Here’s What a Microbiologist Thinks of the Seine Olympic Swimming Situation

In the months leading up to the Paris Games, one of the biggest questions wasn’t only which athletes would be competing, but whether the Seine river would end up being safe enough to host them for some of the swimming events.

As the Games progressed, speculation only intensified. In advance of the men’s and women’s individual triathlons, the swimming portions of practice sessions were canceled due to water quality concerns, and when the events finally did take place on July 31, athletes were…less than thrilled by the conditions. Then, news broke over the weekend that multiple competitors had fallen ill after swimming in the Seine.

On August 4, Belgium even pulled out of the mixed-relay event scheduled for the next day after 35-year-old Claire Michel came down with GI symptoms. While Michel later clarified in an Instagram post that the cause was a virus, the spate of illnesses only underscored the safety concerns athletes and public health experts have voiced ever since it was announced that some Olympic events would be held in the iconic Paris waterway.

“Swimming’s been banned in that river for about a hundred years because it has, notoriously, been dirty,” Bill Sullivan, PhD, a microbiologist and professor at the Indiana University School of Medicine, tells SELF. “There’s a lot of sewage that creeps into the river. And there’s a lot of erosion that occurs with the soil, washing potential animal feces into the water as well.” Combined, those two factors can make for a lot of nasties—like E. coli, the parasites Cryptosporidium and Giardia, and a whole slew of other bugs, according to Dr. Sullivan.

And while you might think that the risk decreases after a downpour—fresh water is good, right?!—the opposite is actually true, according to Dr. Sullivan. Heavy rain is a really big risk factor, he says, especially in old cities like Paris that have more antiquated sewer systems, because the resulting flooding washes all the waste on land into the river. With that in mind, it’s worth noting that rain fell in the days preceding the triathlons, even at the opening ceremony.

However, athletes were able to swim in the Seine after last-minute test results were deemed “compliant” by World Triathlon, the sport’s governing body, according to NBC. Then the mixed-relay event scheduled for August 5 went ahead as planned when officials—including representatives for World Triathlon and the International Olympic Committee—met over the weekend to review the latest water quality tests, which showed the bacteria count to be within the necessary limits, as the Associated Press reported.

But even a stellar test result isn’t a conclusive guarantee of swimming safety: Tests really only target certain species of fecal bacteria, effectively using E. coli as a proxy for other bugs, Dr. Sullivan says. Under most circumstances, that’s “probably okay, but it‘s not a 100% assurance that the water is free of some of these other pathogens as well.” Overall, he says, the whole swimming situation makes him “pretty nervous.”

So, short of simply opting not to compete (and let’s be honest, how many athletes are going to willingly sacrifice a shot at an Olympic medal?), can swimmers do anything to protect themselves from E. coli, Cryptosporidium, Giardia, and all the other bugs bobbing in the Seine? For one, swimmers should ideally try to avoid swallowing any of the water, Dr. Sullivan says, though he understands that’s a pretty tall order. “If you swam in contaminated water and you believe you swallowed some of it,” he says, “be prepared to potentially develop a stomach illness.”

But there are some measures that can help if you do gulp down some. Some teams—like Great Britain, for example—had their swimmers take probiotics before swimming in the Seine, and other athletes, like Team USA’s Taylor Spivey, have mentioned doing similar.

This can be helpful: Essentially, Dr. Sullivan explains, the probiotics work by “outcompeting” any harmful bacteria the swimmers might ingest. “When microbes get into your gut, they need resources in order to replicate and divide, and if you overwhelm your gut with some probiotics that are beneficial—bacteria that don’t harm you—they will usurp those resources in your gut and deprive the bad guys of those resources,” he says. While the probiotics won’t prevent you from getting sick in the first place, they may reduce the severity and duration of any symptoms.

Team Great Britain also mentioned their triathletes would be thoroughly scrubbed down with disinfectant after swimming, another strategy Dr. Sullivan greenlights—and the sooner post-swim, the better. This can help prevent skin infections, he says. Even if you’re not rocking any fresh tattoos, new piercings, or open wounds (and if you are, make sure to cover them with a waterproof bandage before taking the plunge in the first place), “showering and washing your hands thoroughly with soap and water is always a good idea,” he says. “Your 10 fingers are the most common sources of contagion. If you wash them before eating or touching your face, that’s going to minimize transmitting pathogens into your body.”

One strategy Dr. Sullivan doesn’t recommend? Trying to build up your immunity by, say, not washing your hands after going to the bathroom, as one US triathlete jokingly endorsed. “That’s not how it works,” he says.

Despite all the furor, the marathon swim scheduled for August 8 and 9 (also in the Seine) seems set to go ahead. But in the event that the water quality drops, there’s already a contingency plan in place: relocating the event to the Vaires-sur-Marne Nautical Stadium right outside of Paris if necessary.

Editor’s note: This story was updated on August 7 to clarify that Belgian triathlete Claire Michel has since said her illness was caused by a virus.

SELF is your one-click source for all things Summer Olympics. Read our latest coverage of the Paris Games here.

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