Egg output stability and the epidemiology of Schistosoma haematobium Part II. An analysis of the epidemiology of endemic S. haematobium

Abstract

On the basis of observations of school children described in Part I, a model for the infective process of S. haematobium in man in the Misungwi area of Tanzania is put forward. On the basis of steady challenge by cercariae, effective concomitant immunity, exponential death of worm pairs, the observed variance of egg output at age 10, and constant ranking by infection level in early life, the behaviour of egg output patterns with age in the community is predicted. Expected curves of output are compared with those observed in cross-sectional survey of over 4,000 people at Misungwi. Agreement is close. This provides additional evidence for the occurrence of concomitant immunity to S. haematobium in man and suggests that immunity cannot be disregarded in epidemiological models of the infection. It is suggested that there are 3 phases in an age-cohort's experience of endemic infection: acquisition and increase of infection, a phase of decreasing worm load combined with concomitant immunity, and a steady state phase, with loss of immunity and reinfection balancing, in later life. The need for better data on the nature of human immunity to schistosomiasis becomes apparent.