This paper describes the design of a study on immunity to
reinfection after treatment of children with
Schistosoma mansoni infections, the initial observations on transmission that led to the selection of the study population, the effects of treatment, and the results of immunological tests carried out before and at five weeks
after treatment. Iietune village in Machakos District, Kenya, was selected on the basis of high prevalence and intensities of
infection in a small preliminary survey, a stable population living in a small area amenable to detailed study, and a lack of previous intervention in the area. Subsequent observations over a pretreatment period of one year confirmed that prevalence and intensities of
infection among children attending the local primary school were high. This was associated with extensive contact of members of the community with water-bodies shown to contain large numbers of infected snails. Analysis of pretreatment intensities of
infection and water contact patterns in the schoolchildren allowed the selection of 129 children showing a broad scatter between: (a) high intensity, low water contact, and predicted to be non-immune, and (b) low intensity, high water contact, and predicted to be immune. These children were treated with
oxamniquine, 30 mg/kg in divided doses. Five weeks
after treatment, 70% of children showed apparent complete cure, and the over-all reduction in geometric mean egg output was 98.9%. Since these children represented only a small proportion of the whole community, there was no obvious reduction in transmission, as reflected by snail
infection rates, during the following five-month period. Thus, we are in a position to determine whether successfully treated children do or do not become reinfected in a high transmission environment in which it will be possible to make direct estimates of exposure. Immunological tests carried out immediately before treatment were consistent with a pattern of high exposure leading to the early expression of immune responses in most infected children. Eosinophil levels were elevated in 61% of the children, all of whom showed detectable levels of
antibodies against adult worm and egg
antigens, as measured by ELISA. In addition, all patients showed
antibodies capable of mediating eosinophil-dependent killing of schistosomula. At five weeks
after treatment, eosinophil counts and anti-adult worm antibody levels had risen, whereas anti-egg
antibodies remained grossly unchanged. The wide variation in the levels of responses shown by different individuals will allow us to test whether such responses are associated with resistance to
reinfection during the follow-up period.