Heterogeneities in water contact patterns and the epidemiology of Schistosoma haematobium.

Variations in the amount of water contact made by individuals and in the amount of water contact made at different sites may have significant impacts on patterns of human schistosome infection. Previous studies have reported variations in the rate of water contact and differences in the sites used between age/sex classes, but there is limited information on variations in individual water contact behaviour. In this paper we report and analyse observations of essentially all water contacts made over a two week period by all individuals in a rural community in eastern Zimbabwe. The mean rate of water contact was 0.43 contacts/person/day. These data were over-dispersed, ranging from zero to 3.3 contacts/person/day; 90% of contacts were made by only 37% of the population. Contact rates were related to age (highest in 8 to 10-year-olds) but not sex, with substantial variation unaccounted for by these variables. Age and sex classes differed in types of water-related activities and the time of day of contact. A greater diversity of sites was used by children than by adults and by males than by females. Individual contact rates were correlated with intensities of infection, although the risk of infection per contact was estimated to be highest in 2 to 4-year-old children and higher for males than females. Five contact sites were used during the study period, with more than 50% of contacts occurring at just 2 sites. Different age and sex classes used different sites and there were additional site-related differences in types of activity and the time of day of use. The implications of these water contact patterns for schistosome epidemiology are discussed. In particular the results provide strong quantitative support for control programmes aimed at heavily used sites (e.g. focal mollusciciding) or at the minority of individuals making most water contacts (e.g. targeted chemotherapy).