Human helminth co-infection: no evidence of common genetic control of hookworm and Schistosoma mansoni infection intensity in a Brazilian community

Strong statistical associations between soil transmitted helminths and schistosomes are frequently observed in co-endemic human populations, although the underlying explanations remain poorly understood. This study investigates the contribution of host genetics and domestic environment to hookworm and Schistosoma mansoniinfection intensity and evaluates the role of genetic and non-genetic factors in covariation of infection intensity. Detailed genealogical information allowed assignment of 1,303 individuals living in the Brazilian community of Americaninhas, Minas Gerais state, to 25 pedigrees (containing between two and 1,159 members) residing in 303 households. The prevalence of co-infection with both hookworms and schistosomes was high (38.5%), with significant correlation between Necator americanusand S. mansonifaecal egg counts (r = 0.242; P< 0.0001). Bivariate variance component analysis demonstrated a modest but significant species-specific heritability for intensity of N. americanus(h 2 = 0.196) and S. mansoni infection (h 2 = 0.230). However, after accounting for demographic, socio-economic and household risk factors, no evidence for common genetic control of intensity of hookworm and schistosome infection was observed (genetic correlation ?? G = 0.15; P= 0.52). There was some evidence for residual clustering within households (household correlation ?? C = 0.45; P= 0.06), but the majority (63%) of the covariance between N. americanusand S. mansoniinfection intensity remained specific to the individual and could not be explained by shared genes, shared environment or other shared demographic, socio-economic or environmental risk factors. Our results emphasize the importance of exposure to hookworm and schistosome infection in driving the association between levels of infection with these species in hosts resident in areas of high transmission and suggest that much of this common exposure occurs outside the home