Study area and data collection
In early 2010, we invited all 204 schoolchildren attending grades 4-6 of Grand Moutcho primary school in Agboville, south Côte d'Ivoire, to participate in a cross-sectional epidemiological survey. In a first step, district health and education authorities, village leaders and teachers were informed about the purpose, procedures and potential risks of the study. After obtaining their oral agreement, written informed consent was sought from the parents or legal guardians of the children, whereas children assented orally.
In a next step, children were asked to fill out a brief questionnaire, assisted by their class teachers if needed. The questionnaire was based on two sections about physical functioning (PF) and physical role (PR) from the most widely used generic SF-36v2 questionnaire (Medical Outcome Trust, Boston, MA, USA; Health Assessment Lab, Boston, MA, USA; QualityMetric, Lincoln, RI, USA) [8, 14, 18]. Fourteen questions were included and readily adapted to the specific study setting. The questionnaire was pre-tested with support of the head of the school and further revised (see Additional file 1: Questionnaire used to assess self-reported physical fitness in the present study; in French).
Next, participating children were given plastic containers and invited to submit, on the next day, a small portion of their fresh morning stool. After stool collection, from 10:00 hours onwards, children were given a second plastic container and asked to bring a urine sample by 14:00 hours the latest. This procedure was repeated over two consecutive days. Stool and urine samples were transferred to the nearby hospital laboratory of the district town Agboville. Duplicate Kato-Katz thick smears were prepared from each of the two stool samples and quantitatively examined by experienced laboratory technicians for eggs of Schistosoma mansoni and soil-transmitted helminths (i.e. Ascaris lumbricoides, hookworm and Trichuris trichiura) on the same day . Urine samples were subjected to the filtration method and the number of Schistosoma haematobium eggs in a filtrate of 10 ml of urine counted under a microscope [20, 21]. Ten percent of all parasitological results were re-examined by a senior technician for quality control. In case of disagreement with initial findings, the results were discussed with the respective technician and the corresponding sample re-analysed until agreement was reached.
After the helminthological screening, children were clinically examined by a physician to check their general state of health. Additionally, a rapid diagnostic test (RDT) for malaria was performed (ICT ML01 malaria Pf kit, ICT Diagnostics; Cape Town, South Africa). Children with clinical malaria (defined as positive RDT plus recent history of fever), asthma (assessed by stethoscopy), anaemia (assessed by observing conjunctival vasculature ) or dyspnoea (assessed by stethoscopy), according to the physician's appraisal, were excluded from the subsequent fitness test, as participation was considered potentially harmful to them.
Finally, all remaining children were invited to participate in a maximal multistage 20 m shuttle run test to assess the cumulatively covered distance and the aerobe capacity, as measured by their maximal oxygen uptake, the so-called VO2max (expressed in ml kg-1 min-1) [23, 24]. The shuttle run test was conducted in groups of not more than 10 children. The obtained results were used as objectively determined proxies for the children's physical fitness. To ensure that children really tried to reach their maximal physical capacity, their heart rate was observed with a Polar RS400 watch (Polar Electro Europe BV; Zug, Switzerland) and only results of children with more than 180 heart beats per min were considered valid. Throughout the shuttle run test, we monitored ambient air temperature and humidity, as these external factors might influence children's test performance.
The study was approved by the institutional research commission of the Swiss Tropical and Public Health Institute (Basel, Switzerland) and received clearance from the ethics committees of Basel (EKBB, reference no. 377/09) and Côte d'Ivoire (reference no. 1993 MSHP/CNER). Insurance coverage was obtained from GNA Assurance (Abidjan, Côte d'Ivoire; policy no. 30105811010001).
At the end of the study, all children attending Grand Moutcho primary school were administered praziquantel (single 40 mg/kg oral dose) and albendazole (single 400 mg oral dose) free of charge, irrespective of their helminth infection status and whether or not they participated in the study. Children who required further medical treatment were referred to the local health service.
Data were double-entered and cross-checked in Access version 2007 (Microsoft Corporation; Redmond, WA, USA) and analysed in STATA version 10.1 (STATA Corporation; College Station, TX, USA). Questionnaire answers were coded as 1, 2 or 3 (for some questions also 4) with lower scores given to reports of more problems in a certain activity. The individual scores from questions 1 to 10 and 11 to 14, respectively (see Additional file 1
: Questionnaire used to assess self-reported physical fitness in the present study) were summed up in order to obtain a summary measure on PF (questions 1 to 10) and PR (questions 11 to 14). While PF is a summary measure for the ability to fulfil distinct physical tasks (e.g. walking, running and climbing), PR pertains to the physical potential to handle certain (social) roles (e.g. learning, helping and playing). According to this procedure, higher values for PF and PR indicate fewer problems in the respective domain. In a last step, scores for PF and PR were transformed to values between 0 and 100, according to equation (1
Helminth infection intensities were classified as light, moderate and heavy, using readily available guidelines from the World Health Organization (WHO) [26
]. Children's VO2
max was derived from equation (2
), considering age (X1
= age in years), the achieved maximal shuttle running speed (X2
= speed in km/h) and a linear relation according to Léger and Mercier [23
Two different samples were considered in the final analysis in order to assess also the value added by a QoL questionnaire. Sample 1 consisted of all children with complete questionnaire, parasitological and clinical data records. Sample 2 included all children from sample 1 who had not only complete questionnaire, parasitological and clinical data, but also valid physical fitness test results. An attrition analysis was carried out with those children who were included in sample 1, but not in sample 2, i.e. who had complete data records except for the physical fitness test. Besides descriptive statistics, Spearman rank correlation coefficient, uni- and multivariable linear regression models adjusted for participants' age and sex, ambient air temperature and humidity, Fisher's test, χ² and t-test statistics were employed as appropriate to assess statistical significance (p < 0.05).