Study of 2,368 Swimmers Links E. Coli to Higher Stomach Illness Risk in Lakes and Rivers

Millions of people wade into lakes and rivers every summer, trusting that if the water looks clean and no warning signs are posted, it’s safe to swim. A new trial out of Hungary suggests that trust might be resting on the wrong test, at least according to the newest set of international guidelines meant to protect swimmers.

Researchers tracked more than 2,300 volunteers who were randomly assigned to either swim in freshwater or stay on the shore, then checked back a week later to see who got sick. The bacteria that the World Health Organization removed from its 2021 freshwater guidelines, called E. coli, was tied to a real jump in stomach illness among swimmers. The germ the WHO kept as its main freshwater marker, intestinal enterococci, showed no such connection.

Published in the International Journal of Hygiene and Environmental Health, the trial does more than poke a hole in a technical rulebook. It raises a bigger question: Does the science behind swim safety guidelines around the world match what happens to real people in real lakes and rivers?

How Scientists Tested Freshwater Safety Guidelines

This research offers a fresh look at data collected in 2006 and 2007 as part of a larger project called the Epibathe study, carried out at four freshwater sites in Hungary: two lake-like river channels and two flowing river sites. The data itself had never been published until now.

Researchers recruited 2,368 people from nearby communities and randomly split them into two groups. One group, made up of 1,133 people, was told to swim for 10 minutes and dunk their heads underwater at least three times. The other group, 1,235 people, stayed on the beach the whole time and had no contact with the water. Everyone got the same lunch, and both groups had access to activities on shore, though only swimmers were allowed in the water zone.

While swimming, scientists collected water samples every 20 minutes throughout the three-hour trial session and tested for three markers of contamination: E. coli, intestinal enterococci, and somatic coliphages, a type of virus that infects bacteria and serves as a stand-in for sewage contamination. Because samples were taken from specific zones and times, researchers could match each swimmer’s individual exposure level to the water they were actually in, rather than relying on a single site-wide average.

One week later, participants were interviewed by phone or in person about any illness symptoms. Researchers used set definitions to decide what counted as a stomach illness (things like diarrhea, vomiting, or nausea paired with fever), a skin ailment (rash, ulcers, or itching), and other categories like eye, ear, or respiratory infections.

What the Data Revealed About Swimming Risks

Stomach illness and skin problems were the two most common health issues reported, though both were still fairly rare overall. Out of the full group, 48 people (about 2 percent) reported stomach illness, and 47 people (also about 2 percent) reported skin ailments.

For stomach illness, the pattern tracked closely with water contamination. People exposed to higher levels of E. coli had a 73 percent higher risk of getting sick (a relative risk of 1.73), and higher levels of somatic coliphages were linked to a 46 percent higher risk (a relative risk of 1.46). Enterococci levels showed no meaningful connection to stomach illness. Simply being a swimmer instead of a non-swimmer didn’t predict stomach illness on its own either, suggesting that specific germ levels mattered more than just getting in the water.

Skin ailments told a different story. Bacteria levels didn’t predict rashes and itching; only swimming did. Swimmers had more than double the risk of developing skin problems compared to people who stayed onshore (a relative risk of 2.17), yet none of the three water contamination markers showed a link to skin issues. That mismatch hints that something other than the fecal bacteria being tracked, perhaps other organisms living in the water or irritants unrelated to sewage, might be behind the skin reactions.

Other health problems, including eye infections, ear infections, and respiratory illness, were too rare in this group, with only 9 to 13 cases each, for researchers to draw reliable conclusions about what caused them.

Why the WHO’s Freshwater Safety Guidelines May Need a Second Look

European Union bathing water rules currently test for both E. coli and enterococci in freshwater. But the WHO’s most recent global guidelines, updated in 2021, dropped E. coli entirely and kept only enterococci as the freshwater standard. This trial’s authors wrote plainly that “our results do not support such consolidation,” pointing out that enterococci showed no relationship with stomach illness risk in freshwater, while E. coli remained a consistent predictor even after the researchers ran extra checks, including removing the most contaminated site from the analysis and filling in missing data using statistical methods.

According to the researchers, the split makes biological sense. Past lab studies have found that E. coli and coliphages tend to survive longer in freshwater than enterococci do, while the opposite happens in ocean water, where enterococci stick around and E. coli breaks down faster. That difference could explain why a bacterial strain that works well as a warning sign at the beach doesn’t work nearly as well at the lake.

The Bigger Picture for Swimmers

This kind of study is rare for a good reason: randomly assigning people to swim or not swim, then tracking their exposure to contamination levels in real time, is expensive and hard to pull off. According to the authors, only one other trial like this has ever been published, a German study from 2005 that found similar results linking E. coli to stomach illness risk. The consistency between a Hungarian trial and a German trial, conducted in different countries with different water systems, adds real weight to the idea that E. coli deserves a spot in freshwater safety testing, not a demotion.

None of this means lakes and rivers are unusually dangerous. Stomach illness affected only about 2 percent of participants, and most cases were likely mild. But for the health agencies and governments that set the rules for what gets tested before a beach opens for the season, this trial raises a real question. The germ that got cut from the guidebook may have been the one worth watching all along.

Disclaimer: This article summarizes findings from a peer-reviewed study for a general audience and is intended for informational purposes only. It is not medical or public health advice. Individual water quality, health conditions, and local advisories vary, and readers should follow guidance from their local health authorities before swimming in natural waters. The associations described reflect what the researchers observed in their study and do not establish that any single organism directly causes illness.