Malaria morbidity among children exposed to low seasonal transmission in Dakar, Senegal and its implications for malaria control in tropical Africa

To measure morbidity due to malaria and to study its relationship with transmission and parasitemia in children living in an area of low malaria endemicity, a cohort study of 343 schoolchildren was undertaken during a one- year period in Dakar, Senegal. From parallel investigations on transmission and the frequency of malaria as a cause for outpatient visits, three different seasons were chosen for close monitoring of different clinical, parasitologic, and sero-immunologic parameters. The daily incidence rates of malaria parasitemia and primary attacks were at a maximum level during the high transmission season (0.00198 and 0.00185 new cases/person/day, respectively) and decreased considerably during the season of low transmission. For each given period, the values of these two rates were close to each other, suggesting that each new infection was followed by a clinical attack. During the period of maximum transmission, clinical malaria prevalence was 1.36% and malaria was responsible for 36% of school absences due to medical reasons. At the end of the period of minimum transmission, clinical malaria prevalence was 0.15% and malaria was responsible for 3% of school absences due to medical reasons. In contrast, parasite prevalence hardly varied with the season (minimum 3.6%, maximum 7.5%). In a one-year period, the total number of new malarial infections was estimated between 173 and 230. Because of the existence of a vector density gradient in the area concerned, the annual malaria incidence varied considerably according to the children's place of residence. Although this rate reached one infection per year in children living near a marsh where Anopheles breeding sites were localized, we did not observe a higher clinical tolerance in these children than in those less exposed to malaria. These findings show that schoolchildren in Dakar have no protective immunity and that for them, malaria is a major cause of morbidity despite low endemicity. The implications for malaria control strategies based on the reduction of human- vector contact are discussed.