Natural history of Plasmodium falciparum malaria and determining factors of the acquisition of antimalaria immunity in two endemic areas, Dielmo and Ndiop (Senegal).

Development of new anti malaria strategies and particularly vaccines, needs an in-depth understanding of the relationships between transmission, infection, immunity, morbidity and mortality. The intensive and longitudinal collection of entomological, parasitological and clinical data from the Senegalese populations of Dielmo (250-300 inhab.), exposed to a perennial and intense transmission (about 200 infective bites/person/year) and of Ndiop (300-350 inhab.), exposed to a seasonal transmission (about 20 infective bites/person/year), allows to respond to many questions about this subject. The existence of a pyrogenic threshold effect of parasitaemia allows the individual diagnosis of malaria attacks. The initial intensity of clinical manifestations does not differ perceptibly among children and adults, is not related to the duration of the attacks, is not predictive of their severity, and the clearance of parasites and manifestations is longer among youngest persons. The risk of malaria attacks is lower as one gets older and among carriers of AS haemoglobin, is higher when transmission increases and during pregnancy up to three month after delivery, and vary between children. The risk of malaria attack per infective bite is negatively related to the intensity of transmission. Because of their high sensitivity in malaria case detection, this type of small community-based studies are powerful and useful for the identification of protective immunological mechanisms as well as for testing rapidly and cheaply the clinical efficacy of any intervention such as antimalarial vaccines and drug therapy or prophylaxis.