FACTORS CONTROLLING RESISTANCE TO SCHISTOSOMIASIS IN ENDEMIC AREA

In endemic area of schistosomiasis mansoni, environmental factors account for 20 to 25 % of infection variance while 30 to 40 % of it are accounted for by the effects of a major co-dominant gene. Furthermore, resistance develops slowly in children and is altered in children and young adolescents with high water contacts. These factors, major gene and age, that act on resistance account for more than half of the variance of infection intensities indicating that control programmes must give a priority to measures aimed at increasing resistance. Immunological studies carried out in Brazil, in Kenya and in Gambia all suggest that the variations of resistance in children and adolescents could result from the slow development of a protective immunity mediated by IgE and antagonized by the blocking and competing action of IgG4. Though potent schistosomicids are available, prophylaxis, especially vaccination, is better suited for the control of this endemy. Antigens were identified in the analysis of the antibody response of resistant subjects and their vaccinating properties are evaluated. The implications of these findings for the control of schistosomiasis are discussed.