The distribution of Schistosoma mansoni in the Rusizi plain (Burundi)

A regional survey of Schistosoma mansoni infection has been performed in the Rusizi Plain (Burundi), on a geographically evenly distributed 5% population sample (6203 subjects) in 41 localities. The overall prevalence was 33%, with 58% of the cases excreting less than 100 eggs per gram of faeces and a mean egg load of positives of 98 eggs per gram. The geographical distribution is very focal; local prevalences range from 3% to 63%, but vary also considerably on a sublocal level. Eight eco-epidemiological zones can be recognized. The infection rates are over 40% in the marshy cotton paysannats near Lake Tanganyika, the irrigated cotton paysannats of the northern plain and a deteriorated rice cultivation area in the south; 30-40% in the other cotton and rice areas of the south; 20-30% in the dry cotton paysannats of the mid-plain and the hors-paysannats at the foot of the mountains. Natural foci certainly occur, but most often man-made hydraulic systems are the basis of high prevalences. Overall the infection rates are maximal in individuals aged 15 to 20, but they remain relatively high in adults. The age-prevalence curves vary from one area to another, however. In rice culture areas the prevalences show an unexpectedly strong decline in adults, indicating that the impact of irrigation on schistosomiasis endemicity is not a problem of occupational exposure.