The epidemiology of schistosomiasis in Burundi and its consequences for control

Abstract

This paper summarizes the results of a series of studies on the epidemiology, morbidity and transmission of Schistosoma mansoni in Burundi, and discusses their consequences for control. The main endemic area is the Imbo lowland, consisting of the Rusizi plain, the urban focus of Bujumbura, and the shores of lake Tanganyika; a small, new focus was discovered in the highlands, around lake Cohoha. Distribution studies on 5–10% population samples with duplicate 28 mg Kato smears in these 4 foci showed prevalences of 33%, 26%, 17%, 19% and mean (positive) egg loads of 110, 105, 92, 144 eggs/g, respectively. The combined population at risk was estimated to be 400 000 people, the total number of detectable cases 90 000. Prevalences and intensities varied greatly at the subregional, local and even sublocal level. The age- and sex-related prevalences and intensities of infection showed typical peaks in children and adolescents, but remained relatively high in adults in many areas; these patterns varied from one area to another and could be related to ecology and water contact. Morbidity studies showed that, in children as well as in adults, schistosomiasis-related morbidity such as (bloody) diarrhoea, hepatomegaly and splenomegaly was apparent mainly in areas with prevalences over 30–40%. The intermediate hosts were Biomphalaria pfeifferi (Imbo), B. sudanica (Tanganyika marshes) and B. stanleyi (Cohoha). Population dynamic studies showed strong seasonal variations, the patterns of which were focal and even erratic in space and time. Snail densities and cercarial infection rates (0·85% overall in B. pfeifferi) were low. Behavioural studies showed that human contacts were most frequent in young adults, but their mean duration was highest in children. Most contacts were due to fording and to domestic activities. It is concluded that the most appropriate strategies for control are the following, (i) Targetting priority areas with prevalences over 30%. (ii) Selective chemotherapy based on the presence, not the intensity, of infection in all age groups. (iii) Sanitation aiming particularly at the reduction of fording and domestic water contact. (iv) Focal mollusciciding only in well selected sites. (v) Close monitoring and surveillance.