River and lake swimming linked to stomach bugs and skin rashes

Open-water swimming is often presented as a way to reconnect with nature, improve fitness and support mental health. Yet a large randomized controlled trial conducted at four freshwater bathing sites in Hungary suggests that rivers and lakes can also expose swimmers to a measurable risk of illness, even when the water officially meets European bathing standards. The study found that people who entered the water were more than twice as likely to report skin problems, including rashes, ulcers and itching, than participants who remained on shore. Gastrointestinal illness was less common, but its risk increased as concentrations of microbial indicators associated with faecal pollution rose. The findings provide unusually direct evidence that contamination in inland recreational waters can translate into illness among bathers, adding scientific weight to growing concerns about the safety of popular swimming sites.

The research involved 2,368 volunteers who participated in a rare randomized trial designed to separate the effects of entering the water from the characteristics of the people who choose to swim. Participants were assigned either to spend 10 minutes bathing, with at least three complete immersions of the head, or to remain on the shore without going into the water. This design is important because observational studies can be distorted by differences between swimmers and nonswimmers. People who swim may differ in age, health, behaviour or exposure to other environmental risks. Random assignment helps ensure that such factors are distributed between the groups, allowing researchers to examine more directly whether contact with freshwater itself is associated with subsequent illness.

The strongest association concerned skin ailments. Swimmers were more than twice as likely to develop symptoms compared with those who stayed dry, and the pattern appeared across the study locations. Unlike gastrointestinal illness, however, the occurrence of skin complaints did not correspond clearly with the measured levels of faecal contamination. That result suggests that the causes may not be limited to sewage-derived microbes. Freshwater contains a complex biological community, including parasites, bacteria, fungi and other organisms that can interact with human skin. Exposure can also be affected by submerged vegetation, sediments, aquatic animals and chemical pollutants, none of which are necessarily captured by routine bathing-water tests.

One possible explanation is cercarial dermatitis, commonly known as swimmer’s itch. The condition is caused by microscopic parasitic larvae released by infected waterfowl. The larvae normally seek aquatic hosts, such as snails, but may accidentally penetrate human skin when people enter infested water. They cannot complete their life cycle in humans, but their entry can trigger an itchy inflammatory rash. Swimmer’s itch is not evidence of faecal pollution, which could explain why skin symptoms rose independently of the indicators measured in the trial. Agricultural runoff, industrial chemicals or naturally occurring irritants may also contribute. The study did not establish which specific agents caused the skin reactions, but its findings highlight the limitations of judging freshwater safety through a narrow set of microbial measurements.

The risk of gastrointestinal illness showed a different and more chemically interpretable pattern. For every tenfold increase in the concentration of Escherichia coli, the risk of gastrointestinal disease rose by approximately 73 percent. E. coli is a diverse species, most strains of which are harmless inhabitants of the intestinal tract, but its presence in recreational water is widely used as a signal that faecal material may have entered the environment. Faecal pollution can carry a mixture of pathogens, including viruses, bacteria and protozoa, so E. coli is generally treated as an indicator rather than the sole cause of illness. Swallowing even small amounts of contaminated water can provide a route for these organisms to reach the digestive system.

The researchers also examined somatic coliphages, viruses that infect bacteria found in the human gut. These bacteriophages do not usually cause disease in people, but their persistence and behaviour in water can resemble that of some human enteric viruses. Their presence may therefore offer information about contamination that conventional bacterial indicators miss. In the trial, a tenfold increase in somatic coliphage levels was associated with an estimated 45 percent increase in gastrointestinal illness risk. The researchers argue that E. coli and coliphages may be more informative indicators of health risk in rivers and lakes than intestinal enterococci, a bacterial group emphasized in some international water-quality regulations. The results also suggest that monitoring systems designed for coastal waters may not transfer directly to inland environments.

All four bathing sites met European bathing-water standards at the time of the trial, a finding that illustrates the difference between regulatory compliance and zero risk. Water-quality standards are designed to keep hazards below defined thresholds, not to guarantee that no swimmer will become ill. They also depend on which organisms are measured, how frequently samples are collected and how accurately those measurements reflect conditions at the moment people enter the water. Microbial concentrations can change rapidly after rainfall, sewage overflow, agricultural runoff or disturbance of contaminated sediment. A single sample may therefore fail to represent short-lived peaks in contamination. The trial’s results indicate that even within waters considered acceptable under existing rules, differences in microbial concentration can be associated with differences in illness risk.

The findings have particular relevance in Britain, where open-water swimming has expanded rapidly and public debate over river pollution has intensified. Natural bathing sites are promoted for their recreational, psychological and physical benefits, while campaigners have drawn attention to sewage discharges and other sources of contamination. According to the research team, the consistency of the results across four Hungarian sites with different water types and quality conditions, together with findings from an earlier German study, suggests that the observed relationships may also apply to UK waters. The researchers nevertheless emphasized that the absolute frequency of illness was low. Just over 2 percent of participants developed gastrointestinal illness, and a similar proportion reported skin ailments. Respiratory, ear and eye infections were rare, while most illnesses were mild and short-lived.

The study was conducted as part of Epibathe, a collaboration among European institutions investigating infectious risks in marine and freshwater recreational environments. Hungary was selected for the inland-water research because it offered suitable bathing sites and experienced local investigators. The final analysis was led by the University of East Anglia and supported by funding from the European Union’s Sixth Research Framework Programme, together with the National Institute for Health and Care Research and the UK Health Security Agency. By using random assignment rather than relying only on surveys of habitual swimmers, the investigators were able to provide stronger evidence than is typically available for environmental exposure studies. Even so, the work cannot identify every pathogen or pollutant responsible for illness, and it does not imply that all freshwater swimming is unsafe.

Instead, the research points to a more detailed approach to assessing recreational water. Testing for E. coli and somatic coliphages could help authorities identify conditions associated with gastrointestinal risk, while separate surveillance may be needed for parasites, algal products, chemical contaminants and other causes of skin disease. Swimmers may encounter changing hazards that are difficult to detect through infrequent sampling, particularly after heavy rain or known pollution events. The broader message is not that rivers and lakes should be avoided, but that their apparent clarity and regulatory status do not tell the whole story. As open-water swimming continues to grow, understanding how microbial indicators correspond to real-world illness will be essential for setting standards that reflect the diverse risks of inland water.