Associations of water contact frequency, duration, and activities with schistosome infection risk: A systematic review and meta-analysis

Abstract

Background Schistosomiasis is a water-borne parasitic disease which affects over 230 million people globally. The relationship between contact with open freshwater bodies and the likelihood of schistosome infection remains poorly quantified despite its importance for understanding transmission and parametrising transmission models. Methods We conducted a systematic review to estimate the average effect of water contact duration, frequency, and activities on schistosome infection likelihood. We searched Embase, MEDLINE (including PubMed), Global Health, Global Index Medicus, Web of Science, and the Cochrane Central Register of Controlled Trials from inception until May 13, 2022. Observational and interventional studies reporting odds ratios (OR), hazard ratios (HR), or sufficient information to reconstruct effect sizes on individual-level associations between water contact and infection with any Schistosoma species were eligible for inclusion. Random-effects meta-analysis with inverse variance weighting was used to calculate pooled ORs and 95% confidence intervals (CIs). Results We screened 1,411 studies and included 101 studies which represented 192,691 participants across Africa, Asia, and South America. Included studies mostly reported on water contact activities (69%; 70/101) and having any water contact (33%; 33/101). Ninety-six percent of studies (97/101) used surveys to measure exposure. A meta-analysis of 33 studies showed that individuals with water contact were 3.14 times more likely to be infected (OR 3.14; 95% CI: 2.08–4.75) when compared to individuals with no water contact. Subgroup analyses showed that the positive association of water contact with infection was significantly weaker in children compared to studies which included adults and children (OR 1.67; 95% CI: 1.04–2.69 vs. OR 4.24; 95% CI: 2.59–6.97). An association of water contact with infection was only found in communities with ≥10% schistosome prevalence. Overall heterogeneity was substantial (I2 = 93%) and remained high across all subgroups, except in direct observation studies (I2 range = 44%–98%). We did not find that occupational water contact such as fishing and agriculture (OR 2.57; 95% CI: 1.89–3.51) conferred a significantly higher risk of schistosome infection compared to recreational water contact (OR 2.13; 95% CI: 1.75–2.60) or domestic water contact (OR 1.91; 95% CI: 1.47–2.48). Higher duration or frequency of water contact did not significantly modify infection likelihood. Study quality across analyses was largely moderate or poor. Conclusions Any current water contact was robustly associated with schistosome infection status, and this relationship held across adults and children, and schistosomiasis-endemic areas with prevalence greater than 10%. Substantial gaps remain in published studies for understanding interactions of water contact with age and gender, and the influence of these interactions for infection likelihood. As such, more empirical studies are needed to accurately parametrise exposure in transmission models. Our results imply the need for population-wide treatment and prevention strategies in endemic settings as exposure within these communities was not confined to currently prioritised high-risk groups such as fishing populations.

CurrentSchistosoma mansoniexposure and infection have distinct determinants: a data-driven population-based study in rural Uganda

“…Schistosome transmission is complex and driven by human behaviour, accessibility of safe water and sanitation, occupation, and ecological conditions for freshwater snails that are the intermediate host of the parasite. Exposure to schistosomes occurs during water contact with lakes, rivers, or streams through activities including swimming, bathing, or fetching drinking water [3]. During water contact, cercariae—the free-living stage of the parasite—enter a human host by burrowing through the skin.…”

“…Water contact has become a well-established proxy indicator for exposure due to the difficulties in directly measuring cercarial exposure. Previous studies have largely been cross-sectional and used self-reported water contact activities or constructed crude binary indicators of water contact for the purposes of predicting current infection [3]. Among 101 studies in a recent systematic review and meta-analysis by Reitzug et al, only 21.8% (22/101) collected snail abundance data to account for environmental factors relevant for translating water contact into parasite acquisition risk [3].…”

“…Previous studies have largely been cross-sectional and used self-reported water contact activities or constructed crude binary indicators of water contact for the purposes of predicting current infection [3]. Among 101 studies in a recent systematic review and meta-analysis by Reitzug et al, only 21.8% (22/101) collected snail abundance data to account for environmental factors relevant for translating water contact into parasite acquisition risk [3]. Attempts to integrate water contact with environmental variables have not consistently improved the ability to predict infection [13–16].…”

“…Attempts to integrate water contact with environmental variables have not consistently improved the ability to predict infection [13–16]. This may be due to limitations in data such as rough estimates of bodily immersion data [13,15], assumptions made about cercarial density [13,16], or methodological limitations including no systematic variable selection and no out-of-sample validation [3]. Due to regular MDA, studying the role of exposure for infection is further complicated by immunity which can be acquired through past infection or successful treatment with praziquantel [17–19].…”

“…Due to regular MDA, studying the role of exposure for infection is further complicated by immunity which can be acquired through past infection or successful treatment with praziquantel [17–19]. Understanding exposure also has proven difficult as no standardised exposure measurement tools exist and there is high heterogeneity across existing studies [3].…”