The value of incidence for the assessment of schistosomiasis control; a study in Southern Rhodesia.

Rapid assessment of the rate of transmission of schistosomiasis is essential in any control programme. It has been demonstrated that, by following the pattern of infection developing during a longitudinal study in a cohort of children under 6 years of age, it is possible to calculate the incidence of schistosomiasis in the community represented by that cohort. Children who were found by survey to be free from active infection with schistosomes were examined at intervals of 1, 3, or 4 months to determine the rate at which infections of either Schistosoma mansoni or S. haematobium occurred. This gave an immediate measure of the rate of transmission during the period under study. The rate can be calculated for unit time- in this study figures were extrapolated to 120 days. Data are presented to show how the incidence measured reflects the efficiency of snail control procedures carried out in the different farming areas.