A large controlled study conducted in Hungary has found links between wild swimming in freshwater sites and an increased risk of stomach illnesses and skin conditions. The research, which examined health outcomes among 2,368 participants at five river and two lake-like locations, observed that swimmers faced more than double the risk of developing skin complaints such as rashes, ulcers, or itching compared to non-swimmers. Additionally, exposure to higher concentrations of Escherichia coli (E coli) and other faecal pollution indicators corresponded with a greater likelihood of gastrointestinal illness.

Participants were randomly assigned to either immerse themselves in the water for 10 minutes, submerging their faces multiple times, or avoid entering the water altogether. Water samples taken during the trials measured levels of E coli, intestinal enterococci, and somatic coliphages—viruses that infect bacteria and act as markers for faecal contamination. One week after exposure, swimmers reported gastrointestinal symptoms, including diarrhoea, vomiting, and nausea accompanied by fever, at a slightly higher rate (2.1%) than non-swimmers (1.9%). While bathing in general was not statistically associated with stomach illness, individuals who encountered elevated concentrations of E coli and coliphages faced significantly increased risk.

Skin issues were reported by 2.7% of swimmers compared with 1.3% of those who stayed out of the water, yielding a more than twofold rise in skin-related complaints. However, these skin conditions did not appear to correlate with measures of faecal contamination, prompting researchers to suggest other causes such as avian parasites responsible for cercarial dermatitis or irritation from algae.

The study’s lead author, Paul Hunter, a professor of medicine at the University of East Anglia, highlighted the implications of the findings for water quality monitoring practices. Current guidelines from the World Health Organization (WHO) focus on enterococci bacteria as the primary indicator of pollution in both marine and freshwater bathing sites, having phased out E coli monitoring since roughly 2003. Hunter expressed concern about this shift, noting that while enterococci were not linked to increased illness risk in the study, E coli levels were significantly associated with stomach ailments. The research further suggested that somatic coliphages could serve as an additional useful indicator of unsafe water conditions.

Hunter emphasized that the study’s results are likely relevant beyond Hungary, citing a similar 2006 German investigation with comparable conclusions. Both countries share climatic similarities with the UK, where Hunter anticipates parallel risks for wild swimmers.

Though all tested sites met the European Union’s bathing water standards in place at the time, the authors cautioned the investigation was limited to single-day trials at each location, and some participants may have engaged in swimming elsewhere during the follow-up period. Researchers also noted an increase in respiratory, ear, and eye infections among swimmers, but sample sizes were too small to establish statistically significant associations.

The findings raise questions about the adequacy of current international guidelines for freshwater bathing water quality and reinforce calls to retain E coli monitoring to better protect public health amid the rising popularity of outdoor swimming.