Exposure, hazard and vulnerability all contribute to Schistosoma haematobium re-infection in northern Senegal

The risk of infectious diseases, including snail-borne schistosomiasis, is
determined by three interrelated components: exposure, hazard and
vulnerability. For schistosomiasis, exposure occurs through behaviors
involving water contact, but not without the environmental hazard of
snails and parasites in the water. Socio-economic 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. We used cross-sectional
parasitological data from 821 school-aged children 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
fit 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. Multi-model 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. The most important component of S. haematobium
presence was hazard (Ʃwi = 0.95), followed by vulnerability (Ʃwi = 0.91),
while the most important component of S. haematobium intensity was
exposure (Ʃwi = 1.00), followed by hazard (Ʃwi = 0.77). Hazard was
positively associated with infection presence (OR = 1.49, 95% CI 1.12,
1.97), while exposure was positively associated with infection intensity.
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.