Schistosomes (primarily Schistosoma mansoni, S. haematobium, and S. japonicum) and soil-transmitted helminths (STHs: Ascaris lumbricoides, Trichuris trichiura, and hookworms) are prevalent parasites in many tropical countries. Their life cycles suggest that water, sanitation, and hygiene (WASH) might reduce their transmission. However, this field has suffered from a lack of rigorous studies with sufficient statistical power.
A systematic review and meta-analysis determined that people with access to safe water and adequate sanitation have significantly lower odds of schistosome infection, although there was a risk of socioeconomic confounding (that is, people of higher socioeconomic status having better WASH and being protected from infection for other reasons). A more qualitative review identified many sources of complexity and non-linearity between WASH exposures and schistosome infection outcomes.
Next, in a survey in and around 30 schools in southern Ethiopia, children were tested for the parasites, and school-, household-, and child-level WASH facilities and practices were assessed. Child- and household-level sanitation risk factors were compared with hookworm infection (the other helminths being very rare), but no significant associations were found. Finally, a school-level WASH survey was integrated into an Ethiopian national mapping programme for schistosomes and STHs, and data were collected from 1,645 schools. School-level scores were constructed, reflecting exposure to potentially schistosome-infested water during the collection of water for school, and the adequacy of school sanitation and hygiene facilities. These were compared with school-level arithmetic mean infection intensities for S. mansoni and the STHs, using Kendall's τb. Statistically significant associations were found for water and S. mansoni, sanitation and A. lumbricoides, and hygiene and hookworm, suggesting that these are the WASH elements best suited to the control of the respective parasites.