What diseases can you get when swimming in a lake, river, ocean or pool?

Being bitten by infected ticks and mosquitoes, alas, is not the only germ danger you face while enjoying the great outdoors.

In fact, just going for a swim can make you sick, even if you never get near an insect. Cryptosporidium, Giardia, and Shigella are out to get you, and they’re lurking where you swim, summer or winter, ocean, lake, river, or pool.

Not to mention the Jacuzzi at the spa, the decorative fountain the toddlers wade in, and the water park where the kids had such a great time. Sometimes it can take several weeks after swimming in polluted water for a diarrheal illness to begin, and therefore people don’t often connect the swimming with the illness.

Many people don’t even realize that swimming in a chlorinated pool can make you ill. In addition, diarrhea is fairly common, and most people wouldn’t bother reporting diarrhea even to their own doctor, much less to the health department. So the CDC’s statistics are almost certainly an underestimate of the actual number of outbreaks.

Swimming is the second most popular exercise in the United States, and there are about 400 million pool visits every year. In 2000, there were five outbreaks of swimming pool–linked cryptosporidiosis reported to the CDC. This made several thousand people sick, as such outbreaks do every year. Cryptosporidium isn’t a bacterium and it isn’t a virus. It’s a protozoan parasite.

As you know, crypto can get into water supplies, and when people drink water infested with crypto, they get bad diarrhea. People usually don’t die from the illness (there are some deaths among the very old, the very young, and the immunocompromised), but it’s still no fun.

You might think that a well-chlorinated swimming pool would make such an infection impossible, but you would be wrong. In fact, crypto is highly resistant to chlorination, and even the best-maintained swimming pools can be full of it. If you go swimming in one of these pools, and if the water gets into your mouth, you can be infected, too.

How does crypto get into a swimming pool in the first place? People’s rear ends, to put it delicately, are typically not very clean places. And when people go swimming, their rear ends get washed off in the water. If a person goes swimming with diarrhea and has a “fecal accident,” a large number of cryptosporidium can be released into the water, where people have a good chance of swallowing some of them. It isn’t easy to know when someone has had one of these accidents, and crypto can survive for several days in chlorinated water, so you can’t really know when you’re entering a pool that is infected.

Crypto has a clever way of protecting itself in the outside world until it can find a new home inside another host. The parasite is excreted from an infected person in an oocyst, a kind of protective shell. It doesn’t “hatch” until it is ingested by another person, in whose gastrointestinal tract the oocyst breaks open and the sporozoites inside attach themselves to the epithelial cells.

Parasitizing these cells, the protozoan undergoes asexual multiplication by cell division, and then sexual multiplication in which female macrogamonts are fertilized by male microgametes. Next oocysts are generated, which contain the new young sporozoites. The sporozoites, encased in their oocyst armor, wait in the host’s feces until they are excreted to the outside world, where they can seek a new host. Chlorination in the concentrations used in swimming pools is no barrier to crypto’s continuing reproduction.

One “fecal accident” can pollute an entire swimming pool. And very few organisms are required to infect a human. If you swallow just one or two mouthfuls of water from that polluted pool, chlorinated or not, you can get a case of cryptosporidiosis.

Where else is crypto found? Since crypto is resistant even to chlorinated water, you can easily imagine that unchlorinated water is an even safer haven for the bug. If you swim in lakes or rivers, you can still get crypto. Bathing in Jacuzzis, decorative fountains, or hot tubs is also a source of infection. And since crypto is resistant to salt water as well, you can even get it by swimming in polluted ocean water. All you have to do is swallow some of the water.

The disease itself, cryptosporidiosis, is very unpleasant, but it usually goes away by itself without treatment. It can last for up to two weeks, with symptoms disappearing and then returning during that period, and the diarrhea, stomach cramps, and fever that can be associated with it can be debilitating. There’s no medicine that cures it, and no treatment other than drinking plenty of fluids.

Usually the disease is so mild that people get crypto without even knowing it, it goes away, and they are none the worse for wear. Unfortunately, crypto has yet another neat survival trick: it can stay in you for up to two months after the illness has disappeared, and during this period, you can pass the disease on to others. Since you never knew you had it in the first place, you don’t know you’re still infectious. This is great for the parasite, but not so good for people you go swimming with.

Can you test the water for crypto before you swim in it? Well, there are tests, but it is very unlikely that you are equipped to carry them out. For one thing, the tests require many hours of analysis by a specially trained microbiologist.

For another, you’ll have to supply that microbiologist with somewhere between 100 and 400 gallons of water to do the tests. And of course testing the water one day doesn’t mean that it can’t be infected the next.

Public municipal water systems do periodically test for crypto, but a home test for your own swimming pool or hot tub is simply not realistic. Your best bet is to prevent crypto from getting into your swimming water in the first place, and helping to protect others by not entering public swimming areas if you have diarrhea, and not allowing your kids to swim if they have diarrhea. Infants who are not toilet trained should not bathe or swim with other people.

There are two ways the reproductive cycle of crypto can be stopped: crypto can’t survive boiling for more than a minute, and the parasite is big enough to be filtered out of water, provided you have the right kind of filter, but conventional swimming pool filters are not the right kind.

So boiling and filtering can prevent you from getting crypto from drinking water, but they obviously can’t help at all when you go swimming.

About Karen Hill

Karen Hill is a freelance writer, editor, and columnist. Born in New York, her work has appeared in the Examiner, Yahoo News, Buzzfeed, among others.

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