A STOCHASTIC-MODEL OF SCHISTOSOMIASIS IMMUNO-EPIDEMIOLOGY

Schistosomiasis is a helminth (worm) infection with approximately 200 million people infected worldwide. There is still controversy on wheth er differing worm burdens between individuals is a result of differing contact rates or of acquired immunity. In this paper, we present a st ochastic modelling framework to address these issues. By using appropr iate approximations for the higher moments of the joint distributions, differential equations for the means, variances and co-variances of i nfection and immunity can be obtained. Numerical solutions of these eq uations to obtain age profiles of the above properties were compared w ith Monte Carlo simulations of the stochastic process. Simulations sho wed that the results depended on whether between host heterogeneity wa s generated by differing contact rates or differing immune responses. Heterogeneity in contact rates produced a highly aggregated distributi on of parasites with a large variance/mean ratio. Heterogeneity in the immune response had very little effect on the overall dynamics. This agrees with the predominantfield evidence which would suggest that inf ection is mainly determined by ecology with a smaller contribution of immunity. (C) 1998 Elsevier Science Inc. All rights reserved.