A model of schistosomiasis incorporating the searching capacity of the miracidium

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Abstract
This paper describes a modification of the model for the transmission of schistosomiasis proposed by Macdonald (1965), which allows the snail host-locating efficiency of the miracidium to be taken into account. The modified model allows for a choice of miracidia efficiency. If it is assumed that the miracidia are inefficient hunters then the resulting model is very sensitive to perturbations; relatively small changes in snail numbers not only cause appreciable changes in the percentage of snails infected but result in attainment of the threshold (the point at which schistosomiasis ceases to be endemic). If, however, the miracidia are efficient snail hunters then the model is much less sensitive to perturbations; here the percentage of snails infected is much less affected by changing snail numbers, and the threshold is much more difficult to reach by reducing snail numbers. It is argued that, on the whole, the empirical data give more support to the latter model, although there are some circumstances under which the former model might apply. However, even with these modifications it is considered that the Macdonald model does not provide a satisfactory explanation of field observations.