Swimming in lakes, rivers, and other natural bodies can raise your risk of coming into contact with pathogens that can make you sick

Natural Water Can Look Safe—But Is It? How to Quickly Assess Before You Dive In Here are precautions you can take to lower your risk

• Lakes, streams, and other natural bodies of water contain pathogens that can sometimes be dangerous to human health.
• Experts outline the waterborne viruses, bacteria, parasites, and toxic algal blooms that can make people sick.
• People should check for any online health notices before they go swimming, and can also follow simple precautions to lower their risk of getting sick.

On hot summer days, lakes, streams, and ponds can seem particularly enticing. But as inviting as they may appear, fresh bodies of water occasionally contain dangerous pathogens that can bring your summertime plans to a screeching halt.

Every year, around this time, headlines start surfacing about people dying after contracting brain-eating amoebas or winding up in the hospital after ingesting toxic algae.

“These bodies of water aren’t filtered or disinfected like pools, so they’re part of living ecosystems where bacteria, parasites, and other pathogens can grow,” Don Thushara Galbadage, PhD, MPH, a public health professor at Texas Christian University whose research focuses on microbiology and infectious disease pathogenesis, told Health.

While these illnesses are rare, these reports are an important reminder that it’s always a good idea to think twice before taking a dip. Here’s what to know about the pathogens that can hide in fresh water—and a few tips to help you assess whether your local swimming hole is safe.

What Are the Risks?

As mentioned, getting sick from swimming in freshwater is uncommon. Still, there’s always some level of risk, said Bobbi Pritt, MD, the director of the Clinical Parasitology Laboratory at Mayo Clinic and a spokesperson for the College of American Pathologists.

That’s especially true in warm, stagnant water, and after heavy rains. 

During rainstorms, agricultural runoff, sewage, or decaying organic matter are pulled into the water, leading to high levels of nutrients that allow microorganisms to thrive, according to Galbadage. Then, you can accidentally get sick if you happen to swallow contaminated water, splash it into your eyes, or expose an open wound to it.

Most healthy people who contract waterborne infections recover without medical treatment, Galbadage said. But for vulnerable people, including young children, older adults, and immunocompromised individuals, these infections can quickly escalate into serious complications.

Here are a few germs to be aware of this summer.

Bacteria 

Leptospira, spiral-shaped bacteria that cause a flu-like illness called leptospirosis, are shed in the urine of infected animals like rodents and livestock. Leptospira thrive in warm, stagnant freshwater, Pritt told Health, and can enter the body through the eyes or mouth.

While mild cases typically resolve on their own, severe cases may require hospitalization and can lead to kidney damage, she said. Between 100 and 200 people in the U.S. get leptospirosis every year.

Vibrio is another bacterium that resides in both saltwater and brackish water, where freshwater mixes with seawater, Pritt said. The U.S. sees about 80,000 annual cases of vibriosis.

There are two main types, Pritt explained: Vibrio vulnificus, which causes severe wound infections and bloodstream infections, and Vibrio parahaemolyticus, which triggers gastrointestinal (GI) symptoms such as diarrhea. While the bacteria often get into the body from raw or undercooked shellfish, they can also get in through open wounds, according to the Centers for Disease Control and Prevention (CDC).

These infections tend to progress quickly and can be treated with antibiotics, urgent wound care, and, most seriously, surgery.

Parasites

As for parasites, there are many that lurk around freshwater swimming spots.

Perhaps the most common is Giardia, which can cause giardiasis, a gastrointestinal disease that triggers diarrhea, stomach cramps, bloating, and gas. More than 1 million people in the U.S. get sick from Giardia annually. While some people who contract the parasite may remain asymptomatic, others may have symptoms for up to six weeks, per the CDC.

Like the bacteria mentioned above, Pritt said you can get sick if you ingest contaminated water, usually from affected streams or lakes. Antiparasitic medications like metronidazole are available to help clear the infection.

Cryptosporidium is another parasite that causes watery diarrhea. This one, which leads to roughly 823,000 illnesses each year, is spread via fecal contamination from infected individuals, said Pritt, and can lead to diarrhea, nausea, and fatigue. “[Cryptosporidium] are hardy and persist in many water types,” she said.

Perhaps the scariest—but also the rarest, with fewer than 10 cases in the U.S. annually—is Naegleria fowleri (N. fowleri), an amoeba that can cause a fatal brain infection. The infection takes root not by simply ingesting contaminated fresh water but after people forcefully enter it, like by diving or submerging their head. Unlike some other waterborne illnesses, swallowing water containing N. fowleri does not cause infection, according to the CDC. Instead, it happens when water goes up the nose.

The illness starts with a severe headache, nausea, vomiting, and fever before progressing to neck stiffness, confusion, and hallucinations. Within five days, it can lead to coma and death.

“It is commonly called the ‘brain-eating ameba’ because it destroys brain tissue during infection, and it is almost always fatal,” Pritt said. Only four people have survived the illness between 1962 and 2023.

Toxic Algal Blooms

Algae, specifically blue-green algae, can quickly grow and form scum on the surface of fresh water, which may be green, blue, or other colors. Again, these blooms love warm, slow-moving waters packed with nutrients.

“Some of these organisms produce toxins which can cause skin rashes, stomach upset, liver injury, and rarely neurologic effects,” Pritt said. 

Viruses 

Finally, viruses—including adenovirus and norovirus—can fester in rivers and lakes. You can get sick if you accidentally ingest droplets that contain one of these hardy viruses. Both cause viral gastroenteritis—a diarrheal sickness that, in most cases, is mild and short-lived.

What to Look For

If you see a promising freshwater swimming spot and have a few minutes, take a few minutes to look up the body of water. Popular watering holes are often monitored or undergo regular testing by local public health authorities, Pastula said. If pathogens have been detected or reported, the agency will post warnings urging people to not go in.

You can also look up the local park authority or ask a nearby park ranger to see if they have any information on local bodies of water, Pastula said.

As far as eyeballing it, unfortunately, there often aren’t any glaring clues that the water you’re about to wade into is dangerous.

But, in general, Daniel Pastula, MD, MHS, a professor of neurology, infectious diseases, and epidemiology at the University of ColoradoSchool of Medicine, recommends steering clear of brackish water, stagnant water, or foul-smelling water as they’re likely to have a higher concentration of pathogens compared to clear, moving, scent-free water.

What might contaminated water smell like? “Sewage, sulfur, or decomposing vegetation,” Galbadage said. He also recommends keeping a distance from water that has a layer of foam or scum on the surface.

Murky water may be free of pathogens, but come with additional risks—the cloudiness may conceal other hazards like submerged debris or dead animals that present their own set of health risks, Galbadage said.

That said, even water that looks clean and clear can contain pathogens. And suspicious water—such as a cloudy appearance or green algal bloom—may not actually be harmful.

When in doubt, consider delaying your swimming plans if it recently rained, especially after a heavy storm, Galbadage said.

How to Reduce Risk—Even If Water Seems Safe

If the water looks safe and you can’t find any health warnings, being careful is still a good idea. To minimize your risk of contracting a waterborne illness, Pastula told Health he recommends practicing these easy steps:

• Don’t swim with open cuts
• Avoid swallowing water
• Try not to get water up your nose
• Don’t stir up the sediment below
• Avoid swimming near sewage or drainage pipes
• Wash your hands after you are finished swimming

Galbadage also recommends rinsing off after your swim, wearing swim goggles to protect your eyes, and keeping your head above water. Just because the pond or lake looks clean doesn’t mean it is.

“Natural water can carry hidden health threats even on the sunniest day,” Galbadage said.

Edited by Health with a background in health, science, and investigative reporting. Previously, she wrote full time about parenting issues for the app Parent Lab. Before that, she worked as a reporter for National Geographic covering wildlife crime and exploitation." tabindex="0" data-inline-tooltip="true"> Jani Hall Jani Hall Jani Hall is a news editor for Health with a background in health, science, and investigative reporting. Previously, she wrote full time about parenting issues for the app Parent Lab. Before that, she worked as a reporter for National Geographic covering wildlife crime and exploitation. learn more Read more:

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This story originally appeared on: Health News - Author:Julia Ries