Introduction

The development and use of ONCHOSIM for studying epidemiology and control of onchocerciasis is a joint effort of the Onchocerciasis Control Programme (OCP) and the Department of Public Health of the Erasmus University, Rotterdam. ONCHOSIM uses the so-called microsimulation technique for modelling stochastic systems (Habbema et al. 1995). This technique is characterized by mimicking individual life histories of humans and – in the case of ONCHOSIM – parasites. Biological factors and characteristics of control measures can be modelled in detail. Output of microsimulation models can be detailed (age- and sex-specific tables for comparison with detailed data sets) and simple (trends in prevalence during control). New insights can readily be incorporated by redefining relationships in the model and adapting the computer program which is used to perform simulations with the model.

A model that is built and quantified using the ONCHOSIM computer program has two types of assumptions. One concerns the deep model of the transmission cycle and disease process of onchocerciasis. The other concerns the description of the relevant characteristics (‘experimental setting’) of the village or region under study and of the control measures. The degree of complexity of both the deep model and the descriptive part depends on the aims of the model use, the available data and other considerations. When compared to most other current epidemiological models, the most pronounced difference is probably the level of detail in which the descriptive part is modelled. This is a reflection of the primary reason for involvement of modelling in OCP: supporting evaluation and decision making in a particular control programme (Remme et al. 1995).