Urogenital schistosomiasis caused by Schistosoma haematobium, is one of the major neglected tropical diseases (1–3). The World Health Organization (WHO) revealed that 251.4 million cases of schistosomiasis are recorded annually at a global scale, and more than 50% of the cases are predominantly children living in Africa (4, 5). The need for a well-understood pathophysiological mechanism of schistosomiasis, together with accurate disease diagnosis, are crucial elements for the successful control and elimination of the disease. In this regard, there has been an increase in concerted efforts aimed at identifying infection biomarkers to be utilised in the development of diagnostic tools and identification of therapeutic targets (6, 7).

Despite the availability of evidence on the rising morbidity and mortality rates of urogenital schistosomiasis in children (8), there has been a dearth of studies on schistosomiasis in pre-school aged children (PSAC) under the age of five. This knowledge gap has largely been attributed to a number of factors including, difficulties in obtaining parasitological samples, poor diagnostic tools and limited knowledge on risk factors for the age group (9, 10). One of the questions that remains unanswered within this age group is the effect of schistosome infection on the clinically relevant metabolic processes (11). Furthermore, these challenges have been exacerbated by the non-specificity of morbidity biomarkers associated with schistosomiasis in paediatrics (6). The non-specificity of these biomarkers is due to physiological, biochemical and immunological changes and their determination is even more complex in co-infections (12).

Currently, the gold standard method for diagnosing urogenital schistosomiasis is microscopic detection of eggs in urine (13). A major caveat of this technique is the low sensitivity in the detection of light infections that are prevalent in the pre-school aged group and in low schistosome transmission areas (14, 15). Severe metabolic alterations may have occurred to the paediatric patient by the time the eggs are detected (16). On the other hand the standard technique is not able to identify single worm or same sex schistosome infections, as the eggs are only produced when both male and female pair up (17). To mitigate the shortcomings that are associated with the microscopic technique, several methods such as, the polymerase chain reaction and detection of circulating anodic and cathodic antigens have since been developed (18, 19). However, these methods are expensive and often require expertise and complex instrumentation to execute making them unsuitable for use in resource-limited settings. Therefore, finding diagnostic biomarkers with high sensitivity that can detect schistosome infection earlier would be key in reducing the burden of the disease (6).

Spectrometry based metabolomics approaches have expanded rapidly in the field of science for the systematic identification, visualisation and characterisation of infection biomarkers (20, 21). The feasibility of metabolomics in the discovery of biomarkers falls on the premise that metabolites are the key elements in biological pathways and in the presence of a disease, notable changes will occur in these metabolic pathways (22). Consequently, parallel assessment of multiple metabolites during infection enables identification of potent biomarkers that can be used in the development of cost-effective and highly sensitive diagnostic tools (7, 23).

The analysis of host metabolite composition is a holistic approach in identifying novel or potential infection biomarkers. These biomarkers may be indicative of the host’s response to infection and hence, shed light on the underlying pathophysiological mechanisms in urogenital schistosomiasis in pre-school aged children (PSAC) (24). The scarcity of information pertaining to the pathological alterations at the metabolic level in S. haematobium infection in PSAC has provided an impetus for the current investigation. This study aims to profile plasma metabolites that can be used as infection biomarkers in PSAC below the age five.

Metabolomics is a more holistic approach in the identification of biomarkers. In this study, selected plasma metabolites were analysed in samples from PSAC with urogenital schistosomiasis and those without the infection. Additionally, their impact on various biochemical pathways was determined.

Five plasma metabolites (glucose-6-phosphate, succinate, phosphatidylcholine, alanine and creatinine) were associated with S. haematobium infection in PSAC at the population level. However, the ranges in plasma concentration of these metabolites showed considerable overlap between infected and uninfected individual children which precludes their use as robust biomarkers of infection. The concentration of glucose-6-phospate was significantly increased during schistosome infection. Glucose-6-phosphate is a crucial metabolite in the mainstream of carbohydrate metabolism; particularly in the glycolytic pathway, the pentose phosphate pathway, glycogen synthesis, hexosamine pathway, and glucose production depending on dietary or hormonal conditions. Consequently, its alteration has a significant impact on all its connected metabolic pathways (33). The findings obtained in this study are strikingly similar to those obtained in a study conducted by Osakunor and colleagues, who reported a 2-fold increase in glycolytic pathway metabolites glucose-6-phosphate and 3-phoshoglycerate. Osakunor and colleagues concluded that these metabolite changes are consistent with parasite-related clinical manifestations of malnutrition and poor growth observed in schistosome-infected children (34).

Phosphatidylcholine was the second most upregulated metabolite during urogenital schistosomiasis. Phosphatidylcholine is a phospholipid that is found ubiquitously in mammalian cell membranes and plays a pivotal role in both proliferative growth and programmed cell death (35, 36). Several mechanistic investigations on the choline phospholipids revealed that the metabolite promotes carcinogenesis through induction of genomic and autophagy processes that allow the survival of tumour cells using minimum energetic sources (37, 38). During chronic urogenital schistosomiasis, bladder cancer manifestations may occur as a result of inflammation and healing of granulomas thus damaging ureteric musculature (39).

Amongst the most downregulated metabolites, and associated with urogenital schistosomiasis, was creatinine which is a catabolic product formed by the degradation of creatine (N-aminoiminomethyl-N-methyl glycine), a nitrogen-containing compound synthesised from amino acids containing guanidine and phosphate groups that are mainly found in the muscles. Creatine plays a critical role in providing energy through the phosphoryl transferase activity of creatine kinase. Poor development of cognitive skills is one of the effects of urogenital schistosomiasis in paediatrics. These clinical manifestations can be attributed to the downregulation of creatine. This is underscored by reports from several studies indicating that low brain creatine content is associated with voluntary or involuntary movement disorders that can lead to autism, epilepsy, cognitive and motor development delays (40–43). This is espoused by a number of different studies which have further demonstrated an improvement in brain bioenergetics and neuroprotective benefits, including cognition upon dietary supplementation with creatine (44–47). Therefore, the addition of creatine in chemotherapeutic treatment of urogenital schistosomiasis may attenuate mental fatigue and improve cognition.

High-density lipoproteins are a family of complex metabolites that can exhibit fundamentally diverse metabolic functions that include, transportation of cholesterol from peripheral tissues to the liver for disposal and steroidogenic tissues for hormone production (48, 49). The current data in our study revealed a down regulation of high-density lipoproteins. Our findings are similar to a separate lipodomics study conducted by Zinsou and colleagues (50), who reported that circulating levels of lipid species associated with cholesterol-rich lipoprotein particles were significantly reduced in schistosome-infected individuals in an intensity-dependent manner. Identical findings were also reported in several investigations in humans with S. mansoni species which showed the occurrence of low lipid profile or dyslipidemia in patients as compared with healthy groups (51, 52).

Succinate is an intermediate metabolite of the Krebs cycle which is one the critical process in cellular respiration (53). In the current study, succinate was down regulated and statistically different between schistosome infected and uninfected PSAC. Severe inflammation of the bladder is one of the clinical symptoms of urogenital schistosomiasis and succinate has been reported to accumulate in certain pathophysiological situations associated with inflammation and metabolic stress (54, 55). Furthermore, the metabolite has been shown to be a mediator between microbial metabolism and intestinal mucosal immune cell development (56). Moderate concentrations of succinate are required to stimulate inflammation to clear pathogens, however down regulation of the metabolite results in poor activation of the innate immunity that drives inflammatory responses during infection (57).

It is clear from the metabolic impact data that urogenital schistosomiasis in PSAC, dysregulates more than six metabolic pathways. This metabolic dysregulation may consequently lead to severe pathological symptoms. Our observations show that sucrose, glucose and tricarboxylic acid cycle (Kreb’s cycle) were predominantly affected. Similarly, in a separate study investigating metabolic changes during schistosomiasis, using mice models, Wu et al. (58) reported significant elevation of urinary 3-ureidopropionate and most importantly the disturbance of glycolysis where glucose and sucrose are the main initiating metabolites and depression of tricarboxylic acid cycle. In another separate metabolomics study, Wang et al. (59) revealed that tricarboxylic acid cycle and its intermediates such as citrate and succinate and pyruvate were dysregulated upon infection. Taken together, these findings from elsewhere, are in agreement with the metabolomic impact assessment observations of our current investigation.

It is noteworthy that purine synthesis and steroid synthesis were amongst the dysregulated metabolic pathways observed in this study during urogenital schistosomiasis. Previous studies have highlighted the production of orthologous enzymes involved in the conversion of progesterone and pregnenolone to estriol and estrone androsterone in schistosome infection (60–63). Oliveira et al. (64) reported disturbance in oestradiol, testosterone and progesterone in hamster models during schistosome infection and these hormones may largely contribute to sterility and infertility in female urogenital schistosomiasis.

The current study assessed the diagnostic performance of selected plasma metabolites during S. haematobium infection in PSAC and five indicators of infection were identified namely; glucose-6-phosphate, phosphatidylcholine, succinate, alanine and creatinine. Glucose-6-phosphate had highest specificity, sensitivity and AUC value thus a valuable infection indicator in the discrimination between S. haematobium infected and healthy individuals at a population level. Thus, there is a need to explore this metabolite and its dynamics during schistosomiasis for it to be inclusively used as a biomarker in PSAC. Considering the poor sensitivity that is associated with the diagnostic tools that are currently applied in parasitological diagnosis for low intensity schistosome infections and the challenges bedevilling the cost prohibitive, more sensitive tools such as PCR, metabolomics provides a channel for the development of cost effective and highly sensitive metabolite based rapid diagnostic tools (65, 66).

In this current study, we acknowledge confounding factors such as co-infections, diet and environmental conditions have been reported to impact plasma metabolite composition and concentration (67–69). Additionally, variations in fold concentration change for metabolites have been shown to be influenced by type of biofluid sample and metabolomic technique used for the analysis (70, 71). Nevertheless, this study reveals potential biomarkers of infection during urogenital schistosomiasis in PSAC. Furthermore, our investigation provides insight on alteration of metabolic pathways as a consequence of S. haematobium infection which helps to explain some of the pathophysiological changes and clinical symptom manifestations in the disease state for this particular young age group.

In this study, five plasma metabolites were significantly associated with urogenital schistosomiasis with glucose-6-phosphate and creatinine being most upregulated and downregulated respectively. As this field of metabolomics is gaining momentum in helminths research and biomarker discovery, we recommend further research to determine metabolite profiles in urine in PSAC and to further validate plasma metabolites especially glucose-6-phosphate as it showed potential as an indicator for use at the population level.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving humans were approved by Medical Research Council of Zimbabwe (MRCZ). The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent for participation in this study was provided by the participants’ legal guardians/next of kin.

HM: Conceptualisation, Data curation, Formal analysis, Investigation, Methodology, Software, Writing – original draft, Writing – review & editing, Resources. TN: Investigation, Resources, Supervision, Writing – original draft, Writing – review & editing. AV: Data curation, Formal analysis, Investigation, Methodology, Writing – original draft, Writing – review & editing. EC: Formal analysis, Investigation, Writing – review & editing. PM: Data curation, Formal analysis, Investigation, Visualisation, Writing – original draft, Writing – review & editing. MK: Formal analysis, Investigation, Methodology, Writing – review & editing. TM-J: Conceptualisation, Data curation, Formal analysis, Investigation, Writing – review & editing. VM: Formal analysis, Software, Visualisation, Writing – review & editing, Methodology. EN: Data curation, Funding acquisition, Methodology, Project administration, Resources, Software, Writing – review & editing. DK: Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Writing – review & editing. LM: Data curation, Funding acquisition, Methodology, Project administration, Resources, Software, Writing – review & editing. FM: Funding acquisition, Investigation, Methodology, Resources, Supervision, Validation, Visualisation, Writing – review & editing. TM: Conceptualisation, Funding acquisition, Investigation, Resources, Supervision, Validation, Writing – original draft, Writing – review & editing, Methodology.