Variation and stability in Schistosoma haematobium egg counts: a four-year study of Gambian children

Abstract

Individual changes in egg counts of a group of Gambian children were observed over a four-year period by obtaining replicate specimens on several successive days. In a focus where transmission begins in late July and increases in intensity until it stops in October or November, the egg counts of children less than 10 years old rose between November/December and March/April, and fell during subsequent months. These apparent annual cycles of superinfection were less obvious in older children. Some children of all ages showed large changes in counts in a few months and 7% of observations over a 12-month period were of falls by over 90%. Despite these fluctuations there was a significant degree of relative stability in subjects' counts when compared with those of other members of the group. The observations suggest that the worm burdens of some children are in a dynamic but steady state. Among the factors regulating this may be the acquisition of a degree of immunity to superinfection. The results also prompt speculation whether man's immune response may, in some circumstances, affect the egg-laying worms of the established infection. The role of protective immunity in the epidemiology of the infection appears complex and needs further study. While the findings are compatible with the hypothesis that concomitant immunity occurs in man, they suggest that it is unlikely to be solely responsible for the lower prevalence and intensity of infection in adults.