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Research

Wild Swimming Linked to Higher Risk of Stomach Bugs and Skin Rashes, Study Finds

The largest controlled study found wild swimming raises risks of stomach bugs and skin rashes, challenging WHO freshwater guidelines.

AI News Feed · 2026-08-27 21:13

The study involved 2,368 people at two river and two lake-like bathing sites in Hungary. Participants were randomly allocated either to spend 10 minutes in the water, immersing their face at least three times, or to remain on the shore. Water was sampled throughout the trials to estimate each swimmer's exposure to E coli, intestinal enterococci and somatic coliphages, viruses that indicate faecal contamination.

About 2.1% of swimmers reported gastrointestinal illness, including diarrhoea, vomiting or nausea accompanied by fever, compared with 1.9% of non-swimmers. Although bathing alone was not statistically linked to stomach illness, the risk increased significantly among swimmers exposed to higher concentrations of E coli and coliphages. Skin complaints, defined as a rash, ulcer or itching, were reported by 2.7% of swimmers and 1.3% of people who stayed out of the water, making bathers more than twice as likely to develop a skin ailment.

The findings raise questions about the World Health Organization's decision to drop E coli as a recommended indicator of pollution in freshwater bathing sites. Paul Hunter, a professor of medicine at the University of East Anglia who led the study, said the original 2003 guidelines recommended monitoring both E coli and enterococci, but WHO's new guidelines said to focus only on enterococci. "Rather embarrassingly, what we found is that enterococci aren't associated with increased risk, but E coli is," he said.

The skin problems were not associated with any of the three measures of faecal contamination, suggesting other organisms or pollutants may be responsible. Hunter said possible explanations include cercarial dermatitis, where avian parasites burrow into human skin, or irritation from algae, but added that was speculation. Cases of respiratory, ear and eye infections were also more frequent among swimmers, though the numbers were too small for reliable conclusions.

Hunter said he was "fairly confident" the findings would apply to the UK, noting that a separate German study in 2006 produced similar results. All four sites complied with EU bathing-water standards in force during the trials. The researchers said the randomised design provided stronger evidence than observational studies, but cautioned that results were based on a single trial day at each site and that some participants reported swimming elsewhere during the study period.

Hunter said the results supported retaining E coli in freshwater monitoring and suggested somatic coliphages could also help identify unsafe conditions. "I think there is a very strong argument that E coli, particularly for fresh waters rather than marine waters, should be part of the standards and should be part of the guidelines," he added.