Studies on human schistosomiasis: chemotherapy, immunity and morbidity.

At present, chemotherapy is the mainstay of schistosomiasis control programmes, but the most appropriate method of drug delivery frequently remains uncertain. In a six-year study in Machakos District, Kenya, designed to compare different drug delivery protocols, we have found that treatment only of infected primary school children is an effective, inexpensive and logistically practicable method of achieving a long-term reduction in intensity of infection in an area of high transmission but low morbidity. However, it is argued that even this simple approach to drug delivery is still not ideal--especially in areas of high morbidity in which intensity of infection may not be the only determinant of severe disease--and that a long-term approach to alternative control measures, especially vaccination, is desirable. Extensive studies have shown that a range of experimental hosts can be substantially protected against a challenge infection by immunisation not only with live attenuated larvae but also with individual recombinant antigens. The demonstration that an age-dependent acquired resistance to reinfection after chemotherapy also occurs in man offers encouragement that immunisation should theoretically be feasible: and attempts have been made to relate an observed resistance to reinfection in older individuals to various immune responses both to whole parasite antigens and to individual recombinant peptides. However, many questions remain unanswered, especially in the context of understanding what factors, other than intensity of infection, contribute to the development of severe morbidity in particular geographical foci, and some possibilities are discussed.