Getty Images Many sewerage systems dump overflow into waterways after a heavy rain event, raising the risk of pathogens (Credit: Getty Images)Getty Images
Many sewerage systems dump overflow into waterways after a heavy rain event, raising the risk of pathogens (Credit: Getty Images)

Substantial outbreaks of infectious disease have been reported among wild swimmers, with risk linked to swallowing river water. Even when routine monitoring seems reassuring, river swims can trigger significant illness depending on the conditions on the day.

Studies on health and wild swimming show a consistent pattern: the more contact people have with the water (especially if they swallow it), the higher the risk of gastrointestinal illness.

The clearest increases are seen in sports such as surfing, coasteering and extreme kayaking. Children tend to be at higher risk than adults, partly because they swallow more water when they swim.

Most of these illnesses are short‑lived bouts of diarrhoea or stomach cramps. Occasionally, the pathogens are more serious.

Leptospirosis, a bacterial disease carried in the urine of rodents and livestock, can cause organ failure and death. It remains uncommon in temperate countries, but there have been outbreaks among triathletes in the UK, for example, who swim in open water as part of their competitions.

Public health agencies advise avoiding floodwater and rodent-infested areas. You should try to swallowing as little water as possible, shower after swimming, cover cuts and scratches before swimming and seek medical advice if you develop symptoms such as fevers, chills and headaches after a river swim.

Slow-moving or warm stretches of river can also support blooms of cyanobacteria, sometimes called blue‑green algae. These can release toxins that irritate skin and eyes and, more rarely, affect the liver or nervous system of people.

Signs that it may be unsafe to swim

Precautionary guidance is simple: don't swim if the water is visibly "bloomy" – if it's cloudy and opaque green, blue‑green or pea-soup green, looks as if there is green paint swirling in the water, or if there are floating clumps, streaks, or a scum or foam along the edges or surface. Visible surface scums most likely to coincide with potentially harmful toxin levels.

Avoiding wild swimming in the days after a storm or heavy rain may also be a safer choice. Check the water for blooms, brown flow, sewage or chemical odour. Any sign of dead fish provides another clue that you should skip a dip.

Antimicrobial resistance adds another risk to wild swimming and other wild water sports. For example, research suggests that regular surfers are much more likely to carry antibiotic-resistant Escherichia coli (E. coli) bacteria in their gut than non‑surfers. (Read more on the antimicrobial resistance crisis in rivers and seas.) These superbugs are present not just in seas but in rivers too.

"Antibiotic-resistant bacteria are present in river environments, which could have come from pollution sources like wastewater treatment plants," says Isobel Stanton, an environmental microbiologist from the UK Centre for Ecology and Hydrology.

This antibiotic resistance makes any infections harder to treat, adds Douterelo Soler.

Getty Images With the right precautions, it's possible to lower your risk of becoming unwell from wild swimming (Credit: Getty Images)Getty Images
With the right precautions, it's possible to lower your risk of becoming unwell from wild swimming (Credit: Getty Images)

Tests don't tell the whole story

Many swimmers assume that official tests will capture most of these risks. The reality is more complicated, especially for rivers.

Under European Bathing Water rules, designated sites are monitored for two main faecal indicator bacteria: E. coli and intestinal Enterococci. In England, the Environment Agency typically takes around 20 samples each season at each site. Classifications such as "excellent", "good", "sufficient" or "poor" are based on the previous four years of data rather than a single result.

In the US, the picture is similar but varies by state. Federal guidance focuses on the same kind of faecal indicator bacteria – E. coli (mainly in freshwater) and Enterococci (especially in marine waters) – rather than testing routinely for every pathogen.

Many coastal and Great Lakes beaches in the US are monitored, with sampling frequency varying by location (often weekly, sometimes more often). However, it usually takes about 24 to 48 hours to produce results – so public warnings can arrive after people have already swum. Faster same‑day DNA‑based detection methods exist but are not universally deployed.

Essentially, the current approach to monitoring is designed to track longer‑term trends, and tends to work best at more stable coastal sites. It's less useful for on-the-spot decisions, such as deciding whether a river is safe the morning after heavy rain – because short pollution events can be missed by routine sampling.

Because pollution is so influenced by changing weather conditions, even an official, designated bathing site may at times actually not be safe to swim in. Heavy rain often stirs up sewage and farm run-off and can overwhelm sewage system, leading to spikes in pollution.