Antischistosomal treatment and the measurement of incidence of schistosomiasis in a community of high transmission.

There is disagreement in the measurement of incidence in schistosomiasis concerning which of two categories of negatives is representative of the population of an endemic area: naturally uninfected individuals (untreated negatives) and those made negative following treatment (treated negatives). The incidence of schistosomiasis was studied in the two categories of negatives for two periods among 208 school children (6-11 years old) living in a community of high transmission in Zimbabwe. In both periods incidences of Schistosoma haematobium and S. mansoni in the treated negative group were significantly higher than in the untreated negative group. These findings suggest that host factors, water contact behaviour, innate or acquired immunity for the two groups of negatives were probably different. It is important to measure incidence in both treated and untreated individuals of an endemic community.