Human water contacts patterns in Schistosoma mansoni epidemic foci in northern Senegal change according to age, sex and place of residence, but are not related to intensity of infection

Abstract

SummaryIn an epidemic focus in northern Senegal, adults had lower intensities of infection than adolescents, a phenomenon that could not be attributed to immunity acquired over the previous 10-15 years of exposure to the parasite because all age groups had had the same number of years' experience of the worm. This article considers whether this pattern could have been because of higher levels of exposure to the parasite in younger age groups. Personal contact with infected water was recorded using a questionnaire in Schistosoma mansoni foci not more than 3 years old and in another, 10-year-old focus. Many aspects of contact (e.g. frequency, duration or time of day of contact) may contribute to the number of encounters with infective cercariae (true exposure), so various assumptions regarding the relationship between water contact and true exposure were tested resulting in a range of exposure indices. People reported a mean of 4.4 separate contacts, and spent a median of 57 min per day in water. Patterns of water contact differed depending on the exposure index used, e.g. considering duration, males spent a longer time in water than females (P < 0.001). But using frequency, females had more contacts with water than males in most villages (P < 0.001). Generally, exposure levels dropped as people become aged (P < 0.001) and residents of the older focus were more exposed than residents of other foci (P < 0.002). Intensity of (re)infection was not related to exposure either alone or in models incorporating age, sex and/or village irrespective of the index used. There is therefore evidence that age, sex and place of residence determine exposure but none to suggest that exposure had an influence on the relationship between these factors and intensity of infection. We propose therefore that in this population other factors have principal importance in determining intensity of infection.keywords Schistosoma mansoni, human, Senegal, water contact correspondence J. Scott,

Inbreeding within human Schistosoma mansoni: do host-specific factors shape the genetic composition of parasite populations?

“…On the other hand, children may be more exposed to related parasites than adults owing to differences in water contact behavior. Children in Senegal (Scott et al, 2003;Sow et al, 2011) and elsewhere (for example, Fulford et al, 1996) showed more and longer water contact than adults, and a study in Kenya showed that children visit fewer transmission sites than adults (Kloos et al, 1997). Children who visit the same transmission site frequently would have an increased risk of re-infection by S. mansoni strains that are related to the ones that previously contributed to the gene pool (that is, sib transmission; Anderson et al, 1995;Criscione et al, 2005).…”

Exploring the Contribution of Host Susceptibility to Epidemiological Patterns of Schistosoma japonicum Infection Using an Individual-Based Model

“…Both in our earlier epidemiological studies and those of others, infection intensity was found to be relatively insensitive to degree of water contact. 27,33 This was also the case in these simulation studies (see Supplemental Material Item 8 [SM8]). Hence, in the simulations, and very likely in the field, cercarial density and individual susceptibility are the principal determinants of the population distribution of EPG and of prevalence.…”

Bayesian geostatistical modelling for mapping schistosomiasis transmission

“…Three decades ago, the use of the negative binomial distribution was proposed to model the excessive variation, known as overdispersion, in helminth egg counts (Cohen, 1977). In the meantime, negative binomial regression models have been widely used to analyse helminth infection intensity data (Utzinger et al 2002 ;Scott et al 2003 ;Yapi et al 2005 ;Brooker et al 2006).…”