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Laura Reineke is an avid open water swimmer. Her passion has taken her from crossing the English Channel to circumnavigating Manhattan Island. But the hobby has also made her feel poorly on several occasions. “They’re always severe. They always take you by surprise. Joint pain, full body aches, diarrhea, vomiting,” recalled Reineke, who has founded Henley Mermaids, a grassroots campaign against sewage pollution, and Friends of the Thames, a charity organization that aims to protect the English river.

As temperatures warm across Europe, many more open water swimming enthusiasts are venturing into rivers, lakes, reservoirs, and seas. While offering significant physical and mental health benefits, the burgeoning pastime increasingly exposed people to waterborne contaminants. But the risks are difficult to quantify.

Therese Westrell, PhD, principal expert for Food and Waterborne Diseases at the European Centre for Disease Prevention and Control (ECDC), told Medscape Medical News, “We don’t really know how big of a problem there is across Europe. There are risks that we know of when there is high contamination.”

The Microbial Menace

Europe’s bathing waters are routinely monitored for Escherichia coli and intestinal enterococci, which serve as bacterial indicators of fecal contamination, as mandated under the European Union (EU) Bathing Water Directive. The latest figures for the 2023 season show that 85.4% of bathing waters in the EU were classified as “excellent,” indicating high overall quality. However, a persistent 1.5% of sites, amounting to 321 locations, were still classified as “poor.”

A significant disparity in water quality exists between inland and coastal areas. Inland bathing waters, including rivers and lakes, generally exhibit poorer quality, with 2.4% of inland sites categorized as “poor” in 2023 compared with 1% of coastal sites. The European Environment Agency wrote to Medscape Medical News in an email that these inland waters “are more susceptible than coastal areas to short-term pollution caused by heavy rains or droughts.”

The primary sources of microbial contamination are human activities and animal waste. Inadequate urban wastewater treatment, agricultural runoff carrying livestock waste, and even direct contamination from waterfowl like mallards can elevate bacterial levels in water. In some big, older cities, such as London and Paris, combined sewer overflows are used as safety release valves to prevent urban flooding during heavy rainfall. However, their frequent activation leads to the direct discharge of enormous volumes of untreated sewage into waterways. In England alone, more than 400,000 such discharges occurred in 2020.

Common illnesses associated with microbial contamination include gastrointestinal infections caused by pathogens like Norovirus, E. coli, Giardia, and Cryptosporidium, which give symptoms such as diarrhea, vomiting, and abdominal pain. Other manifestations include respiratory, dermatologic, ear, nose, and throat infections. A more severe, albeit rarer, bacterial infection is leptospirosis (Weil’s disease), which can cause significant liver and kidney deterioration.

Quantifying the exact extent of these problems across Europe remains challenging due to the complexities of attributing specific transmission modes to surveillance data. “The information we have on which cases come from swimming and which are from food is not really there,” Westrell said.

However, specific microbes are closely monitored. Vibrio species, for instance, are naturally occurring bacteria in coastal waters that thrive in elevated temperatures and brackish water. Ingestion can cause mild gastrointestinal symptoms, but wound infections are more concerning. “If you get it through a wound, you can get a very severe infection. You can also get the blood infection, which could be fatal,” Westrell explained.

The Baltic Sea has been a hot spot for these bacteria, with a significant increase in reported cases from several countries during the 2018 heatwave, Westrell said. With global temperatures rising, Vibrio species are expanding beyond the Baltic area, and the ECDC is actively increasing its surveillance on the microbe.

European Member States report on the occurrence and management of local infectious disease outbreaks linked to water swimming, but comprehensive pan-European data is fragmented and not routinely collected in all waters.

Chemical Contaminants

European waters also contain a “cocktail of chemicals,” said Helena Rapp Wright, PhD, a research associate at Imperial College London, London, England. These compounds originate from many sources, including pharmaceuticals and personal care products, industrial discharges, agricultural pesticides, and vehicle emissions.

Most wastewater treatment plants remove pathogens but are not designed to remove these chemicals. Consequently, many of these substances pass through treatment processes and are discharged directly into rivers and other water bodies. While their concentration is generally low in large bodies of water, the continuous input means they are persistently present.

Rapp Wright and her colleagues have found “loads of chemicals” in English and Irish waters. While many pose no risk at current concentrations, some compounds are “quite toxic,” she said. Particularly concerning are pesticides detected at high-risk levels, such as imidacloprid, which was banned in 2018 but is still used for pet flea treatments, and antibiotics due to their potential contribution to antimicrobial resistance, she explained.

But for swimmers like Reineke, who trains at least 1 hour in the Thames 6 days a week, the primary health concern related to chemical contaminants is often long-term exposure and the potential of developing chronic conditions, rather than immediate acute effects, which are more commonly associated with microbial pathogens.

What General Practitioners (GPs) Should Be Aware of and Do

To reap the health benefits of open water swimming while minimizing the possible hazards associated with exposure to pathogens and pollutants, GPs can help their patients practice this sport in the safest way:

• Understand risk profiles: While many coastal bathing areas are designated and monitored, rivers and inland lakes often lack this official status, implying a higher inherent health risk for swimmers in these less-regulated environments.
• Advise on avoidance: It is important to advise patients to avoid open water swimming during and immediately after periods of heavy rainfall, especially if these follow dry spells. Such conditions significantly increase the likelihood of combined sewer overflows discharging untreated sewage directly into waterways.
• Encourage informed decisions: Encourage patients to utilize readily available public resources before swimming. These include official online bathing water profiles provided by environmental agencies, such as the Environment Agency’s Swimfo in the United Kingdom, and local real-time pollution maps, such as Wild Swim Map, which visually indicate safe (green) vs recently polluted (red) areas.
• Clinical suspicion: When patients present with unexplained gastrointestinal issues, skin rashes, respiratory symptoms, or ear, nose, and throat infections, GPs should always inquire about recent open water swimming exposure in their history taking. For severe wound infections, particularly after swimming in warmer brackish waters, consider Vibrio as a potential pathogen.

Ultimately, Rapp Wright said that the goal is not to deter enthusiasts from the therapeutic joys of open water bathing and swimming. “We don’t want people to stop [swimming], we want to make sure that they do it safely.”

As for Reineke, the potential of falling ill has not stopped her from dipping into the water. “I want to do Oceans Seven and become the first British female to do that.”

Medscape Medical News reached out to the European Commission spokesperson responsible for health, but they declined an interview.

Reineke, Westrell, and Rapp Wright disclosed no relevant financial relationships. 

Manuela Callari is a freelance science journalist specializing in human and planetary health. Her work has been published in The Medical Republic, Rare Disease Advisor, The Guardian, MIT Technology Review, and others.