patterns of incidence of severe and m ild disease may be explained by assuming that a majority of these stra ins are associated with mild disease and that although severe malarial anemia is a complication occurring in a certain proportion of early i nfections 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 p ossible by the absence of competition between the various parasite str ains, arising from weak shared immune responses. The theoretical frame work Presented in this paper can explain other epidemiological observa tions, such as the results of interventions with insecticide-impregnat ed bednets.