A semi-structured questionnaire was pre-tested on another community with similar characteristics as the study community and the questions were modified accordingly before being administered. This pre-tested questionnaire of approximately 5 min or less in length per respondent was administered to each participant by a trained research assistant with the aid of the teachers to obtain information on demography, personal hygiene and practices, health status and possible risk factors of Plasmodium and helminth infections as well as malnutrition and anaemia. The questionnaire was administered in English language and with a few exceptions in Pidgin English, the most widely spoken language in the area. The ages of participants were obtained from the school register.

The axillary temperature was measured using a digital thermometer and a participant was considered febrile, if the body temperature was ≥ 37.5 °C. The height was measured to the nearest 0.1 cm using a graduated ruler of length 2 m. The body mass was measured to the nearest 0.5 kg using a mechanical scale of capacity 120 kg (KINLEE® model BR9310, Guangdong, China), and mid upper arm circumference (MUAC) was measured using a graduated tape. These measurements were used to calculate an array of anthropometric indices used as proxies for malnutrition: weight-for-age (WA: under-weight); height-for-age (HA: stunting) and weight-for height (WH: wasting). Anthropometric indices were computed as z-scores based on the WHO growth reference curves using the WHO AnthroPlus for personal computers manual [29]. Underweight was defined as a weight-for-age z (WAZ) score of <  − 2, wasting as a weight-for-height z (WHZ) score of <  − 2 and stunting as height-for-age z (HAZ) score of <  − 2. A child was identified as being malnourished if he or she scored <  − 2 in one of the anthropometric indices of WA, HA and WH indices [29].

Approximately 2 ml of venous blood was collected in ethylenediamine tetra-acetate tubes for malaria parasite detection and haematological analysis. Thick and thin blood films were prepared in situ, following standard operational protocol [30]. Thin blood films fixed in methanol and thick blood films were Giemsa stained and examined microscopically following standard procedures [30]. Slides were considered positive when asexual forms and/or gametocytes of any Plasmodium species were observed on the blood film. All the slides were read twice by two independent microscopists. Malaria parasite per µl of blood was determined by counting the number of parasites per 200 leukocytes and multiplying by the individuals white blood cell (WBC) count. Parasitaemia was classified as low (≤ 500 parasite/µl of blood), moderate (501–5000 parasites/µl of blood) and high (> 5000 parasites/µl of blood).

A complete blood count was ran using a Beckman Coulter counter (Urit 3300, Guilin Botest Medical Electronic Co., Ltd., Guilin, China) that automatically gave values for red blood cell (RBC), WBC and platelet counts, haemoglobin (Hb), haematocrit (Hct), mean cell volume (MCV), mean cell haemoglobin (MCH), mean cell haemoglobin concentration (MCHC) following the manufacturer’s instructions. The classification of anaemia (Hb concentration below the WHO reference values for age or gender) and its severity was done following WHO standards (mild anaemia = 100–109 g/L, moderate anaemia = 70–99 g/L and severe anaemia < 70 g/L) [25, 26]. Leucopenia was defined as WBC < 4.5 × 109/L, hypochromasia as MCHC < 32 g/L [31], microcytosis as MCV < 73 fl and thrombocytopenia was defined as platelet count < 150 000/μl.

About 25 ml of midstream urine was collected into plastic screw cap vials after a brisk exercise between 10:00 am and 2:00 pm. Gross haematuria was determined by visual observation while micro haematuria was determined with the aid of reagent strips (Combistix) following the manufacturers guide (CYBOW™ 11 M a series of Health Mate Ref 0974, DFI Co., Ltd, Gomo-ro, 388–25 Korea). Following agitation, 10 ml of urine was drawn using a syringe and filtered through a polycarbonate membrane filter (STERLITECH corporation, Washington, USA). The filter membrane was examined microscopically for the presence of schistosome eggs as described by Cheesbrough [25]. Schistosome egg density was expressed as the number of eggs in 10 ml urine (eggs/10 ml) and the intensity of infection was categorised as either light (< 50 eggs/10 ml) or heavy infection (≥ 50 eggs/10 ml) [32, 33].

Fresh stool samples were collected, smears were prepared and examined using the Kato-Katz thick smear method, as described by Cheesbrough [30]. Duplicate smears were prepared for each specimen using a 41.7 mg Kato-Katz template. Each slide was allowed to clear for 30 min, and then examined at 100 × total magnification within one hour of preparation to avoid missing hookworm eggs. The number of eggs counted per slide was multiplied by 24 to obtain the egg count per gram (epg) of faeces. As a quality control measure, all positive slides and 10% of randomly selected negative smears were re-examined by a third parasitologist who had no knowledge of the previous results. An average of the counts was utilised. Children were classified as having light (1–4999; 1–999 epg), moderate (5000–49 999; 1000–9999 epg) or heavy (≥ 50 000; ≥ 10 000 epg) infections for A. lumbricoides and T. trichiura respectively [34].

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