Uncertainties in the epidemiology and control of schistosomiasis.

In this paper, gaps in our understanding of the dynamics of infection, transmission, pathology, and control of schistosomiasis, and the possible contribution of modeling are briefly discussed. The measurement of prevalences and intensities of infection by egg counts has shortcomings; a recently developed model of egg count variations contributes to a better interpretation of survey data, and suggests that true prevalences and worm loads in endemic communities may be considerably underestimated. The question as to whether schistosome populations are regulated by host-dependent or transmission-related factors is still being debated; recent scientific advances and operational control experiences tend to favor the first mechanism, with important implications for control and research strategies. However, there is still a lack of field data to feed models of the dynamics of transmission. We still know little about the dynamics and even the importance of schistosomiasis morbidity, although more objective data are now becoming available through ultrasound studies. Models of the development of early- and late-stage morbidity could substantially contribute to more cost-effective strategies of passive and active chemotherapy. Modeling can also contribute to a better understanding and improvement of results of ongoing control efforts, particularly concerning the impact of repeated chemotherapy, and its complex interaction with many biological and social factors on infection, transmission, and morbidity.