That glistening swimming hole might look—and feel—refreshing on a sweltering day. But writhing in pain from stomach cramps, nausea, and vomiting? Staring at the bathroom ceiling instead of the blue, sunny sky? Not so much.
Jumping into even the prettiest and clearest rivers, lakes, creeks, and other natural bodies of water can expose you to a cesspool of unpleasant and invisible fellow swimmers—most commonly bacteria, viruses, and parasites. “These germs are microscopic, so you’re not going to be able to tell they’re there,” says Bill Sullivan, a professor of microbiology and immunology at the Indiana University School of Medicine and author of Pleased to Meet Me: Genes, Germs, and the Curious Forces That Make Us Who We Are. “Swimming is a great, fun activity—don’t get me wrong. But you do need to be mindful that there are dangers that lurk out there.”
That’s especially true for certain people. If you’re mostly healthy, you’ll fare better than some; if you’re exposed to bacteria or other nefarious germs, your body should recover fairly successfully. The people most susceptible to severe illness are “the very young, the very old, and those with preexisting conditions that weaken their immune system,” Sullivan says. “If you’re in one of those vulnerable populations, you should definitely reconsider swimming in questionable water.”
We asked experts what threats might be lurking in your favorite swimming hole, and how to stay safe.
The problem with inland water
Environmental scientists have a reliable way of monitoring the water quality at popular coastal beaches, often by taking samples on a daily or weekly basis. If bacteria levels surpass a certain threshold, they post an advisory online and put up signs that warn swimmers the water might be contaminated. “The problem with inland locations is that it’s hard to know where people are going to try to jump in,” says Rachel Noble, a professor in the Institute of Marine Science at the University of North Carolina who leads a laboratory that studies bacteria and viral pathogens in recreational and other types of water. Someone who’s kayaking at a local lake, for example, might make their way to a small island away from the main swimming area and decide to dive in to cool off.
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Still, resources exist. There’s data available for many inland bodies of water, though monitoring is less frequent than at coastal spots. Where Noble lives in North Carolina, for example, she can pull up a recreational water quality website and review data—and a helpful map—from the last week, six months, or year. Every state has a similar resource, she adds, and it’s a good idea to check yours regularly.
The most common health threats
You might think of E. coli and Salmonella in relation to undercooked meat or tainted lettuce. But they’re also the likeliest types of bacteria you’ll encounter in a swimming hole, thanks to fecal contamination from humans or, more likely, animals, Sullivan says. Imagine livestock grazing in a tributary that feeds into the lake you’re swimming in, for example. All it takes is one heavy rainstorm to push their manure and animal waste into the water. “It washes downstream to where people might be swimming,” he says. If you end up with E. coli or Salmonella, you could experience potentially severe gastrointestinal symptoms—which sometimes get so bad, people land in the hospital with dehydration.
A bacteria called Pseudomonas aeruginosa, meanwhile, causes swimmer’s ear, which can lead to ear pain, swelling, and discharge. “It’s not an extremely dangerous infection, and sometimes it resolves on its own,” Sullivan says. “Sometimes a shot of antibiotics will clear it right up, but it can still be a nuisance.”
In addition to bacteria, rotavirus and norovirus could be lurking in the water. Both cause “rapid-onset” diarrhea and vomiting, Sullivan says. Norovirus is particularly contagious, he adds, and can survive outside of the body for one to two weeks. If you get one of these viruses, your symptoms will likely last a few miserable days.
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Another threat, parasites, can cause longer-lasting distress. Cryptosporidium, which is excreted by cows, commonly contaminates natural bodies of water—and if you get sick, you’ll experience watery diarrhea, stomach cramps, and vomiting. “If you have any reason to believe that cattle are near the body of water you’re swimming in, that would give me pause as to whether I want to dip my toe into those waters,” Sullivan says. Giardia, which is shed in the feces of animals like beavers and muskrats, is also common and leads to similar symptoms.
Typically, people need pharmaceutical assistance to recover from parasites, Sullivan says, but there aren’t as many treatments available for Giardia as there are Cryptosporidium. “You’ve usually just gotta ride it out, and it’s a really unpleasant experience that lasts about one to two weeks,” he says. Parasites tend to linger, especially if you’re immunocompromised—and in addition to the physical symptoms, “there’s a psychological element, in that there are these creepy things living inside you,” he says.
Rarer but more extreme risks
You’ve probably seen headlines about the dramatic-sounding “brain-eating amoeba,” or Naegleria fowleri—a parasite found in warm freshwater. It can enter swimmers’ noses and then make its way to their brain, causing an infection called primary amebic meningoencephalitis that’s almost always fatal. “It can feel scary because it’s probably essentially everywhere, but it doesn’t cause disease very often,” says Dr. Daniel D. Rhoads, chair of the College of American Pathologists’ microbiology committee, and section head of microbiology at the Cleveland Clinic. “I’m sure pretty much all of us have been exposed to it, but it doesn’t usually cause a problem because it’s physically challenging to get all the way into the brain just from swimming in water.” When people do get infected, they almost always die within a week.
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Meanwhile, global warming has caused a spike in “flesh-eating disease,” more formally known as necrotizing fasciitis. It’s caused by bacteria in saltwater or brackish water and eats away at soft tissue and muscles. “What happens is the bacteria get inside the body through a break in the skin—some kind of wound,” Sullivan says. “Once it starts taking root, it happens pretty quickly. It causes extreme pain, and it looks like it’s eating away at the flesh, but what’s really happening is the bacteria is destroying muscle and nerves underneath the wound.”
That prevents your immune cells from reaching the site where they would need to be to fight the bacteria—and keeps medicine from getting there, too. That’s why the most common treatment is “either carving away the infected tissue or even amputating the limb,” Sullivan says.
The path to infection
The most common way people swimming inland get sick is by swallowing the water. How much you consume matters: “If you swallow just a teeny-tiny bit, you might be able to get away with it,” Sullivan says. “But if you swallow a good mouthful, there’s a reasonable chance that’s enough to make you sick.”
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Keeping your head above water can help reduce the likelihood of gulping down germs. But it’s not foolproof. When people emerge from the water, their skin might be covered with bacteria or viruses. The next thing you know, they’re chowing down on a ham sandwich or passing around bottles of water, and those germs are hopping from person to person. That’s why you should always sanitize or wash your hands with soap and water after swimming, Sullivan says; if possible, rinsing off in an outdoor shower is a good idea, too.
When should you see a doctor? If you sense your gastrointestinal distress isn’t typical, if you have a high fever, if your symptoms last beyond a few days, or if you see blood in your diarrhea, schedule an appointment, Rhoads says.
How to stay safe
If you’re determined to swim in a river or lake, there are some ways to make the experience safer. Here’s what experts recommend.
Heed posted warnings
Take signs about swimming risks and possible contamination seriously. “If somebody’s taking the time to provide health guidance, it’s worth listening to it,” Rhoads says. In addition to checking local water-quality reports, click over to your destination’s social-media account. Some parks, for example, post warnings about the water, especially after severe storms or if there’s been a sewage leak.
Use all of your senses
Most of the time, the water you’re swimming in won’t have any noticeable signs of what’s lurking beneath the surface. But if you do happen to detect a foul odor or notice a strange color in the water, stay out. Discoloration could indicate “there’s discharge from a pipe or a nearby golf course,” Noble says. “If you see a lot of cloudiness, or if a lake is otherwise clear blue and it looks brown and covered with algae, then the likelihood is that there’s some sort of runoff that’s getting to that location,” she says. “The runoff can contain things that make you sick, so use your senses.”
Pay attention to the weather
The risk of infection spikes in the aftermath of a storm. Heavy rain can wash all kinds of contaminants into the water: dog waste from nearby paths, debris, chemicals, human sewage, and more. It’s impossible to say exactly how long the risk remains elevated, Noble says, because many factors influence it. But if you notice active runoff—or rainwater flowing across the land into the water—it’s best to stay on dry land.
Opt for well-maintained areas
Always do a sweep of the area where you’re swimming to get a pulse on its cleanliness, Noble advises. Is there visible dog waste? Overflowing garbage cans? “If the trash cans aren’t regularly emptied, you can get a lot of birds in them, and those birds will defecate,” she says. “Then that feces will make its way into the water.”
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While birds will naturally inhabit areas with swimming holes, poorly maintained garbage disposal makes it more likely they’ll congregate in high numbers, and that can affect water quality.
Never drink the water
Whether you get thirsty kayaking down the river—or while you’re biking and happen to pass a nice stream—do not drink the water, however tempting it may look. One of the classic case reports Rhoads hears about is people who spotted nice-looking water in a mountain stream and filled up their water bottles—not realizing there were beavers upstream going to the bathroom. Then, he says, they end up with a parasite.
Use ear plugs and nose plugs
Wearing ear plugs when you swim can help protect you from the bacteria that causes swimmer’s ear. And nose plugs (sometimes called clips) act like “a clamp for the outside of the nose,” Sullivan says, which can block water from entering your nostrils. The U.S. Centers for Disease Control and Prevention recommends employing them to help ward off serious infection.
Don’t swim if you have open wounds
This can help protect you from flesh-eating bacteria, Sullivan says. You should also cover any cuts, scrapes, or fresh tattoos or piercings with a waterproof bandage, he advises. Stay away from sharp objects while you’re swimming, too: “If there’s a lot of rocks or shells, and they cut you while you’re swimming, the flesh-eating bacteria can get into your body that way.”
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