Swimming in a lake or river for ten minutes was associated with more than twice the risk of a skin rash or itching compared with standing on the shore, according to a randomized trial of 2,368 people published in the International Journal of Hygiene and Environmental Health.
The timing lands right before the last big swimming weekend of the American summer. The finding is not a reason to skip the lake, and the researchers do not frame it that way. It is a reason to understand which risk actually went up, because the headline result and the result most people assume are not the same.
Skin problems rose. Stomach illness, on its own, did not.
Swimmers Versus People Who Stayed on Shore
The design is what makes this study unusual. Most research on recreational water illness follows people who chose to swim, which makes it hard to separate the water from everything else about the kind of person who jumps in.
Here, researchers led by a team from the University of East Anglia randomly assigned participants at four freshwater bathing areas in Hungary either to swim for ten minutes or to stay on shore with no water contact. Roughly half went into the water. The two groups were well balanced on demographics and behavior, with no statistically significant differences in prior illness. Water samples were tested for E. coli, intestinal enterococci, and somatic coliphages, viruses that indicate fecal contamination.
After one week, swimmers were more than twice as likely as non-swimmers to report a skin complaint such as a rash or itching, a relative risk of 2.17. Respiratory, ear, and eye infections were uncommon in both groups, with 9, 13, and 13 cases respectively.
Paul Hunter, a professor of medicine at the University of East Anglia and a study author, said in a university news release that the work provides some of the strongest evidence yet that contamination at freshwater recreation sites translates into real-world illness among bathers.
The Stomach Finding Is Not the One People Expect
Gastrointestinal illness was reported by 24 swimmers and 24 non-swimmers, working out to 2.12 percent and 1.94 percent. The difference was not statistically significant. Simply getting in the water did not raise stomach illness risk on its own.
What did raise it was contamination. Among swimmers, each one-unit increase in E. coli concentration was associated with a 77 percent higher risk of gastrointestinal illness after adjustment for age, recent prior illness, and behavior. Each one-unit increase in somatic coliphage concentration was associated with a 48 percent higher risk.
That is a more useful finding than a blanket warning, because it means the water quality reading at a specific beach on a specific day carries real information. A clean lake and a contaminated one are not the same swim.
The skin result works the other way. None of the three microbial indicators was significantly linked to skin symptoms, which suggests whatever irritated swimmers' skin was not the sewage-related bacteria being measured. Hunter has suggested naturally occurring organisms, or agricultural and industrial contaminants, may be involved. That remains unresolved.
Hungarian Lakes and American Ones
The study was conducted entirely in Hungary. That limitation belongs early rather than at the end.
Hunter argues for broader relevance on the grounds that results were consistent across four sites with different water types and quality levels, and consistent with an earlier study in Germany, and he applies that argument specifically to waters in the United Kingdom. That is a reasonable case for generalizability, but it has not been tested in U.S. waters, and American freshwater sites carry hazards the Hungarian sites may not, including harmful algal blooms and, in warm southern lakes, rarer organisms with much more serious consequences.
This is also an association study for the contamination findings, embedded in a randomized design for the swim versus no swim comparison. The randomization supports the skin conclusion well. The dose-response contamination findings are observational within the swimming group and cannot by themselves establish cause.
MedicalDaily previously reported on a CDC investigation of a lake-linked norovirus outbreak, which reflects a different pathway, person-to-person contamination of the water, than the fecal indicators measured here.
Sensible Precautions for the Last Weekends of Summer
Nothing in this trial supports avoiding open water. The practical takeaways are small and easy.
Check whether the beach posts water quality results or advisories before going, since local health departments in most states monitor designated swimming areas and post closures. Avoid swimming for a day or two after heavy rain, when runoff pushes contamination levels up. Rinse off with clean water afterward, which is the most direct response to a skin finding that appears driven by something in the water rather than by swallowing it. Keep heads above water and mouths closed where you can, which addresses the contamination-linked stomach risk.
Young children, older adults, pregnant people, and anyone with a weakened immune system have less margin if they do get sick, and open wounds or fresh tattoos are a reason to stay out regardless of water quality.
Most skin irritation after swimming is mild and settles on its own within a few days without treatment. Reasons to contact a clinician include a rash that spreads or worsens rather than fading, diarrhea lasting more than a couple of days, blood in stool, signs of dehydration, or fever. Sudden confusion, severe headache, or a rapidly worsening illness after freshwater swimming warrants urgent evaluation.
The researchers say the results support using E. coli levels to gauge gastrointestinal risk in freshwater and suggest somatic coliphages may be a useful additional water quality marker. Whether U.S. agencies adopt that second measure is an open question, and no American replication of this trial has been announced.
Key Questions Answered
What did the study find? People randomly assigned to swim ten minutes in a lake or river were more than twice as likely to report a skin rash or itching a week later than people assigned to stay on shore.
Did swimming cause stomach illness? Not by itself. Rates were nearly identical between swimmers and non-swimmers. Among swimmers, stomach illness risk rose as E. coli and somatic coliphage levels in the water rose.
Where was the research done? Four freshwater bathing sites in Hungary. It has not been replicated in U.S. waters.
Is this strong evidence? The swim versus no-swim comparison was randomized, which is unusually strong for this field. The contamination findings are observational within the swimming group.
Who should be most careful? Young children, older adults, pregnant people, anyone immunocompromised, and anyone with an open wound or a fresh tattoo.
What can I do before swimming? Check posted water quality advisories, skip swimming for a day or two after heavy rain, keep your mouth closed, and rinse with clean water afterward.
When should I see a doctor? A rash that spreads or worsens, diarrhea lasting more than a couple of days, blood in stool, dehydration, or fever. Sudden confusion or rapid worsening needs urgent care.
