Plasmodium falciparum clinical malaria: lessons from longitudinal studies in Senegal

Development of new antimalaria 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 inhabitants), exposed to a perennial and intense transmission (about 200 infective bites/person/year) and of Ndiop (300-350 inhabitants) exposed to a seasonal transmission (about 20 infective bites/person/year), allows to respond to many questions about this subject. The acquisition of an antimalaria immunity as one gets older appears to reduce parasite density, complexity of infection, risk of new patent infection after a suppressive treatment but does not reduce the prevalence (as assessed by PCR) of infection which is commonly chronic and asymptomatic. The existence of a pyrogenic threshold effect of parasitaemia allows the individual diagnosis of malaria attacks. P. falciparum genotyping suggests that successive malaria attacks are due to distinct recently inoculated parasite populations that multiply initially without restriction, a dominant population is generally responsible of the clinical manifestations and all new populations do not trigger systematically attacks. The initial intensity of clinical manifestations does not differ perceptibly among children and adults, is not related to the duration of the attacks, does not allow the distinction between several types of 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 months 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. As a lot of vaccine candidates and drug combinations will be screened or tested in the perspective of the 'Roll-Back Malaria' programme, more attention must be given to longitudinal studies of this type.