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Cahiers d'études et de recherches francophones / Santé

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Malaria in the central health district of Dakar (Senegal). Entomological, parasitological and clinical data Volume 10, issue 3, Mai - Juin 2000

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Authors
Service de parasitologie, Faculté de médecine, Dakar, Sénégal.
  • Page(s) : 221-9
  • Published in: 2000

We previously investigated malaria in the southern health district of the city of Dakar, which includes the oldest neighborhoods. In this study, we investigated malaria in the central health district, corresponding to the central area of the conurbation. The study was carried out at 12 sites, from March 1996 to February 1997. The sites were selected such that the entire district was covered and included 2 sites in the shanty town and three in an old village that has been absorbed into the city. We carried out prospective monthly entomological analyses with a view to identifying the vectors and the mode of transmission of malaria. We also carried out clinical and parasitological follow up to determine the incidence of parasitemia and of bouts of malaria. Insects were collected overnight from humans and the insects remaining the next morning in 10 bedrooms in the health district were collected. For clinical and parasitological follow up, families were visited at home once per week and their clinical state was assessed. Blood smears were taken to facilitate the detection of bouts of malaria. Body temperature was measured and we checked for the presence of organisms in the blood systematically during the last weekly visit of each month. For a total of 308 collections at night from human volunteers and 1,395 bedroom collections of residual fauna, we obtained 12,879 Culicidae females, 199 (1.5%) of which were anopheles mosquitoes, with Culex quinquefasciatus accounting for 98% of the remaining mosquitoes. As in the southern district, A. arabiensis was the only species of the A. gambiae complex collected. Anopheles mosquitoes accounted for only 0.3 bites per man per night and 0.07 females per room. They were therefore poorly represented in this district and were not detected at all at five sites. They were found in large numbers only during the rainy season, especially in September, when they accounted for 2.25 bites per man per night and 0.3 females per room at 3 sites in an undeveloped zone in which 81.4% of all the anopheles mosquitoes were collected. The parturition frequency of the biting females was 32.6% and that of the females collected in houses was 50.0%. None of the A. arabiensis females dissected (98.5% of those collected) carried Plasmodium sporozoites. The clinical and parasitological follow up concerned 2,583 individuals, aged from 1 month to 80 years, from 285 families resident in Dakar who volunteered for the study; 41.9% of these individuals were less than 15 years old and 92.2% had been living in Dakar for more than 2 years. Thick and thin blood smears taken monthly showed the frequency of the parasite to be 1.0% and that of gametocytes to be 0.1%. P. falciparum was the only parasite detected in the subjects. Plasmodium infections were observed in all age groups, with a frequency of 0.4% (adults over the age of 20 years) to 1.6% (children under two years of age). Parasitized subjects were detected in every month of the study, with a frequency of 0.4% (in January) to 1.9% (in December). The largest number of cases detected in a three-month period (38.8% of all cases) was that for October to December, the three-month period immediately after the rainy season (July to September). Parasite frequency, which was no higher than 1.2% at 10 sites, was clearly higher at two sites in the shanty town (3.8 and 6.8%), mostly inhabited by immigrants from rural areas. At the end of the study year, satisfactory weekly follow up was considered to have been achieved for 1,067 of the participants. The annual incidence of parasitemia in this cohort was 5.1% and that of malaria was 2.4%. Incidence did not vary significantly with age and was between 1.8% and 7.6% for parasitemia and between 0.8% and 3.5% for malaria. However, significant differences in incidence were observed between areas. Incidence was higher at the two sites in the shanty town, with rates of 12.1% and 36.5% for parasitemia and 6.1% and 15.9% for malaria. We were unable to determine the rate of malaria transmission from the entomological data collected. However, based on the number of parasitized subjects, we estimate the rate to be one infected anopheles mosquito bite per man every 20 years for the district as a whole and one infected bite per man every 3, 8 and 10 years respectively for the 3 sites at which the highest parasite frequencies were recorded. As for other West African urban centers, the low prevalence in the center and the south of this city is probably due to urban development and easier access to medical care. It may also be due to the geographical situation of the city, in the north of the sahelian area, with a total annual rainfall of less than 400 mm.