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

Abstract

Background The risk of infectious diseases, including snail-borne schistosomiasis, is determined by three interrelated 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. MethodsWe 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.

Associations of water contact frequency, duration, and activities with schistosome infection risk: A systematic review and meta-analysis

“…Almost any type of water contact activity was linked to increased infection risk despite expected differences in activities regarding contact duration and degree of bodily immersion [ 4 , 11 , 29 ]. Our results contrast with a previous systematic review (without meta-analysis) by Grimes et al which considered laundry, bathing and recreational swimming as the activities with most exposure and getting water as unimportant [ 46 ].…”

“…Such an occupational shift has been observed in Nigeria where fishing became an important water contact activity after dam construction [ 71 ]. More research is needed to understand the aquatic biology and behavioural patterns which shape infection outcomes associated with different waterbodies [ 4 , 6 ]. It is unclear if our finding of differential risk of different waterbodies (i.e., between rivers, lakes, and streams)–rather than reflecting substantial risk differences in these settings—may be spurious as some waterbodies are more well-studied than others.…”

“…Infection occurs when the free-swimming stage of the parasite penetrates a human host by boring through the skin. Local transmission depends on human water contact behaviour and environmental conditions including climate and the characteristics of waterbodies and their suitability for the parasite’s intermediate host; competent freshwater snail species [ 4 , 5 ]. While it is known that the high focality of schistosomiasis is due to the setting-specific interplay between humans and the environment, the factors determining local prevalence are complex and remain incompletely understood [ 4 , 6 ].…”