Malaria in Dakar and Implications for Control

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 malana 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 (0.00 198 and 0.00 185 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 O. 15% and malaria was responsible for 3% ofschool absences due to medical reasons. In contrast, parasite prevalence hardly varied with the season (minimum 3.69'0, maximum 7.50/0. 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. The level of malaria endemicity in tropical Africa is generally much lower in urban areas than in rural ones.'.* The urbanization process tends to reduce transmission by eliminating Anopheles breeding sites, by spacing out the persistent anopheline populations within a denser human population, and by limiting their dispersion from breeding place^.^-^ This phenomenon and its consequences have been well-documented by recent studies in regard to entomologic, parasitologic, and Sero-immunologic features. In contrast, except for impact on severe malaria,6 the clinical consequences of urbanization have not been studied. The relationship between malaria morbidity the entomologic inoculation rate, incidence and recovery rates of malaria parasitemia, and the incidence of malaria attacks in stable endemic areas. Transmissionlevels in tropical Africa vary considerably, according to ecologic conditions, from approximately to IO3 infective bites per person per year. At a given age, the degree of acquired protective immunity differs according to the level of transmission and has marked consequences on the absolute and relative importance of malaria morbidity (Trape JF and others, unpublished data). Thus, whereas in areas of high transmission the incidence of malaria attacks is approximately 40 times higher in children less than five years of age than in adults, and urbanization in tropical Africa occurs within the general framework of relationships among the differences among the age groups tend to decrease in areas of low transmission, and in older 748 children and adults, the incidence of clinical attacks is liable to be higher in areas of low transmission than in those of high transmission (Trape JF and others, unpublished data). In fact, despite their importaxit implications for malaria control, the effects of transmission level on the incidence of malaria attacks in persons of a given age are not well known. This knowledge is necessary, however, to appreciate-the global impact of malaria pathology, to decide on the advisability of a campaign to reduce transmission, and to assess the short-, medium-, and long-term effectiveness of all forms of malaria control. The first objective of this study, camed out in Dakar, Senegal, was to accurately measure ma!aria morbidity in older children living in a town in tropical Africa where malaria is hypoendemic. The second was to use the phenomenon of vector density gradients observed in urban areass to carry out simultaneously at different transmission levels a quantitative analysis of the relationships among transmission, parasitemia, änd morbidity in these children. MATERIALS AND METHODS