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There’s no better cure for a hot Pacific Northwest summer day than a jump in a cold body of water. But before you jump off that stand-up paddleboard in Lake Washington or cannonball into the public pool at the park, look before you leap.
In addition to the risk of drowning (the most serious danger for swimmers, by far) lots of things are lurking in the depths that have the potential to make you sick.
“One of the great things about living in Seattle is all these beautiful bodies of water all around us,” says Paul Pottinger, MD, a physician and director of the Infectious Diseases & Tropical Medicine Clinic at UW Medical Center – Montlake. “Please go into the water with a buddy and a flotation device. And, know that you’ll never find a body of water that’s totally free of germs — it’s about which germs are there, and how many you’ll encounter.”
Here’s what you need to know about the gross things lurking in the depths.
Germs from other people’s poop
Given our chilly ocean waters, most people in the Pacific Northwest swim in pools, lakes and rivers. The easiest way to get sick in fresh water? Poop from other people (and critters).
“The water that runs into those lakes and rivers has human waste material in it, and that waste material can be a threat if we ingest it,” says Pottinger.
That’s not to mention dogs, geese and other animals who poo near swimming holes. The most common feces-related bacteria, viruses and parasites are:
Stomach bugs
All that poop can contain bacteria and viruses like E. coli, Salmonella and norovirus, which will make you run to the bathroom with diarrhea or vomiting.
One fact to keep in mind? Not all bodies of water carry the same risk.
“The solution to pollution is dilution, so a small stagnant pool is going to be higher risk than something that flows or larger bodies of water,” says Pottinger. “But even in large bodies of water, including Lake Washington, if there’s a lot of people there and it’s a hot day, it might have unacceptably high amounts of coliform bacteria.”
Cryptosporidium
Another place to be careful is public pools and water parks. The parasite Cryptosporidium is another germ from human and animal waste that especially likes these warm environments. Unfortunately, it is relatively resistant to chlorine, so it can survive for more than a week even in a well-disinfected pool. If you get infected by swallowing it, you can expect bad diarrhea for up to two weeks — longer if your immune system is reduced.
Giardia
If you’re a hiker and backpacker, you’ve probably thought about Giardia, a parasite that often gets into rivers and lakes from animal droppings, most famously beavers. Truth is, human waste entering the water upstream can spread Giardia, too. That’s why experienced backcountry travelers know to boil their water or bring a water filtration system when camping: The consequences of getting Giardia, especially while hiking, are not pretty — think some of the worst, stinkiest diarrhea of your life.
You might be more surprised that water parks can also experience outbreaks of Giardia, since, again, it can be spread through human poop, as well. If a pool is well-maintained and treated, though, Giardia isn't likely to survive.
Hot tub itch
Yet another germ lurking in pools, hot tubs and hot springs is called Pseudomonas aeruginosa, which can cause painful, pus-filled bumps on your skin, often called “hot tub itch” or “hot tub folliculitis.” This microbe loves warm water, but proper maintenance and disinfection will kill it. So if you’re at a vacation rental and you’re not sure when that hot tub was last cleaned, maybe think twice before hopping in.
Swimmer's itch
You might have heard of another water-related problem: swimmer’s itch. It is actually caused by a parasite that infects waterfowl like ducks. Humans aren’t a good host for it, but you can still get a rash when the organisms get onto your skin and try to infect you.
“It causes a red, raised, bumpy rash, which is very itchy,” says Pottinger. “It's not serious, but if you scratch at it, that can cause a secondary infection.”
Germs that live in the water
Some microbes aren’t human or animal-caused, they just call oceans, lakes and rivers home. And you don’t want to be unlucky enough to encounter them there.
Algae
One of the most common environmental contaminants in fresh water is toxic algae.
“If a beach is closed because of a toxic algae bloom, please do not go in the water,” says Pottinger. “That's not just a risk of diarrheal illness, other health effects can happen.”
People (and pets) who’ve ingested a large amount of toxic algae (including through eating shellfish) can experience neurological issues like muscle weakness, confusion, dizziness and short-term memory loss, in addition to symptoms like vomiting, diarrhea and cramping.
Vibrio
Other microbes live in salty and brackish water and can be especially harmful if you have an open wound. One of the nastiest? Bacteria called Vibrio vulnificus, which can cause a serious infection if it gets into a cut or a scratch.
“If somebody notices that they have an infection of their foot or their ankle after they've been on the beach or tide pooling, or in the ocean, they should see a doctor right away because these can become very serious, very quickly,” says Pottinger.
Signs of an infection include redness, swelling, fluid-filled blisters and fever.
Brain-eating amoebas
Let’s talk about the scariest thing last. You might have heard horror stories about brain-eating amoebas. The good news is that they’re very rare, especially in Washington state. The bad news is that they are extremely lethal.
Naegleria fowleri is the main culprit for swimmers. It’s a free-living amoeba (aka, it doesn’t need a host) that thrives in shallow fresh water in warm places, like hot springs and stagnant pools. Once it gets into your nasal passages, say through some splashed water, it moves with terrifying speed into the brain.
“It’s a catastrophe,” says Pottinger. “They come in through the nose and they destroy the brain. It's usually a life-threatening or fatal condition.”
Only 157 Naegleria fowleri cases have been reported between 1962 and 2022 in the U.S., and none in Washington state, but they do exist in our lakes and oceans, and global warming could make infections more likely in the PNW in the future.
How to stay safe
While it’s important to stay informed, don’t get too freaked out by these germs. If you take the right precautions, you can enjoy a dip in a lake, ocean or pool with low risk of getting sick.
Check the local health department website
The best way to prevent getting sick from swimming is to not go into contaminated water to begin with.
“ You should know whether it's safe to get in the water based on the amount of germs that are being detected,” says Pottinger.
That’s made easy thanks to local health departments, including King County’s, who conduct regular wastewater surveillance at swimming areas. King County posts a map of lake beaches in the region that are closed because of unsafe levels of bacteria or algae. And Washington State has a similar map for saltwater beaches. Be sure to check those before heading out for a swim, especially if there’s recently been a lot of rain, which could lead to contaminated storm runoff going into lakes and rivers.
Try not to swallow too much water
When you do jump into a lake, ocean or pool, a gulp of water might be inevitable — but even so, try to keep water out of your mouth. If you’re at a water park, keep your face away from jets of water.
“I think it's impossible to avoid some water getting in the mouth, but it is true that if you drink less water, your risk also goes down regardless of the kind of pathogen,” says Pottinger.
If it’s a particularly warm body of water, you may want to just keep your head out of the water altogether. Brain-eating amoebas are very rare, but you may still decide it’s not worth the risk.
Avoid swimming with an open wound
You now know about Vibrio, so it goes without saying that if you recently got a cut or scrape, it’s wise to wait until it scabs up before going for an outdoor swim.
Towel off quickly
Parasites that live on your skin, like the kind that cause swimmer’s itch, can be wiped off with a towel if you dry off shortly after getting out of the water. Even better? Take a shower right after swimming if one is available.
If you’re having stomach issues, stay home
Many of the gross germs listed above are spread by other humans — you don’t have to be one of them. If you’re having stomach issues, stay home or at least on the beach.
“If you've recently been sick, please stay out of the water as a courtesy to other people,” says Pottinger.
If you’re worried, go to the doctor
So you’re unlucky enough to get diarrhea after your swimming hole excursion — or worse, a wound is getting gross-looking. Don’t hesitate to go to primary care or urgent care.
“If you get diarrhea after being in the water, this may be one of these infections, but it's hard to know if it's a parasite or a virus or algae or bacteria,” says Pottinger. “You should see a doctor to get checked out.”
