by Coach Jeff Sankoff, M.D. (a.k.a. the Tri-Doc)

Earlier this month at an event in Sunderland in the United Kingdom, 57 people were reported to have fallen ill with gastrointestinal ailments after participating in the World Triathlon Championship Series event there. More recently, World Aquatics had to abruptly cancel the Open Water Swimming World Cup event planned for the Seine in Paris and there is concern that the Paris 2024 triathlon test event will not have a swim if the water quality does not improve.

These instances of illness and poor water quality are not new nor are they particularly uncommon when it comes to triathlon. Over the past several years there have been numerous cases of events having to cancel the swim portion of the race or of athletes complaining that they became ill when the swim wasn’t canceled but maybe should have been. What is going on here? Is there an explanation for why water can become particularly dangerous and is there a way that athletes can take precautions to prevent becoming ill when the swim does take place?

A review article in Current Sports Medicine Reports from 2019 addressed these questions and helped shed some light on this issue.[i] The authors point out that along with the potential for infection, open water swims are associated with significant potential environmental hazards. Water temperature is the most commonly considered of these with a long-established association between an increased risk of fatalities with lower water temperatures. Other medical problems that are associated with colder water include hypothermia, exercise induced bronchospasm and swimming induced pulmonary edema (SIPE).

Other environmental hazards also pose risks to swimmers in open water. These can be from rough surf, strong currents or marine animals like jelly fish, sharks or apparently, otters!

Still, infections are a growing concern among public health and medical professionals and are increasingly recognized as an all-too-common occurrence after swimming in open water even when the swim takes place in salt water or in a river with a strong current.

The connection between swimming and illness has been recognized for more than a hundred years. In the early to middle decades of the twentieth century summertime swimming in lakes or ponds all too frequently led to the contraction of polio, a virus transmitted in sewage and then ingested by swimmers leading to disease and occasionally permanent paralysis.

With the advent of the Salk vaccine, the threat of polio receded but other diseases still contaminated the water. These mostly come in the form of other viruses and are generally mild but there are also parasites and bacteria that can be ingested from contaminated water and these can lead to more serious illnesses.

In 2017, researchers published a study looking at a large outbreak of diarrheal illness in swimmers at an event in the Thames River in London.[ii] The authors found that fully one third of participants contracted a gastrointestinal illness after the event and this was despite water samples demonstrating no significant bacterial contamination measured the day before. In addition, local wastewater utility managers reported no known sewage spills into the river on the day of the event. So how did so many people become ill?

It turns out that on the day before the swim took place there was an unusually high amount of rainfall detected in the area surrounding London. All that rainfall the authors reported, washed off farmland and into tributaries of the Thames. By the time the wastewater poured into the river where the open water swim was taking place it was laden with all of the animal waste and fertilizer that it had picked up from the surrounding farmlands and it was this that swimmers then ingested and became ill from.

The impact of rainfall was dramatically shown in another paper from Denmark where participants of the Ironman Copenhagen were compared in years that were dry to those when there was extreme rainfall.[iii] In drier years rates of gastrointestinal illness after the event measured 8% while after a year with extreme rainfall it was as high as 42%!

Rainfall is not the only environmental factor that impacts the likelihood of swimmers becoming ill. Water temperature is another. Higher water temperatures in conjunction with increased runoff from adjoining farmlands can lead to algal blooms and in some cases those algae may be toxic to fish and mammals. In 2019 just such a bloom of toxic blue-green algae in the Ohio river led to the cancellation of the swim for Ironman Louisville. Had it not been cancelled there would have been a significant threat to participants.

More benign but still annoying conditions that can be contracted from the water include swimmers itch, a reaction to parasites deposited by waterfowl. These are generally benign and no more than a nuisance to triathletes but still a nuisance!

Knowing all of this, what can triathletes do to mitigate their risk when participating in an event? One of the most important things that an athlete can do is just be aware. Keeping an eye on the weather leading up to an event can give a reasonable indication of whether you can expect significant runoff from farmland or spillage from sewage plants because of flooding. If there is a lot of rainfall in the forecast leading up to an event this is a warning sign that water contamination is possible.

If rain is in the forecast there isn’t a whole lot that you can do. Water quality is assessed on a regular basis and it is no one’s best interest to allow an event to take place if water quality is out of line with safety standards but because of the delay in results it is possible that a safe result comes from a day before the event and that rain fall since that result was obtained will have dramatically changed the quality of the water in the intervening time period. If you have concerns, you have two options: 1) do not participate or 2) limit the amount of water that you ingest.

Several studies have correlated the amount of water that swimmers ingest during an event with the likelihood that they will become ill. While swallowing water is often completely involuntary, making a conscious effort to NOT swallow any water is a potentially viable strategy.

There are a couple of reassuring thoughts to consider after all this discussion. First, even if you do ingest a quantity of water that is sufficient to cause illness if the water was contaminated, the time for the onset of that illness is in most cases longer than the time it will take to complete the event. In other words, you should get to complete your event before you develop any symptoms. Second, if you do become ill, although the illness is unpleasant these are very much self-limited and rarely need interventions. Aside from treating the symptoms antibiotics are only needed occasionally and most cases last no more than two to three days.

Train hard, train healthy.

[i] Chamberlain M, Marshall AN, Keeler S. Open Water Swimming: Medical and Water Quality Considerations. Curr Sports Med Rep. 2019;18(4):121-128. doi:10.1249/JSR.0000000000000582

[ii] Hall V, Taye A, Walsh B, et al. A large outbreak of gastrointestinal illness at an open-water swimming event in the River Thames, London. Epidemiol Infect. 2017;145(6):1246. doi:10.1017/S0950268816003393

[iii] Harder-Lauridsen NM, Kuhn KG, Erichsen AC, Mølbak K, Ethelberg S. Gastrointestinal Illness among Triathletes Swimming in Non-Polluted versus Polluted Seawater Affected by Heavy Rainfall, Denmark, 2010-2011. PLoS One. 2013;8(11):78371. doi:10.1371/JOURNAL.PONE.0078371