What the Largest-Ever Wild Swimming Study Tells Us About Waterborne Health Risks
The largest controlled study ever conducted on illnesses linked to bathing in rivers and lakes has found that wild swimming carries a higher risk of stomach bugs and skin rashes, with gastrointestinal illness risk rising significantly in water contaminated by faecal bacteria. The randomised controlled trial, led by Paul Hunter, a professor of medicine at the University of East Anglia, involved 2,368 participants at two river and two lake-like bathing sites in Hungary (University of East Anglia).
A randomised controlled trial is a type of study where participants are randomly assigned to different groups — in this case, swimming versus staying on shore — so that researchers can compare outcomes while reducing the influence of other variables. Participants were randomly allocated either to spend 10 minutes in the water, immersing their faces at least three times, or to remain on the shore without entering it. Water was sampled throughout the trials to estimate each swimmer's exposure to E. coli, intestinal enterococci, and somatic coliphages. Follow-up assessed gastrointestinal illness (diarrhoea, vomiting, or nausea accompanied by fever) and skin complaints (rash, ulcer, or itching).
About 2.1% of swimmers reported gastrointestinal illness, 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. Enterococci showed no association with increased illness risk. "The study found enterococci are not associated with increased risk of illness, but E. coli is," Hunter said.
Skin complaints were more than twice as common among swimmers at the Hungarian sites: 2.7% of swimmers reported rashes, ulcers, or itching, compared with 1.3% of those who stayed on shore. Notably, the skin problems were not associated with any of the three faecal contamination measures tracked in the study. Hunter suggested cercarial dermatitis, a condition in which avian parasites burrow into human skin, or irritation from algae as plausible non-faecal explanations for the skin conditions.
Hunter said he was "fairly confident" the findings would be applicable to the UK despite the research being conducted in Hungary. A separate German study published in 2006 produced similar results (PubMed). Between 2016 and 2020, at least eight outbreaks of gastroenteritis potentially linked to wild swimming occurred in one studied region (MDPI Water.
The findings carry direct implications for international water quality standards. The study raised questions about the World Health Organization's decision to drop E. coli as a recommended indicator of pollution in freshwater bathing sites. Given that E. coli exposure was the variable most clearly tied to gastrointestinal risk in this trial, while enterococci showed no such association, the WHO's move away from E. coli monitoring could remove the single most predictive indicator identified by the data. The peer-reviewed journal publication, appearing in International Journal of Hygiene and Environmental Health, confirms that bathers had higher risk of gastrointestinal and skin ailments and that gastrointestinal illness risk increased with E. coli and coliphage densities (ScienceDirect).
Environmental factors complicate the risk picture. Heavy rain, sewage overflows, and farm runoff can lead to disease outbreaks at rivers and lakes where wild swimming takes place (BBC Future). The UK government's own public health guidance acknowledges that open water swimming can increase the risk of gastrointestinal illnesses that may cause diarrhoea and vomiting (gov.uk). E. coli is among the most prevalent waterborne pathogens a person may encounter when wild swimming (The Conversation). Patients with waterborne infections from wild swimming typically present with diarrhoea, vomiting, and abdominal pain within one to three days of exposure (The Pharmacist. The London School of Hygiene & Tropical Medicine has published expert commentary explaining why some people experience illness after swimming in rivers, lakes, and the sea (LSHTM.
The broader context here is one of methodological weight and policy tension. The trial's randomised design gives its findings a credibility that observational studies — which track people who already swim, without controlling who enters the water — cannot easily match. The distinction between faecal and non-faecal pathways matters: gastrointestinal risk tracked cleanly with bacterial contamination, while skin risk did not, suggesting that water quality monitoring focused solely on faecal indicators would capture only part of the health burden. If E. coli is removed from monitoring frameworks, regulators may lose the marker most directly tied to the stomach illnesses the study identified. The result is a tension between simplifying water quality surveillance and retaining indicators with the strongest evidence base for predicting bather illness.
