Popular swimming spots may be screened for bacteria, but they aren't screened for harmful protozoa.
A microscopic view of Giardia.
A microscopic view of Giardia. CDC
Under the Microscope
Microbiologist Thaddeus Graczyk of Johns Hopkins University and his grad students analyzed the water at Maryland's Gunpowder River for several types of protozoa that can cause human disease. (Health officials regularly check for bacteria, but there are no commercial tests in widespread use for protozoa.)
Graczyk and his students found protozoa in 30 percent of the samples they collected on weekdays. And they found them in 59 percent of the samples they collected on weekends, when there were a lot of people in the water kicking up sediment.
Here's a little more about what they found:
Giardia lamblia and Cryptosporidium parvum: When people take in enough of either of these two protozoa to cause illness, the most common problem is diarrhea. People may also suffer stomach cramps, nausea, vomiting or fever. Infection usually isn't serious. But it can cause problems in children, the elderly, and people whose immune systems have been battered by HIV/AIDS or cancer chemotherapy. Giardia infections can be treated with antibiotics.
There's also an antibiotic drug for Cryptosporidium, but people with compromised immune systems have a hard time tolerating it. For them, doctors take an indirect attack: They try to beef up their patients' immune systems.
Microsporidium (Enterocytozoon bieneusi and Encephalitozoon intestinalis): These bugs are difficult to test for, and no one is quite sure how they pass between and among humans and animals. They can inflame the small intestine, and cause diarrhea and other problems. —Joanne Silberner
Sources: Applied and Environmental Microbiology, Water Research. World Health Organization, Centers for Disease Control and Prevention
Recreational water illnesses (RWIs) are spread by swallowing or inhaling contaminated water from swimming pools, hot tubs, lakes, rivers or oceans.
Although chlorine is often strong enough to kill bacteria and protozoa within hours, some germs such as Cryptosporidium, which causes diarrhea, can easily survive for days in pools that follow recommended disinfection protocols.
Rivers can become contaminated with germs from sewage, animal waste and water runoff following rainfalls. Some common RWIs are diarrhea, skin rashes and eye infections.
Learning about RWIs and the germs that cause them can help protect you from illness.
Read more about waterborne diseases at the Centers for Disease Control and Prevention's Web site.
Also at the CDC, find out what you can do to splash around safely this summer, and what questions you should ask your pool operator.
What's in Your Beach Water?
Want to know how safe your beach is? Browse Earth 911's interactive state-by-state map that allows you to monitor your beach's water. You can also find out what your local officials are doing to make sure water-quality levels remain high during the swimming season. —Alejandra Garcia
In the summertime, there's nothing more inviting than refreshing pools of water. Unfortunately, pools, ponds, rivers and oceans are also contaminated with microbes that could make you sick.
At Gunpowder Falls State Park in Maryland, it's easy to think everything is fine. The park is beautiful, with a lush green lawn, a row of shade trees and picnic benches. There's a clean, broad beach and a sparkling, blue river a mile wide. Several dozen people splash about under the watchful eye of a lifeguard.
But Thaddeus Graczyk knows there is at least the potential for problems. In his day job, he's a microbiologist at the Johns Hopkins Bloomberg School of Public Health. For the last three years, he has been coming here on weekends to play.
"This is very nice spot for recreation," Graczyk says. "I use it for windsurfing."
But right from the start, he noticed something: "When there are a lot of people in this water, the water is turbid."
People were kicking up sediments. Sediments can carry organisms called protozoa, and some of those can make you sick. At swimming areas that are monitored, local officials routinely test the water for bacteria. But not for protozoa.
Graczyk got some grad students together. They waded out into the water and collected samples on weekdays, when there were few people in the water, and on weekends, when there were a lot of people.
"More samples came up positive on the weekend," he says. "And concentrations of the pathogens in the weekend samples were higher than the weekday samples."
So how much should you worry about these and other microbes before diving in?
That's a hard question to answer, says Michael Beach. He's head of the Healthy Swimming Program at the Centers for Disease Control and Prevention. There are no national surveys, he says.
"We see 30 to 40 outbreaks per year occurring in recreational waters, but we think that's the tip of the iceberg," Beach says.
Outbreaks can affect a few people or a few thousand. Usually the infections aren't serious. But Beach says people with open wounds should be extra careful. And so should another group.
"People who have compromised or weakened immune systems should consider that swimming is a communal bathing activity," he says. "They need to consider whether swimming is an option for them."
At Gunpowder Falls State Park, Bill Howard and his wife and son are eating their lunch. They aren't fazed by talk about germs.
Though not all parks are monitored, this one is. Park officials have closed the beach a few times in the past few years because of bacteria contamination. So Howard figures he's safe – the closings show officials are paying attention.
"Probably I'm more worried about sun overexposure than what you might get from the water," Howard says.
Another picnicker, Kristin Devaux, is here only because her father-in-law likes it. She prefers her water chlorinated — like in community pools, "because I know what's in them."
She's assuming chlorine takes care of the germs. But does it?
"I think that was reasonable until about two decades ago when we've seen the emergence of a parasite that's actually chlorine resistant, so now we have a bug out there that can bypass the major barrier to disinfectants in swimming pools," says the CDC's Michael Beach.
And nobody checks pools for these chlorine-resistant protozoa. Still, chlorination takes care of most bugs.
Beach says pools should be chlorinated several times a day.
As for rivers, ponds and oceans, think twice before jumping in after a heavy rain, when sewage runoff can contaminate the water.
Beach also advises people to protect your fellow swimmers and stay out of the water if you've had a recent gut-wrenching illness. And parents shouldn't rely on swim diapers for their kids.
"These are not a panacea," he says. "If your child is ill with diarrhea, swim diaper or not, they shouldn't be in the water."
Overall, your chances of getting sick are slim.
"There are hundreds of millions of visits to swimming venues every year," Beach says. "Most of those people are just fine."
Beach regularly swims in a lake — he just tries not to swallow the water.
As for Graczyk, he likes his spot on Maryland's Gunpowder River. You won't see him packing away his windsurfing gear, unless the wind is better somewhere else.