Control of schistosomiasis transmission in newly established smallholder irrigation schemes.

The extent to which schistosomiasis can be controlled through careful design of the irrigation works, the operation schedule, the location of villages and the provision of safe water supplies and sanitation in smallholder irrigation schemes at Mushandike in south-east Zimbabwe is reported. The schemes provide land for 400 peasant families who are monitored for schistosomiasis soon after resettlement. Between April 1986 and August 1987 there was a marked reduction in overall prevalence following treatment. For Schistosoma haematobium, the reductions in 2 schemes were 53% and 82% and for S. mansoni, 82% and 100%, in spite of transmission in settlements upstream from the schemes. In contrast, in a nearby irrigated farm where no control measures were introduced, S. haematobium prevalence declined by only 31% while that for S. mansoni increased by 22% a year following treatment. These results are supported and strengthened by measurements of snail population sizes and their infection rates, cercarial densities in the water and incidence data for the human population, and suggest that the engineering and environmental control measures introduced are significant means of controlling transmission in smallholder irrigation schemes with the lowest recurrent costs.