Additive host genetic factors influence fecal egg excretion rates during Schistosoma mansoni infection in a rural area in Brazil.

This study quantifies the influence of shared household and kinship on egg counts during Schistosoma mansoni infection in a sample from rural Brazil. Detailed genealogic information allowed assignment of 597 individuals to 6 multihousehold pedigrees residing in 145 households. A variance component method was used to partition egg counts into shared household, additive genetic, and individual-specific environmental effects. Host additive genetic effects consistently accounted for a large proportion of the variation in egg counts: 43% in an unadjusted model and 40% in model adjusted for covariates. In a model that examined the confounding of shared household with kinship, additive genetic effects still accounted for 27% of the variation in egg counts and shared household only 12%. The consistently important role for host additive genetic factors on the variation in egg counts points to new ways of modeling and understanding the mechanisms that contribute to trait variation during infection with S. mansoni.

Exposure to Schistosoma mansoni infection in a rural area in Brazil. II: household risk factors.

A number of studies have pointed out the potential importance of the household in the transmission of schistosomiasis. The clustering of domestic activities associated with water collection, storage, and usage can result in the sharing of transmission sites and infective water contact behaviours. In this study, we employed a variance component method to estimate effects due to individual risk factors and shared residence on the variance in faecal egg counts during Schistosoma mansoni infection. A suite of covariates, which included demographic, socioeconomic, water supply, and water contact behaviour terms, contributed 15% to the variance in faecal egg counts. Shared residence alone accounted for 28% of the variance in faecal egg excretion. When both the suite of covariates and shared residence were considered in the same model, shared residence still contributed 22% to the variance in infection intensity. These results point to the importance of shared residence as a means of capturing the complex interrelationship between shared demographic, socioeconomic, physical environmental, and behavioural factors that influence transmission of schistosomiasis at the household level.