The assessment of a large snail control programme over a three-year period in the Gezira Irrigated Area of the Sudan

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Abstract
The control of the snail vectors of schistosomiasis by aerial application of the molluscicide N-trityl morpholine was carried out in a 80 000 ha area from 1974 to 1977. The effect of this molluscicide regimen on transmission of Schistosoma mansoni was monitored by longitudinal surveys in a number of schools and in pre-school-age children. A cross-sectional school survey was carried out in 1977. There was no significant difference in incidence between the treated area and an adjacent untreated area as measured by the longitudinal school survey, the longitudinal village survey, or the cross-sectional survey. Consideration of egg output and prevalence (as determined by ELISA test) also failed to reveal any difference between the two areas. The initial prevalence and intensity in the subjects selected from the treated area were higher than in the untreated area, and some villages showed possible transmission reduction. In general, however, the results showed a great variation in infection rates and intensity within the areas. Any new monitoring of control efforts must take account of these variations.