People swimming in rivers and lakes may face a greater risk of skin conditions and stomach bugs, according to a new study from the University of East Anglia. Researchers found that freshwater swimmers were more than twice as likely to develop skin problems. At the same time, higher levels of bacteria associated with faecal pollution were linked to an increased risk of gastrointestinal illness. The findings come as open-water swimming continues to grow in popularity and concerns about water quality increase.

The researchers analysed data from 2,368 volunteers who participated in a randomised controlled trial at four freshwater bathing sites in Hungary. Participants were randomly assigned either to swim for 10 minutes, including at least three full head immersions, or to remain on shore without entering the water. This allowed researchers to compare illness rates between swimmers and non-swimmers directly.

Prof Paul Hunter, who led the analysis, said swimmers were more than twice as likely to experience skin problems such as rashes, ulcers or itching compared with those who stayed dry. The risk of gastrointestinal illness also increased as levels of E. coli and certain viruses that infect bacteria in the human gut rose in the water, indicating greater faecal contamination.

Importantly, all four bathing sites met European bathing-water standards during the trial. Despite this, researchers still found a relationship between microbial contamination and illness. For every tenfold increase in E. coli concentration, the risk of gastrointestinal illness rose by about 73 per cent. Higher levels of somatic coliphages, viruses that infect bacteria, were associated with a 45 per cent increase in risk.

The researchers said E. coli and somatic coliphages may be better indicators of illness risk in rivers and lakes than intestinal enterococci, a group of bacteria emphasised in some international water-quality guidelines. They also suggested that freshwater and coastal waters may require different approaches to monitoring because bacterial indicators can behave differently in the two environments.

Skin complaints, meanwhile, increased among swimmers regardless of measured levels of faecal contamination. Prof Hunter said this could result from naturally occurring organisms rather than sewage pollution. One possibility is cercarial dermatitis, commonly known as “swimmer’s itch”, an itchy rash caused by parasites associated with waterfowl. Agricultural or industrial contaminants could also contribute to skin problems.

The findings could have implications for the UK, where open-water swimming has become increasingly popular, and sewage discharges into rivers and other waterways have generated public concern. Prof Hunter said the results were consistent across the four Hungarian locations, despite differences in water type and quality, and were also consistent with an earlier German study. This suggests the identified relationships could potentially apply to UK waters as well.

However, the researchers stressed that illness was relatively uncommon and generally mild and short-lived. Just over two per cent of participants developed gastrointestinal illness, while a similar proportion reported skin problems; respiratory, ear and eye infections were rare. The researchers concluded that monitoring E. coli and related microbial indicators remains important for protecting swimmers. The work formed part of Epibathe, a European collaboration investigating health risks associated with marine and freshwater recreational sites.

More information: Edward K.S. Lam et al, A Randomised Controlled Trial Assessing Infectious Disease Risks from Bathing in Inland Recreational Waters, International Journal of Hygiene and Environmental Health. DOI: 10.1016/j.ijheh.2026.114888

Journal information: International Journal of Hygiene and Environmental Health Provided by University of East Anglia