Wild swimming linked to higher risk of stomach bugs and skin rashes, study finds

Study links wild swimming to health risks

Swimming in rivers and lakes has been linked to a higher risk of stomach bugs and skin rashes, according to a new study from the University of East Anglia. Researchers found that people who swam in natural freshwater were more than twice as likely to develop skin problems, while higher levels of bacteria linked to faecal pollution were associated with an increased risk of gastrointestinal illness.

The findings come amid growing public concern over water quality and a boom in open-water swimming.

How the research was conducted

Scientists analysed data from 2,368 volunteers who took part in a randomised controlled trial at four freshwater bathing sites in Hungary. Participants were randomly assigned either to spend 10 minutes swimming, including at least three full head immersions, or to remain on the shore without entering the water.

Water was sampled throughout the trials, allowing researchers to estimate each swimmer's exposure to E. coli, intestinal enterococci, and somatic coliphages – viruses that infect bacteria and can indicate faecal contamination. Participants were questioned about symptoms a week later.

Prof Paul Hunter, who led the analysis, said: "We found that swimmers were more than twice as likely to suffer skin ailments such as rashes, ulcers or itching compared with those who stayed dry."

He added: "We also found that the risk of gastrointestinal illness increased as levels of E. coli and other viruses that infect bacteria found in the human gut rose in the water, which is indicative of faecal pollution."

Key findings

The study found that for each tenfold increase in E. coli concentration, the risk of gastrointestinal illness rose by around 73 per cent. Higher levels of somatic coliphages were associated with a 45 per cent increase in risk. The team said these organisms appear to be better indicators of illness risk in rivers and lakes than intestinal enterococci.

About 2.1% of swimmers reported gastrointestinal illness, including diarrhoea, vomiting or nausea accompanied by fever, compared with 1.9% of non-swimmers. 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.

Although all four study sites met European bathing water standards at the time of the trial, scientists still detected an association between higher levels of microbial contamination and sickness.

Implications for water quality standards

The findings raise questions about current water quality indicators. Prof Hunter noted that the World Health Organization had moved away from recommending E. coli as an indicator in freshwater, focusing instead on enterococci.

"The original guidelines, which were published in about 2003 – and I was part of the group that developed those – recommended [monitoring of] both E coli and enterococci for marine and fresh waters," he said. "WHO produced new guidelines, and they basically said: 'Don't worry about E coli, just focus on enterococci.' Rather embarrassingly, what we found is that enterococci aren't associated with increased risk, but E coli is."

The researchers noted that freshwater and coastal waters may require different standards, as the behaviour of bacterial indicators differs between the two environments.

Context and limitations

The study was funded by the EU's 6th Research Framework Programme and the National Institute for Health and Care Research (NIHR), in partnership with the UK Health Security Agency (UKHSA). It is published in the International Journal of Hygiene and Environmental Health.

The researchers said the randomised design provided stronger evidence than observational studies. They cautioned that the results were based on a single trial day at each site and that some participants reported swimming elsewhere during the study period.

Skin complaints increased among swimmers regardless of measured levels of faecal indicators. Prof Hunter suggested this could be due to exposure to naturally occurring organisms not linked to sewage pollution, such as cercarial dermatitis – commonly known as 'swimmer's itch' – caused by parasites shed by waterfowl. Agricultural or industrial contaminants may also play a role.

Prof Hunter emphasised that the overall rates of illness were low. "Just over two per cent of participants developed gastrointestinal illness and a similar proportion experienced skin ailments," he said. "But it is important to remember that the rates of illness were relatively low overall and most illnesses were mild and short lived."

Open-water swimming has surged in popularity, with advocates praising its mental and physical health benefits. Campaigners have repeatedly raised concerns about sewage discharges and pollution affecting popular bathing locations. The findings add to the debate over the condition of Britain's rivers and lakes, with Prof Hunter noting that the results were consistent across the four Hungarian study sites and suggesting the relationships are generalisable and likely applicable in UK waters.