Exposure, hazard, and vulnerability and their contribution to Schistosoma haematobium re-infection in northern Senegal

Background The risk of infectious diseases, including snail-borne schistosomiasis, is determined by three inter-related components: exposure, hazard, and vulnerability. For schistosomiasis, exposure occurs through behaviours involving water contact, but not without the environmental hazard of snails and parasites in the water. Socioeconomic vulnerability makes it difficult to reduce exposure in the presence of hazard, because it increases reliance on hazardous activities and environments. We aimed to quantify the contributions of exposure, hazard, and vulnerability to schistosome re-infection presence and intensity. Methods We used cross-sectional parasitological data from 821 school-aged children (5–15 years) in 13 villages along the Senegal River, survey data from 411 households where those children resided, and ecological data from all 24 village water contact sites. We fitted mixed-effects logistic and negative binomial regressions with indices of exposure, hazard, and vulnerability as explanatory variables of Schistosoma haematobium infection, along with demographic control variables. Multimodel inference was used to determine the relative importance of each component of risk and model averaging was used to quantify associations between infection outcomes and indices of hazard, exposure, and vulnerability. Findings The most important component of S haematobium presence was hazard (Σwi=0·95), followed by vulnerability (Σwi=0·91), and the most important component of S haematobium intensity was exposure (Σwi=1·00), followed by hazard (Σwi=0·76). Hazard was positively associated with infection presence (odds ratio [OR]=1·49, 95% CI 1·12–1·97), whereas exposure was positively associated with infection intensity. Interpretation Our findings highlight how social (exposure and vulnerability) and environmental (hazard) processes act together to facilitate the acquisition and accumulation of schistosome infection from the environment across time and space. This approach can inform targeting of social and environmental interventions as complements to mass drug administration. Funding National Science Foundation Couple Natural Human Systems programme (1414102), Bill and Melinda Gates Foundation (OPP1114050), James and Nancy Kelso Fellowship through the Stanford Interdisciplinary Graduate Fellowship programme at Stanford University (AJL), Michigan Society of Fellows at the University of Michigan, a Sloan Research Fellowship from the Alfred P Sloan Foundation, a UW Innovation Award and a grant from the National Science Foundation (CLW; OCE-1829509).