On the application of mathematical models of schistosome transmission dynamics. II. Control.

Mathematical models have considerable potential as aids to the design of schistosome control programmes. This is because of the complex nature of the schistosome transmission cycle and the variety of control measures available, which make the comparative effectiveness of different control options extremely difficult to predict. This review aims to demonstrate how control can be incorporated in models of schistosome transmission dynamics, and to make explicit the assumptions and limitations of the models and their relationships with field data. A basic model is described which considers changes in the mean number of schistosomes per person. The criteria for the eradication of endemic infection and the potential for reducing levels of infection are discussed. The treatment of various control measures within this framework is reviewed. These include: chemotherapy (mass, selective and targeted), molluscicide application (blanket and focal) and other snail control measures, larval stage control, improved water supplies and sanitation, and health education. The incorporation of economic variables is also discussed. The choice between different control options depends on the relationships between schistosome epidemiology and the costs and effects of control. The evaluation of cost-effectiveness is a dynamic problem, and the outcome will depend on local conditions and constraints. Very general recommendations for the design of schistosome control programmes are unlikely to prove useful.