Introduction

It is estimated that one quarter of the world population is infected with one or more of the intestinal helminth species Ascaris lumbricoides, Trichuris trichiura and hookworms (Ancyclostoma duodenale and Necator americanus) (WHO 1987, Bundy 1990). Approximately one hundred million people worldwide may suffer morbidity as a result of each of these infections (Chan et al. 1994b). A relatively cost-effective method of controlling the more serious consequences of all these infections is the use of mass or age-targetted chemotherapy.

When designing community based chemotherapy programs, the cost-effectiveness of different treatment strategies must be taken into account. In the absence of adequate field data, the likely benefit of treatment in terms of infections and morbidity prevented can be assessed by epidemiological modelling. An epidemiological model has been developed which incorporates observed patterns of infection in the community and can be used to estimate the rate of reinfection following treatment interventions (Medley et al. 1992). The output from the epidemiological model can be combined with actual cost data from chemotherapy operations to provide a cost-effectiveness analysis of different control options (Guyatt et al. 1993).

The aim of this study is to develop an age structured version of the Medley et al. (1992) model. The rationale for this approach comes from the well known age differences in prevalence and intensity of infection. For Ascaris lumbricoides and Trichuris trichiura, highest prevalence and intensity is found in school age children, whereas with the hookworms, highest prevalence and intensity is found in adults. The model also allows treatment to be targetted at one age group only.