Genetic epidemiology of Schistosoma mansoni infection in a rural area of Brazil

This cross-sectional study examined the contribution of additive genetic effects on fecal egg counts during Schistosomiasis mansoni infection in a sample of 634 individuals living in a rural area of Brazil. All individuals were placed into extend multi-household pedigrees for analysis. Data were collected on the following risk factors: socioeconomics, demographics, physical environment (including water supply and sanitation), and water contact behaviors. A variance decomposition model was used to estimate the effects of shared residence and the above risk factors on the variation in fecal egg counts during Schistosoma mansoni infection. Individual risk factors, including water contact behaviors, gender, occupation, and socioeconomic status contributed 16.6% to the variation in fecal egg counts. Shared residence accounted for 27% of the variance in fecal egg excretion and when adjusted for the above risk factors contributed 21% to the variance in fecal egg counts. A quantitative variance decomposition method was then used to partition the observed variance in fecal egg counts into components attributable to additive genetic effects, shared residence, and risk factors. This analysis indicated that 31% of the residual variance in fecal egg counts was attributable to additive genetic effects, after adjusted for the suite of risk factors. The previously observed effect of shared household no longer remained significant when modeled with additive genetic effects and the suite of risk factors. The apparent variation in fecal egg counts due to shared residence was primarily a consequence of shared residence mimicking the effect of additive genetic factors. The data and results clearly demonstrate the need to explicitly model and evaluate any putative sources of trait variation (e.g. water contact, socioeconomics) rather than (a priori) assuming their importance. The identification of the additive effect of host genes regulating schistosome host-parasite interactions provides new ways of understanding the mechanisms in humans that are critical for determining infection with S. mansoni.