The epidemiology of Schistosoma haematobium and S. mansoni infections in the Egypt-49 project area. 4. Measurement of the incidence of bilharziasis.

The measurement of incidence, or the rate at which people become positive, for Schistosoma haematobium and S. mansoni was carried out in four parts of the Egypt-49 project area near Alexandria. For S. haematobium, rates as high as 22.8% per year were found for children 0-6 years old in a rural area; in the same area, the incidence of S. mansoni was 8.5% per year. The true incidence is underestimated because many cases become negative spontaneously. This loss rate of S. haematobium cases is 0.476 per year for children 0-4 years old, and 0.049 per year for those aged 5 and 6 years; for S. mansoni, the rates are 0.580 and 0.327, respectively. Despite the error, incidence is the most accurate and sensitive method of assessing the success of control operations, and is an important measurable parameter in epidemiology.