Wild swimming has been linked to a higher risk of stomach bugs and skin problems, according to a study. Swimmers at four Hungarian sites were more than twice as likely to report skin complaints.

The risk of stomach illness rose sharply when swimmers encountered higher levels of E. coli and other indicators of faecal pollution. All four sites met EU bathing-water standards when researchers conducted the tests.

The findings challenge World Health Organization guidance that dropped E. coli as a pollution indicator for freshwater sites. They come as outdoor swimming grows in popularity amid concerns over water quality.

Study Details and Findings

The University of East Anglia study involved 2,368 volunteers at four freshwater bathing sites in Hungary. It was a rare randomised controlled trial, offering stronger evidence than observational research.

Participants were randomly assigned to swim for 10 minutes or remain on shore. Swimmers immersed their faces at least three times while researchers tested the water.

Researchers measured E. coli, enterococci and somatic coliphage viruses. Participants were then asked about symptoms a week later.

Bathing itself was not statistically linked to stomach illness, but the risk rose with higher concentrations of E. coli and coliphages. The result pointed to pollution levels, rather than swimming itself, as the key factor.

As E. coli levels increased, the risk of a stomach bug increased by 73%. Higher somatic coliphage levels were linked to a 45% rise in risk.

Skin complaints affected 2.7% of swimmers, compared with 1.3% of non-swimmers. Study leader Professor Paul Hunter said the results were consistent across all four sites.

A separate German study from 2006 produced similar findings. Hunter said he was 'fairly confident' the results would apply to UK waters, despite the study's acknowledged limitations.

The trial took place on only one day at each site. Some participants also reported swimming elsewhere during the study period, while most illnesses were mild and short-lived.

Doubts Over WHO Guidance

The findings raise questions about WHO guidance, which favours enterococci for freshwater pollution. Hunter, who helped develop the original guidelines, said enterococci were not linked to illness in this study.

E. coli, by contrast, showed a clear relationship with stomach illness. Hunter argued there is therefore a strong case for keeping E. coli in freshwater monitoring standards.

He also suggested somatic coliphages could help identify potentially unsafe conditions. Further research would be needed before they could be widely adopted.

The researchers could not establish what caused the skin rashes because none of the three pollution markers explained them. Hunter suggested cercarial dermatitis, caused by parasites released by waterfowl, as one possibility.

Algae in rivers and lakes could also contribute to skin problems, although Hunter stressed that this remains speculation. Further studies are needed to establish the cause.

Wider Evidence From Past Events

The UEA findings echo research from the 2010 Ironman triathlon in Copenhagen. The event took place after heavy rainfall caused sewage overflows, and 42% of swimmers fell ill.

Only 8% became ill during a similar, cleaner event the following year. Illness risk also increased with the amount of water swallowed while swimming.

Stool samples from unwell athletes contained Campylobacter, Giardia lamblia and diarrhoeagenic E. coli. The findings added to evidence linking polluted water with gastrointestinal illness.

Open-water swimming has grown rapidly, partly because of its perceived health benefits. Campaigners have nevertheless raised concerns about sewage discharges affecting rivers and lakes across the UK.

The UEA researchers said their findings support stronger monitoring of freshwater bathing sites. They concluded that tracking E. coli remains an important tool for protecting public health.

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