A Transmission model for Schistosomiasis Mansoni with age dependent exposure

The aim of this project is to formulate a model for the transmission of Schistosomiasis, to obtain values to be used as parameter estimates, and to study the model's behaviour. The use of a different ial equation in modelling the transmission of Schistosomiasis was pioneered by Macdonald (1965a). His paper leaves some aspects unclear and an at.tempt is made herein to clarifyathat model. The basis for Macdonald's assertion that the effects of altering exposure and snail factors are similar is studied, as well as the lack of respo~sivencss to changes in contamination. A graphical technique to explore the asymptotic behaviour of this formulation is outlined. Work by Nasell and Hirsch is discussed and an extension of their model incorporating age dependent exposure is described. From studies in St Lucia, parameter estimates are obtained and their appropriateness discussed. The model's behaviour is explored by comparing observed and predicted results, by investigating the asymptotic levels of infection, and by studying the sensitivity of asymptotic levels to changes in parameters. Macdonald's "breakpoint" phenomenon is not observed, mostly due to the immigration of infection. The proportion of snails patently infected is consistently overestimated, The predicted infection levels for different age groups fail to resemble empirical findings satisfactorily. Tactical questions are raised as to which specific mathematical steps reflect different control strategics. An example is presented whereby different approaches attempting to model mollusciciding produce contradictory recormmendatlons for optimal control strategy. Future work should deal more realistically with snail populations and an allowance for immune effects is recommended.