Malaria and helminth co-infections in children living in endemic countries: A systematic review with meta-analysis

Background Current knowledge on the burden of, and interactions between malaria and helminth co-infections, as well as the impact of the dual infections on anaemia, remains inconclusive. We have conducted a systematic review with meta-analysis to update current knowledge as a first step towards developing and deploying coordinated approaches to the control and, ultimately, elimination of malaria-helminth co-infections among children living in endemic countries. Methodology/Principal findings We searched Medline, Embase, Global Health and Web of Science from each database inception until 16 March 2020, for peer-reviewed articles reporting malaria-helminth co-infections in children living in endemic countries. No language restriction was applied. Following removal of duplicates, two reviewers independently screened the studies for eligibility. We used the summary odds ratio (OR) and 95% confidence intervals (CI) as a measure of association (random-effects model). We also performed Chi-square heterogeneity test based on Cochrane’s Q and evaluated the severity of heterogeneity using I2 statistics. The included studies were examined for publication bias using a funnel plot and statistical significance was assessed using Egger’s test (bias if p<0.1). Fifty-five of the 3,507 citations screened were eligible, 28 of which had sufficient data for meta-analysis. The 28 studies enrolled 22, 114 children in 13 countries across sub-Saharan Africa, Southeast Asia and South America. Overall, the pooled estimates showed a prevalence of Plasmodium-helminth co-infections of 17.7% (95% CI 12.7–23.2%). Summary estimates from 14 studies showed a lower odds of P. falciparum infection in children co-infected with Schistosoma spp (OR: 0.65; 95%CI: 0.37–1.16). Similar lower odds of P. falciparum infection were observed from the summary estimates of 24 studies in children co-infected with soil transmitted helminths (STH) (OR: 0.42; 95%CI: 0.28–0.64). When adjusted for age, gender, socio-economic status, nutritional status and geographic location of the children, the risk of P. falciparum infection in children co-infected with STH was higher compared with children who did not have STH infection (OR = 1.3; 95% CI 1.03–1.65). A subset of 16 studies showed that the odds of anaemia were higher in children co-infected with Plasmodium and STH than in children with Plasmodium infection alone (OR = 1.20; 95% CI: 0.59–2.45), and were almost equal in children co-infected with Plasmodium-Schistosoma spp or Plasmodium infection alone (OR = 0.97, 95% CI: 0.30–3.14). Conclusions/Significance The current review suggests that prevalence of malaria-helminth co-infection is high in children living in endemic countries. The nature of the interactions between malaria and helminth infection and the impact of the co-infection on anaemia remain inconclusive and may be modulated by the immune responses of the affected children.


IMPACT OF MALARIA-HELMINTH CO-INFECTIONS AMONG CHILDREN IN LOW AND MIDDLE INCOME COUNTRIES
STH alone account for a global burden of over 3.3 million disability-adjusted life years (3) and are associated with anaemia, malnutrition, and impaired physical and cognitive development among preschool-aged children (PSAC), and school-aged children (SAC) (6)(7)(8). Furthermore, empirical evidence shows that STH have a negative impact on the clinical outcome of malaria, especially among PSAC and SAC (9) . Infections with Schistosomes and STH have also been reported to exert deleterious effects on the course and outcome of clinical malaria (10). A longitudinal study amongst Gabonese school children showed that Trichuris trichiura or hookworm associated with schistosomiasis increased the risk of Plasmodium falciparum infection; whilst schistosomiasis is independently associated with a malaria increase in young children (11).
Statistical and spatial models support the geographic overlap and co-endemicity of falciparum malaria and hookworm infections in SSA and suggest that about 25% of school-age children are at risk of these two groups of infection (12). Similar spatial distribution has been documented for the association between malaria and schistosomiasis (8). Hookworm infection and schistosomiasis are both known to cause anaemia (2). Similarly, malaria is also a leading cause of severe anaemia.
Because the anaemia results from blood loss, haemolysis, an inflammatory process, and splenic sequestration, the additive low haemoglobin effect of the co-infections probably contributes substantially to mortality from malaria in children in SSA (13). Therefore, in low and mid income countries (LMIC) where malaria and helminths co-exist, there is potential for these infections acting together to cause severe anaemia, a situation previously described as a 'perfect storm of anaemia' (14).

Rationale for review
Despite the obvious effects of helminths on malaria infection, the impact of these co-infections is not clearly elucidated as the few existing studies report conflicting findings on the association between malaria and helminths. Given the variability in the mechanisms of immune activation by helminths and Plasmodium parasites within human host (15), and the fact that helminths downregulate immune responses to Plasmodium pathogens, it can logically be concluded that STH infection increase risk to Plasmodium infection and related clinical outcomes. Nevertheless, the complex interactions between helminths and malaria during co-infections involving multiple pathways make the association and impact of the co-infections unclear (4,5). While some studies reported protective effect of hookworms and S. haematobium infection (11,16) against Plasmodium infection, others reported increased Plasmodium infection in children infected with S.
In 2016, two systematic reviews with meta-analysis (4, 5) were published to address the limitations identified in the findings of two previous narrative reviews on the nature of the interactions and impact of malaria-helminth co-infections (18,19). Most of the studies included in the systematic reviews published in 2016 were cross-sectional in nature, making it difficult to conclude whether the observed high prevalence and density of P. falciparum infections and low prevalence P.
falciparum-related anaemia were due to STH infection. A moderate level of bias was also observed within the studies included in the reviews, thereby leading to an overestimation of the evidence of relationship reported between STH and asymptomatic/uncomplicated P. falciparum malaria in the reviews. Possible diagnostic inaccuracies for STH and malaria in the original studies included in the reviews might also have affected the conclusions, and the original studies may not have fully controlled the effect of different confounders that could affect the nature of relationship of STH and malaria (4,5).
To overcome these limitations, this systematic review will make use of new and innovative methods to generate real-time epidemiologic profiles of malaria and helminth co-infections amongst children in low and mid-income countries. This approach has the potential to augment the traditional database search that is usually fraught with the limitations of original studies included in systematic reviews. This review will improve understanding on the effect of P.falciparum and helminth co-infections on the epidemiology of malaria among children living in LMIC. This would strengthen the evidence towards developing and implementing integrated malaria-helminth control strategies in high-burden settings.

Aim and Objective
The objective of this study is to systematically review available data from new mapping tools on malaria and helminths and the literature from cross-sectional studies, cohort studies and clinical trials on the nature of interactions between P. falciparum malaria and helminth infections (STH and Schistosoma spp) among children living in LMIC.

Search Strategy
A systematic literature search will be conducted for scientific articles published in peer-reviewed journals, which reported P. falciparum malaria and helminth infections (STH and Schistosoma spp) among children living in LMIC. Conference abstracts without published full-texts will be excluded from the study. To allow for more eligible articles, no date and language restrictions will be applied on the search.

Search methods
Electronic databases: The following databases will be searched: Medline, EMBASE, Global Health and Web of Science for publications and conference abstracts using search terms described in To track more relevant citations, grey literature and a list of references of potentially relevant papers will be searched and potentially relevant citations will be included for eligibility screening.
Hand searches: Reference lists of all full-text articles identified for inclusion in the systematic review will be screened, as well as reference lists of relevant review articles.
Compilation of results: Searches of all sources described above will be completed by MA; and submitted for peer review by LSHTM Library Services to confirm that no relevant papers have been left out. Details of each search will be fully documented (including, but not limited to, information Malaria-helminth co-infections in children: systematic review v0.3; 20 Mar 2020 7 on search date, search strategy and number of results). Results (including titles and abstracts) will be compiled in Endnote X9, and any duplicates will be excluded.

Search Terms
A compound search strategy combining related truncated and non-truncated terms or synonyms will be used, and these will be tailored to each of the selected databases. For example, common terms (keywords or Medical Subject Headings-MeSH words) for malariahelminth co-infection will be combined with the other key concepts identifying this condition in children living in LMIC, using the Boolean operator -"OR" or "AND" when appropriate to the specific database. The proposed search strategy is summarized in Table 1.

Selection Criteria
The PICOST framework will be used to aid the selection of published articles relevant to the search question eligible for the review: Population of interest: pre-school and school aged children

Comparator: None
Outcome: anaemia, poor physical and cognitive development, death Setting: low and mid-income countries

Type of study:
Observational studies, interventional studies, systematic reviews, Cochrane reports

Inclusion criteria
1. Articles which are published in peer-reviewed journals.

Articles which described primary data findings from interventional or observational studies.
Exclusion criteria.

Case reports and case series
2. Studies whose full-texts could not be retrieved.
3. Duplicate articles describing the same subjects.

Selection of studies
The literature search will be conducted as described above by MA. All articles returned by the database search will be screened independently for eligibility by two reviewers (ED, BMA) based on titles and abstracts using the checklist summarized in Table 2, to exclude obviously non-relevant articles. There will be no restriction on study designs. Following this step, full-texts of the studies focusing on malaria-helminth co-infections will be accessed through the LSHTM Library and Archives and assessed for eligibility using the above-listed criteria. Table 2. Checklist for elimination of study by abstract screening.

Questions Yes
No Unclear 1. Is the publication in a peer-reviewed journal? 2. Was the article describing malaria and helminth coinfections? 3. If the study was on malaria-helminth co-infections, did it also focus on the impact of the co-infections? 4. Did the study report the risk, clinical course or interactions between malaria and helminth infections 5. Did the study involve paediatric population as defined above?
*If the answer to any of the questions 1-5 above is 'No' then the study is not eligible.
*If 'No' to all, but one 'Unclear', then retrieve the fulltext article for clarity.

Data Extraction
Data will be extracted using a standard form. To ensure there are no errors during the extraction process, the extracted data will be re-checked for accuracy by MA. Irrespective of the study design, the following data will be extracted from all included studies: first author and year of publication, country/setting, study title and objectives; confirmation of eligibility for review; methodologystudy design, sample size, study duration, data collection with study time points; population of interest; interventions; outcome -indicators; results; authors key discussions /comments / limitations; reviewer's comments.

Quality assessment/ risk of bias in individual studies
To ascertain the internal and external validity of the eligible studies, including the risk of bias, the quality of each study will be assessed using a combination of Newcastle Ottawa Scale (NOS) (20) and Effective Public Health Practice Project (EPHHP) tools (21), which are recommended for systematic reviews by The Cochrane Public Health Review Group (22). The NOS has been adapted for use in cross-sectional studies from the Newcastle-Ottawa Quality Assessment Scale for cohort studies to perform a quality assessment of cross-sectional studies for systematic review in other studies.
Therefore, it will be used to assess the cross-sectional studies in this review. Studies will be assessed from three angles: selection of the study groups, comparability of the study groups, and the ascertainment of the outcome of interest. The studies will be awarded a score ─ weak, moderate, or strong, for the overall quality. Each study will be assessed for the following: selection bias, study design, confounding, blinding, data collection methods, and withdrawals and drop-outs. The scale is summarized in Appendix 2.

Data Synthesis
Narrative synthesis: Narrative synthesis will be carried out alongside meta-analysis, using a framework which consists of three elements: 1. developing a preliminary synthesis of findings of included studies; 2. exploring relationships within and between studies; and 3. assessing the robustness of the synthesis. Studies will be grouped together if they compared similar types of outcomes of interest. Where available, references to p-values and confidence intervals of observed associations will be made in relation to their strengths and limitations.
Meta-analysis: Where feasible, considering potential heterogeneity in study designs, we will perform meta-analyses for the outcomes of malaria and helminth co-infections and nature of interactions between malaria and helminths. We will use random-effects meta-analysis to estimate pooled effect estimates across studies, allowing for between-study heterogeneity (23). We will examine heterogeneity using the I 2 statistic and publication bias using funnel plots and Begg's test for correlation between the effect estimate and their variances (24).
An influence analysis will also be performed to assess the robustness of the pooled summary effects by excluding each of the studies from the pooled estimate. Primary analyses will be stratified by gender. Sub-group analyses by (i) age and (ii) species of helminths will be performed to compare pooled effects and heterogeneity. Data will be analyzed using Stata version 16.0 (Stata Statistical Software, College Station. TX: Stata Corporation).

Registration and Reporting
The protocol for this systematic review will be registered on PROSPERO (https://www.crd.york.ac.uk/PROSPERO/). The review will be reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) (25).

Ethical considerations
As this is a systematic review of published articles, ethical approval is not required.

Dissemination of findings
Findings of the data extracted from eligible studies in this systematic review will be disseminated at scientific meetings and in a peer-reviewed journal.

Timelines
Timelines for completion of the work described in this protocol are shown in Table 3 Month  Timelines may vary according to numbers of search hits and full papers to be reviewed.

Rationale
3 Describe the rationale for the review in the context of what is already known.
Objectives 4 Provide an explicit statement of questions being addressed with reference to participants, interventions, comparisons, outcomes, and study design (PI-COS).

METHODS
Protocol and 5 Indicate if a review protocol exists, if and where it can be accessed (e.g., registration Web address), and, if available, provide registration information including registration number.
Eligibility crite-6 Specify study characteristics (e.g., PICOS, length of follow-up) and report ria characteristics (e.g., years considered, language, publication status) used as criteria for eligibility, giving rationale.
Information 7 Describe all information sources (e.g., databases with dates of coverage, sources contact with study authors to identify additional studies) in the search and date last searched.
Search 8 Present full electronic search strategy for at least one database, including any limits used, such that it could be repeated.
Study selec-9 State the process for selecting studies (i.e., screening, eligibility, included in tion systematic review, and, if applicable, included in the meta-analysis).
Data collection 10 Describe method of data extraction from reports (e.g., piloted forms, indeprocess pendently, in duplicate) and any processes for obtaining and confirming data from investigators.
Data items 11 List and define all variables for which data were sought (e.g., PICOS, funding sources) and any assumptions and simplifications made.
Risk of bias in 12 Describe methods used for assessing risk of bias of individual studies (inindividual stud-cluding specification of whether this was done at the study or outcome ies level), and how this information is to be used in any data synthesis.

DISCUSSION
Summary of evi-24 Summarize the main findings including the strength of evidence for dence each main outcome; consider their relevance to key groups (e.g., healthcare providers, users, and policy makers).
Limitations 25 Discuss limitations at study and outcome level (e.g., risk of bias), and at review-level (e.g., incomplete retrieval of identified research, reporting bias).
Conclusions 26 Provide a general interpretation of the results in the context of other evidence, and implications for future research.

FUNDING
Funding 27 Describe sources of funding for the systematic review and other support (e.g., supply of data); role of funders for the systematic review. 2) Sample size: a) Justified and satisfactory. * b) Not justified.
3) Non-respondents: a) Comparability between respondents and non-respondents characteristics is established, and the response rate is satisfactory. * b) The response rate is unsatisfactory, or the comparability between respondents and non-respondents is unsatisfactory. c) No description of the response rate or the characteristics of the responders and the non-responders. 2) Statistical test: a) The statistical test used to analyze the data is clearly described and appropriate, and the measurement of the association is presented, including confidence intervals and the probability level (p value). * b) The statistical test is not appropriate, not described or incomplete.