The population dynamics of onchocerciasis

Abstract

This chapter describes an attempt to fit a mathematical model to epidemiological data relating to onchocerciasis. In the first section I present a brief survey of the life cycle of Onchocerca volvulus and discuss the biological evidence for its regulation. On the basis of this I draw some general conclusions about the structure of the model. Before specifying in detail the model’s assumptions and the definition of the model’s parameters I present the data which have been used for the model construction. The observations come from seven Sudan savanna villages which cover a wide range of endemicity levels. For each of the seven villages entomological observations have been collected for several years such that a direct estimate of the ‘Annual Biting Rate’ and the ‘Annual Transmission Potential’ is available. A cross-sectional survey of the total population gives the age-specific densities of microfilariae and the prevalence of eye lesions and blindness caused by the parasite. Assuming that the measured biting rates have been at this level for a long time, the observed parasitological situation can be considered to represent an equilibrium state. The model constructed allows us to specify in numerical terms a critical level of the rate at which men are bitten; below this rate the infection cannot maintain itself at an endemic level. This critical level will of course depend on the particular characteristics of the vector population, i.e. longevity and rate of feeding on man.