There’s a reason why veteran hikers and campers never drink directly from lakes or streams, no matter how clean and pure the water may look. It’s the same reason why getting a mouthful of water from a swimming pool or a hot tub isn’t such a great idea, either.

Water that isn’t intended for drinking can harbor a nasty surprise: A single-celled protozoan parasite called Giardia. This microscopic creature infects about 200 million people worldwide each year, making it one of the most common causes of waterborne illness. According to the Environmental Protection Agency, Giardia may be found in practically every lake, stream, river, or pond on the planet. That’s enough to give pause to backpackers everywhere. Even more worrisome, the parasite threatens the water supply of many cities, towns, and villages throughout the world.

Giardia — why you want to avoid it

Like all parasites, Giardia survives by feeding off a host. In this case, the parasite feeds off the cells in an animal’s digestive tract. Giardia can infect people, rodents, and a few other mammals, as well as birds, amphibians, and reptiles. The feces of the host contains millions of Giardia cysts, or tiny “eggs” that can survive in water or soil for months. Each oval-shaped cyst is about 10 microns long (one hundredth of a millimeter), far too small to be seen with the naked eye. If the cysts find their way into another welcoming digestive tract, the parasites will emerge to cause another infection.

As you might expect, having a protozoan parasite in your digestive tract is generally not a pleasant experience. Children often — but not always — sail through without any symptoms, but many other people can expect a case of diarrhea, ranging from mild to severe. Young children and pregnant women who get diarrhea from a Giardia infection may be particularly susceptible to becoming dehydrated. The diarrhea can start as soon as three days after infection, but it can show up as much as three weeks later. Bloating, flatulence, cramps, nausea, and greasy or fatty stools are other possible symptoms. The symptoms may disappear within a few days, but some people stay sick for months or even years. The symptoms are usually mild in these persistent cases, which is why people may unknowingly let them linger without getting treated. Anyone can become infected, but people with weakened immune systems — including those with HIV/AIDS — tend to be more symptomatic.

Giardia infections are almost never fatal in the United States, but they’re serious enough to hospitalize 4,600 Americans each year. The patients are typically young children who have become dehydrated.

How Giardia is transmitted

Ingesting some of the cysts is the only way to catch Giardia. Drinking untreated water that has been contaminated with human or animal waste is the most common way to get sick. (Well water is usually safe, unless the well is so shallow that it mixes with contaminated water from the surface.)

You can also get infected by eating contaminated, undercooked food, or by getting Giardiacysts on your hands (after changing a diaper, for example), then putting your fingers in your mouth or eating tainted food. Heavily used or poorly maintained pools, as well as the swimming areas at lakes and ponds, are especially likely to have Giardiain the water. Even pools that are cleaned regularly can be a problem because Giardiacysts are resistant to the concentration of chlorine normally found in pools, and can be passed in the stools of infected people weeks after they no longer have symptoms.

Diagnosis and treatment

If you think you or your child may have a Giardia infection, contact your doctor. You’ll be asked to provide a stool sample, which will be examined in a laboratory for signs of Giardia. Because it can be difficult to make a definite diagnosis, your doctor may ask you to provide stool samples over several days.

Many people don’t need any special treatment to recover from a Giardia infection. As long as they get enough fluids to compensate what was lost from having diarrhea, they’ll be fine. Doctors can usually quickly clear up more serious cases of Giardia infection with antimicrobial drugs such as Flagyl (generic name is metronidazole). Unfortunately, these drugs aren’t always effective for people with weak immune systems. In some such cases, according to the Food and Drug Administration, Giardia may contribute to a shortened life span.


Drink only properly treated water. It’s usually safe to assume that any water coming out of your tap is safe, although there have been periodic outbreaks of Giardia in city water supplies. When you go backpacking, boil all of your water to 158 degrees Fahrenheit for 10 minutes. Using a portable water filter is another option. Make sure the pore size is 1 micron or smaller. You can also look for a label stating that the filter is NSF rated for cyst removal. Chlorine or iodine disinfectant tablets are better than nothing, but they should be used as a last resort because the amount needed to purify the water varies according to the temperature, acidity, and cloudiness of the water.

If you travel to a developing country, you may no longer be able to rely on the tap water, ice cubes, or water served in glasses at restaurants. Stick with bottled beverages if possible, or boil all water before drinking. If you buy bottled water from a vendor, make sure the seal hasn’t been broken; also, avoid vegetables, salads, and garnishes that may have been rinsed with untreated water. Many restaurants will tell you whether they rinse the raw fruit and vegetables with disinfectant.

You can also greatly reduce your risk of contracting Giardia — and lots of other diseases — by practicing simple hygiene. If you go swimming in a lake, river, pond, or swimming pool, take care not to swallow any of the water. Take a shower before swimming and make sure your children wipe their bottoms properly before getting in the pool. Wash your hands thoroughly after using the bathroom, soaking in a hot tub, or changing a diaper, and also before handling, preparing, or eating food.


Centers for Disease Control and Prevention. Giardiasis fact sheet. October 5, 2009.

Environmental Protection Agency. Giardia: Drinking water fact sheet. 2000.

Centers for Disease Control and Prevention. Giardia and drinking water from private wells. December 2, 2009.

World Health Organization. Protozoan parasites.

Food and Drug Administration. Giardia lamblia. May 4, 2009.

Mayo Clinic. Giardia infection (giardiasis). July 8, 2010.

State of California Department of Health Services. Giardiasis.

Center for Health and Hygiene in the Home and Community. Tips for Staying Healthy While Traveling Abroad. Simmons College, Boston, MA. June 2007.

© HealthDay

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