Bacterial Faecal Indicators as Proxies for Viral Gastrointestinal Illness Risk in Inland Recreational Waters: Evidence from the Epibathe Randomised Controlled Trial

Lam, Edward, Vargha, Márta, Thomas, Daniel, Salmon, Roland, Kádár, Mihály, Figueras (URV), Maria-José, Brainard, Julii ORCID: https://orcid.org/0000-0002-5272-7995, Bartram, Jamie, Barna, Zsofia and Hunter, Paul R. ORCID: https://orcid.org/0000-0002-5608-6144 (2026) Bacterial Faecal Indicators as Proxies for Viral Gastrointestinal Illness Risk in Inland Recreational Waters: Evidence from the Epibathe Randomised Controlled Trial. In: UNSPECIFIED.

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Abstract

Introduction: Gastrointestinal illness associated with recreational freshwater bathing is predominantly viral in aetiology, with norovirus, rotavirus and enteric adenoviruses being frequently implicated pathogens. However, as sporadic cases are typically self-limiting and subclinical, the causative agent often goes unidentified. Direct viral quantification in bathing waters remains technically demanding and is not routinely performed in regulatory monitoring. Instead, bacterial faecal indicator organisms (FIOs), such as Escherichia coli, intestinal enterococci and somatic coliphages, are used as proxies for faecal contamination and, by extension, the presence of viral pathogens. The strength of this proxy relationship in freshwater environments dominated by diffuse contamination is poorly characterised due to design limitations in previous studies. Here we report findings from the four freshwater study sites included in the Epibathe study, a large randomised controlled trial carried out across European bathing sites. We quantified the exposure-response relationship between specific faecal indicator concentrations and subsequent illness risk, demonstrating the value of E. coli and somatic coliphage densities as indicators of freshwater quality. Methods: 2368 participants were prospectively recruited in Hungary during 2006-2007 and randomly allocated to bathing (≥3 head immersions, 10 minutes) or shoreside non-bathing groups. Concurrent water sampling enabled the characterisation of individual-level FIO exposure and health outcomes were assessed at one-week follow-up using a validated symptom-based case definition. Population-average relative risks were estimated using generalised estimating equation (GEE) models with exchangeable working correlation, adjusting for sociodemographic covariates and clustering by site. Robustness to missing data was assessed by multiple imputation. Results: Higher E. coli concentrations were significantly associated with increased gastrointestinal illness risk (adjusted RR = 1.73 per log-unit increase in E. coli concentrations; 95% CI 1.13–2.65; Table 1), as were somatic coliphage concentrations (adj. RR = 1.45; 95% CI 1.05–2.04; Table 1). Intestinal enterococci showed no significant association with gastrointestinal illness. Additionally, there was an increased risk for skin ailments amongst bathers (adj. RR = 2.17; 95% CI 1.55-3.03; Table 1), although skin ailments were not significantly associated with any individual microbial indicator, potentially suggesting a non-faecal, non-viral aetiology. These exposure-response relationships were all consistent across complete-case and imputed analyses, demonstrating robustness against missing data. Conclusion: These results, derived from a rigorous RCT design, strengthen the epidemiological basis of E. coli and somatic coliphages as indicators of faecal contamination and predictors of increased risk for viral gastrointestinal illness in freshwater settings. Crucially, the performance of somatic coliphages as a predictor of gastrointestinal illness highlights the need for virology-focused water quality monitoring, given their structural and ecological parallels with other human enteric viruses. Intestinal enterococci, by contrast, appear to be poor predictors in this context, in contrast to their established utility in marine waters. Future studies incorporating direct viral detection, alongside source-specific viral markers and expanded FIO panels, will be vital to fully characterise the viral risks associated with freshwater recreation and underpin the development of evidence-based water quality standards.

Item Type:	Conference or Workshop Item (Paper)
Uncontrolled Keywords:	sdg 3 - good health and well-being,sdg 6 - clean water and sanitation,sdg 14 - life below water ,/dk/atira/pure/sustainabledevelopmentgoals/good_health_and_well_being
Faculty \ School:	Faculty of Medicine and Health Sciences > Norwich Medical School University of East Anglia Research Groups/Centres > Theme - ClimateUEA
UEA Research Groups:	Faculty of Medicine and Health Sciences > Research Centres > Public Health Faculty of Medicine and Health Sciences > Research Groups > Epidemiology and Public Health Faculty of Social Sciences > Research Centres > Water Security Research Centre
Depositing User:	LivePure Connector
Date Deposited:	16 Sep 2026 09:07
Last Modified:	20 Sep 2026 23:00
URI:	https://ueaeprints.uea.ac.uk/id/eprint/104553
DOI:

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