Retrieved Studies ----------------- The search yielded a total of 47,589 articles from PubMed (n = 21,718), Embase (n = 18,188), Web of Knowledge (n = 7,502), and LILACS (n = 181), with 42,882 unique records. Our PRISMA flow diagram is available in [Figure 1][1]. After reviewing titles and abstracts, we examined 397 articles more intensively: 264 were excluded for lacking a relevant effect measure, 30 were excluded for aggregating non-STH infections in the outcome, and 11 were excluded for being review or editorial articles (see [Tables 3][2], [4][3], and [5][4] for included studies and [S1][5] for excluded ones). We contacted 11 authors to obtain additional data [53]–[60], but only three authors responded [61]–[63]. A total of 94 studies ultimately met our inclusion criteria, yielding over 450 estimates of effect. Retrieved data included findings from one unpublished investigation [64] and one publication with information about two related studies [65]. Most included studies were published in English (n = 86), though articles in Portuguese (n = 4), Chinese (n = 2), and Spanish (n = 2) were also included. Studies researched populations in Asia (n = 42), Africa (n = 29), and the Americas (n = 23). Studies investigated access and practices relating to water (n = 56), sanitation (n = 79), and hygiene (n = 53) ([Figure 2][6]); the most commonly explored were access to sanitation (n = 63), access to water (n = 45), handwashing (n = 30), and wearing shoes (n = 27). Studies reported investigating infection with A. lumbricoides (n = 69), T. trichiura (n = 60), hookworm (n = 63), S. stercoralis (n = 12), and any STH collectively (n = 52). [Tables 6][7] and [7][8] illustrate the number of articles in which both specific WASH components and helminth infections were investigated. Of 94 studies, 89 were observational: 75 used a cross-sectional epidemiologic design, 13 were prospective, and the remaining was a case-control study. Most studies investigated multiple potential risk factors for STH infection. Exposure status for WASH access and practices was typically determined through self-report, although 15 studies also used some form of observation to validate self-reported information. All included studies reported the diagnostic method used to assess helminth infection, with the Kato-Katz technique most frequently mentioned (n = 63). To assess the independent effect of WASH components on STH infection, authors typically used multiple regression analysis (n = 68), though adjusted effect estimates were often not reported for WASH covariates if they were not statistically significant. Not all multivariable models were reported with a full list of included covariates either. Slightly more than one-third of the studies (n = 33) reported at least one non-significant adjusted effect estimate. Study bias assessment is presented in [Table S2][9]. Meta-analysis results are available in [Table 8][10] and grades summarized in [Table 9][11]. Water ----- Water-related access and practices were generally associated with lower odds of STH infection. We conducted meta-analyses to examine the association of piped water access and use of treated water on STH infection. Using treated water (filtered or boiled) was associated with lower likelihood of having any STH infection (k = 3, OR 0.46, 95% CI 0.36–0.60). The quality of evidence for the analysis was low, as all three studies were observational ([Figure 3][12]). Use of piped water was not associated with STH infection in general (k = 5, OR 0.93, 95% CI 0.28–3.11). The quality of evidence for the pooled estimate was very low due to high heterogeneity (I2 = 98.6%, 95% CI 98%–99%, Q p-value<0.01) among the studies ([Figure 4][13]). The heterogeneity could have stemmed from multiple factors, as the five studies shared few methodological characteristics. Use of piped water was associated with reduced likelihood of A. lumbricoides infection (k = 4, OR 0.40, 95% CI 0.39–0.41) and T. trichiura infection (k = 3, OR 0.57, 95% CI 0.45–0.72). Evidence quality for these two meta-analyses was low, based on four studies and three studies respectively ([Figures 5][14] and [6][15]). We did not find a sufficient number of studies to conduct a similar meta-analysis for hookworm infection, although Nasr and colleagues found a significantly lower adjusted odds of infection (OR 0.59, 95% CI 0.34–0.91) for Malaysian children with access to piped water [66]. Other researchers found no statistically significant associations between piped water access and hookworm infection [67],[68]. Other water-related exposures for STH infection were reported in the literature, but not with sufficient frequency for meta-analyses. In one study examining storage of water, Quintero and colleagues found a significantly higher adjusted odds of T. trichiura infection for Venezuelan children and adults collecting water in “inappropriate” receptacles (OR 1.12, 95% CI 1.09–1.15) [69]. Limited evidence also was retrieved on the influence of water source location; Belyhun and colleagues [70] found a beneficial association of using an outside water pipe compared to an indoor tap for infection with any STH among Ethiopian infants (OR 0.21, 95% CI 0.09–0.51). Matthys and colleagues [71] found that having a private well significantly increased the odds of hookworm infection for farming households in western Côte d'Ivoire (OR 2.32, 95% CI 1.24–4.05). No evidence was found of an association between public or private water source and S. stercoralis infection [72]. Having “inadequate water supply” in schools was strongly associated with increased infection with any STH among school children living on Pacific islands (OR 4.93, 95% CI 2.24–10.88) [73]. Sanitation ---------- Sanitation access (availability or use of latrines) was associated with lower likelihood of infection with any STH (k = 8, OR 0.66, 95% CI 0.57–0.76), T. trichiura (k = 7, OR 0.61, 95% CI 0.50–0.74), and A. lumbricoides (k = 6, OR 0.62, 95% CI 0.44–0.88) ([Figures 7][16], [8][17], and [9][18]). The quality of evidence for these meta-analyses was low due to the observational nature of included studies. We did not find evidence that sanitation access was associated with hookworm infection (k = 6, OR 0.80, 95% CI 0.61–1.06), which had very low evidence quality due to imprecision ([Figure 10][19]). We found limited evidence that use of shared or private sanitation facilities influenced odds of STH infection. Worrell and colleagues [74] found in Kenya that participants using toilets located outside of their household premises had significantly increased odds of infection with any STH. In contrast, another study found that sharing latrines with neighboring households, compared with private latrine use, was associated with significantly lower odds of hookworm infection [71]. Few details were provided to contextualize this finding. Hygiene ------- Three randomized controlled trials, two carried out in China and one in the Peruvian Amazon, found strong benefits for interventions that focused on promoting hygiene in schools [75]–[77]. Xu and colleagues [75] assessed a randomized intervention that promoted handwashing with soap, both before eating and after defecation among 657 school children in three schools. All infected children were treated at baseline. At the 1-year follow-up, A. lumbricoides prevalence for children in the experimental group had declined by 35.7% (pre-intervention prevalence, 68.3%; post-intervention cumulative infection rate, 43.9%) compared with an increase in the control group of 78% (pre-intervention, 41.4%; post-intervention, 73.7%); this was a statistically significant difference (p<0.01). The study's primary limitation was that schools were the unit of randomization, with two primary schools becoming controls and the third receiving the intervention. With so few clusters, it is highly possible that confounding factors were not comparable between the control and experimental groups. More recently, Bieri and colleagues [76] reported on a single-blind, unmatched, cluster-randomized intervention trial involving 1,718 children (aged 9–10) in 38 schools over the course of one school year. Schools were randomly assigned to a health-education package, which included an entertainment-education cartoon video, or to a control package, which only displayed a health-education poster. All participants were treated with albendazole at baseline. At follow-up at the end of the school year, knowledge about STH was significantly higher in the intervention group, and almost twice as many intervention children (63.3% versus 33.4%, p<0.01) reported washing their hands after defecating. The incidence of STH infection (predominantly T. trichiura and A. lumbricoides) was also significantly improved in the experimental schools: 50% lower in the intervention group than in the control group (4.1% versus 8.4%, p<0.01). Gyorkos and colleagues [77] conducted an open-label, cluster-randomized controlled trial using a hygiene education intervention in Peruvian primary schools. Within paired groups, 18 schools (1,089 fifth grade student participants) were randomly allocated to receive albendazole and the hygiene intervention or albendazole alone. The health intervention included a helminth-oriented class for students, a health curriculum workshop for teachers, and educational print materials. Four months after the intervention, the experimental group showed a significant reduction in A. lumbricoides intensity compared to deworming alone (adjusted incidence rate ratio [IRR] 0.42, 95% CI: 0.21–0.85). T. trichiura and hookworm intensity did not show statistically significant improvements in the experimental group, nor did prevalence of any single STH species. Children in the intervention group showed significant improvements in STH knowledge and water treatment behaviors compared to the control, but not in most other hygiene practices (e.g., handwashing). The authors also noted that the prevalence of hookworm was low (about 5% compared to 30% for A. lumbricoides and 50% for T. trichiura) and that albendazole was less efficacious against T. trichiura than it was against A. lumbricoides. Our meta-analyses of hygiene-related observational evidence provided estimates that are consistent with findings from these randomized controlled trials. Soap use or availability was significantly associated with lower odds of STH infection at the 5% level (k = 3, OR 0.53, 95% CI 0.29–0.98). The quality of the evidence was low, though the possibility of respondents' over-reporting hygiene behaviors could have underestimated the strength of the association ([Figure 11][20]). Handwashing, both before eating (k = 3, OR 0.38, 95% CI 0.26–0.55) and after defecating (k = 3, OR 0.45, 95% CI 0.35–0.58), was associated with lower odds of A. lumbricoides infection ([Figures 12][21] and [13][22]). Both analyses were of low quality due to the observational evidence available. Handwashing after defecation also was associated with reduced odds of any STH infection (k = 5, OR 0.47, 95% CI 0.24–0.90). This meta-analysis had very low evidence quality due to high heterogeneity among estimates from the five pooled studies (I2 = 88%, 95% CI 74%–94%, Q p-value<0.01, [Figure 14][23]). All studies used Kato-Katz for diagnosis, but varied considerably in most other study characteristics, including population age, baseline prevalence, and geographic setting. Balen and colleagues reported limited evidence of a dose-response effect for handwashing; respondents who more frequently washed their hands with soap after defecation had lower odds of infection with any STH, but confidence intervals of the handwashing groups overlapped [78]. Washing vegetables was found to be associated with lower odds of STH infection in two studies. Steinmann and colleagues [79] found washing vegetables to be negatively associated with A. lumbricoides infection in school children (OR 0.69, 95% CI 0.50–0.95), while Hohmann and colleagues [80] found washing was associated with lower odds of T. trichiura (OR 0.50, 95% CI 0.31–0.79) and any STH infection (OR 0.71, 95% CI 0.51–0.99). Our meta-analysis found evidence of a strong association between wearing shoes and lower odds of hookworm infection (k = 5, OR 0.29, 95% CI 0.18–0.47). The quality of the evidence was moderate, upgraded due to the magnitude of effect ([Figure 15][24]). Wearing shoes was also associated with lower odds of infection with any STH (k = 3, OR 0.30, 95% CI 0.11–0.83). The evidence quality for that analysis was low, downgraded by heterogeneity (I2 = 74%, 95% CI 12–92%, Q p-value = 0.02) ([Figure 16][25]) but upgraded by a strong effect magnitude. Heterogeneity could have been introduced by many different factors, as the studies shared few characteristics. Three studies found mostly non-significant associations between geophagy (i.e., consumption of soil) and STH infection [81]–[83]. In adjusted models, households with dirt floors in India and Venezuela were found to have higher odds of T. trichiura and A. lumbricoides infection than were houses with other more elaborate flooring material [69],[84]. Young children living with dirt floors in Colombia also showed higher odds of infection with any STH compared to those with tile or cement floors [85]. Integrated Interventions ------------------------ In a cluster-randomized controlled trial, Freeman and colleagues examined a comprehensive WASH intervention in Kenyan schools that included hygiene promotion, water treatment and storage, and installation of sanitation infrastructure [27]. The intervention reduced reinfection prevalence (OR 0.56, 95% CI 0.31–1.00) and egg count (IRR 0.34, 95% CI 0.15–0.75) of A. lumbricoides, but not of T. trichiura or hookworm. Effects of the intervention differed by sex, with girls in the intervention group showing a significantly reduced A. lumbricoides infection intensity compared to the control group; boys in the intervention group did not show any significant difference from controls. Shoe-wearing and geophagy also emerged as effect modifiers for hookworm and T. trichiura infection intensity, respectively. Dumba and colleagues found no statistically significant benefit of a participatory hygiene and sanitation transformation (PHAST) intervention when compared with a control group that only received deworming [86]. PHAST uses training sessions to encourage communities to identify problems in their own environment, decide what aspects need to be improved, and then implement changes. Parents or guardians of participating children in 19 villages received three PHAST education sessions. Participants in both control and experimental villages received albendazole and showed significant reductions in helminth prevalence compared with baseline, but the prevalence in the experimental group did not decline more than that among the control children. This study grouped Hymenolepis nana and Enterobius vermicularis with STH in analysis, but only a handful of participants were infected by H. nana or E. vermicularis, whereas STH prevalence was very high (>80%). *Strongyloides stercoralis* ------------------------- We found 12 studies that investigated the relationship between WASH and S. stercoralis infection, but only located relevant effect estimates in five. Among school children in Cambodia, Khieu and colleagues found crude associations between infection and handwashing, shoe-wearing, and sanitation access [87]. Hall and colleagues found mixed results for a range of sanitation-related exposures, with some evidence that open defecation and use of community latrines were associated with higher odds of S. stercoralis infection in children [72]. In a multivariable model using data from a rural Peruvian community, Yori and colleagues found that wearing shoes never or occasionally (versus more frequently) was associated with higher odds of infection (OR 1.89, 95% CI 1.10–3.27) [88]. Knopp and colleagues did not find a significant association between S. stercoralis infection and home latrine ownership or handwashing after defecation [89]. [1]: https://osf.io/589gr/ "Figure 1" [2]: https://osf.io/upk2n/ "Table 3" [3]: https://osf.io/98pf3/ "Table 4" [4]: https://osf.io/dvn43/ "Table 5" [5]: https://osf.io/ftjcz/ "Table S1" [6]: https://osf.io/g32w6/ [7]: https://osf.io/2tf4s/ "Table 6" [8]: https://osf.io/2mr5z/ "Table 7" [9]: https://osf.io/kf7gz/ "Table S2" [10]: https://osf.io/42pwr/ "Table 8" [11]: https://osf.io/xm8rg/ "Table 9" [12]: https://osf.io/vgsw3/ [13]: https://osf.io/3b9dp/ [14]: https://osf.io/zk9rg/ [15]: https://osf.io/j7a9p/ [16]: https://osf.io/ngz7f/ [17]: https://osf.io/q4kcd/ [18]: https://osf.io/jrgev/ [19]: https://osf.io/8am5p/ [20]: https://osf.io/bgw4e/ [21]: https://osf.io/zw5xk/ [22]: https://osf.io/mtfxd/ [23]: https://osf.io/2b67k/ [24]: https://osf.io/gqxkr/ [25]: https://osf.io/m2bt7/