A randomized controlled trial assessing infectious disease risk from bathing in inland recreational waters

Evidence to determine the suitability of water quality standards to prevent illness from recreation exposure in inland waters is limited. We present a secondary analysis of previously unpublished data from four Hungarian freshwater sites included in the Epibathe study, a large randomised controlled trial. A total of 2368 participants were randomly allocated either to bathe for 10 min undertaking at least three head immersions, or to remain on the shoreside without water contact. Concurrent water sampling quantified individual-level faecal indicator organism densities and health outcomes were assessed at one-week follow-up. Generalised estimating equation models quantified the relative risk for adverse health outcomes for bathers versus shoreline-only (non-bather) participants, as well as the change in risk per unit increase in FIO concentration; crude, covariate-adjusted and covariate-adjusted models with multiple imputation are reported. Higher concentrations of Escherichia coli and somatic coliphage were each associated with increased risk of gastrointestinal illness (adj. RR = 1.73; 95% CI 1.13-2.65 and RR = 1.46; 95% CI 1.04-2.04 respectively). Bathing itself, independent of any individual microbial indicator, was associated with increased risk for skin ailments (adj. RR = 2.17; 95% CI 1.55-3.03). Low prevalence of eye, ear or respiratory infections precluded reliable estimation of exposure-response relationships. These findings confirm the value of E. coli and potential of somatic coliphage densities as indicators of freshwater quality relevant to recreation-associated gastrointestinal illness risk. In freshwater settings, E. coli and coliphages appear to be more informative than enterococci as predictors of gastrointestinal illness, contrasting with evidence from marine waters.