Covariates

All children completed an interview-questionnaire in native language to collect demographic information, including date of birth and gender. Socioeconomic status (SES) was assessed using six questions as proxy-indicators: ‘Do you have a TV at home?’ (Yes/No), ‘Do you have a car at home?’ (Yes/No), How many brothers do you have?’, ‘How many sisters do you have?’, ‘Did you eat breakfast today?’ (Yes/No) and ‘Did you eat lunch yesterday?’ (Yes/No). Only the number of siblings (family size) was used as an indicator of SES in this study, since the other variables showed little variance. Children were also asked about the presence of mouth problems and abdominal pain at the time of examination.

Data from the WASH survey were used to obtain information on school characteristics, including the number of enrolees per school and the number of handwashing facilities with water and soap available. In addition, data on sanitation were collected, including the number, functionality and cleanliness of toilets, as proxy indicators of school maintenance and cleanliness in general, and as a covariate since there is evidence that access and cleanliness of sanitation facilities influences children’s parasitological health [25]. Data in the WASH survey were collected at baseline and follow-up through observations using an adapted version of the UNICEF WASH in Schools Monitoring Observational Tool [26]. Per country, two researchers of the local research team were trained to conduct the WASH survey in the schools. Toilets were scored as clean, partially clean or not clean, and as functional, partially functional and not functional. Toilets were subsequently classified as both clean and functional, partially clean and/or functional, or not clean and/or functional. Data from the WASH survey were solely used to describe the schools in the study sample, and for inclusion as potentially important covariates in the analysis of children’s health outcomes. The full findings of the WASH survey will be reported in a separate paper.