What Is Giardiasis?


Giardiasis, an intestinal infection of the small bowel caused by the microscopic parasite Giardia lamblia, is the most common parasitic infection in the United States. Most commonly, the parasite passes from the feces of an infected person or animal into a food or water supply and is then ingested.

The parasite can also pass from one person to another through oral-genital sexual contact and, more rarely, by hand contact. The parasites then colonize in the small intestine. Symptoms usually appear within one to two weeks after exposure, but may take as long as 25 days—so you may have trouble pinpointing exactly when you became infected.

Symptoms of Giardiasis

The following symptoms may occur (though about two-thirds of infected people have no symptoms):

  • Mild to severe diarrhea that is foul-smelling (rotten-egg odor) and frequent (2 to 10 stools a day). Diarrhea may last one or more weeks, accompanied by one or more of the following: headache, fatigue, nausea, vomiting, abdominal pain, stomach cramps, dehydration, lethargy, gas, watery stool, bloating, or swelling. (Diarrhea from giardiasis is not associated with fever or blood in the stool.)
  • Loss of appetite and possible weight loss.

What Causes Giardiasis?

Most infections stem from ingesting fecally contaminated water or the fecal-oral transfer of the Giardia lamblia from an infected person. Giardia infection occurs worldwide and is most common in developing countries or areas where sanitation and hygiene are substandard.

In the United States it also turns up among campers and hikers who have drunk unpurified water from streams contaminated by animal and human feces. Up to 80 percent of all water in the wild contains this protozoan, according to the Environmental Protection Agency (EPA). But the most common sites of infection are daycare centers and preschools, which now account for about half of the giardiasis cases. In many centers children’s soiled diapers harbor millions of the parasites, and both staff and children risk becoming infected after diaper changes if they don’t carefully wash their hands.

You can also get giardiasis from food poisoning (caused by someone who is infected handling food without washing his hands). And pets can become infected and then spread the parasite to humans.

What If You Do Nothing?

Giardiasis with mild diarrhea typically clears up by itself without any treatment—usually within one to four weeks. However, in some cases, if left untreated, symptoms become chronic and recur intermittently. With proper medical treatment, recovery is complete.

Home Remedies for Giardiasis

There are no effective home remedies to treat giardiasis. It is usually treated with prescription medication—typically metronidazole (Flagyl) for adults and furazolidone (Furoxone) for children. No alcohol can be consumed while taking metronidazole and it shouldn’t be taken during pregnancy.


  • Wash your hands carefully after toilet visits. Wash thoroughly and often, throughout the day, to avoid catching the infection from other people. Pay special attention to the fingernails, which can harbor the parasite. If you have young children who attend preschool or daycare centers, giardia infection is one more reason to wash your hands well after handling soiled diapers or underwear.
  • Boil water gathered outdoors. When you are camping or hiking, no matter how crystal clear and safe water from a stream, pond, river, or lake may seem, boil any water for three minutes before drinking it. If in doubt, carry your own bottled water or use an appropriate water filter.
  • In developing countries, don’t drink from the tap. If you are traveling in developing areas of Latin America, Asia, or Africa, or if you are in any location where conditions seem unsanitary, drink only bottled water, carbonated drinks, or drinks you open yourself. Stay away from drinks containing caffeine. Don’t brush your teeth or wash dishes with any water that hasn’t been bottled or boiled. Also, don’t add ice cubes to drinks.
  • Wash your food. When traveling in developing countries, be sure to peel all raw fruits and vegetables before eating them in case the foods have been rinsed in contaminated water or handled by an infected person.

Beyond Home Remedies: When To Call Your Doctor

If you are experiencing giardiasis symptoms, contact your doctor. Be aware that many doctors are unfamiliar with this condition. The infection might be mistaken as gallbladder inflammation, irritable bowel syndrome, or a peptic ulcer.

What Your Doctor Will Do

A doctor familiar with the disorder will ascertain if you have traveled or have been camping within the past few weeks, or if you work or have children in a daycare center. A laboratory stool test will be ordered to detect the presence of Giardia lamblia. The skill of the lab technician is important, and the test may need to be repeated as many as three times over a two-week period if early studies are negative, since many of the tests are false-negative (negative even though you may have the infection).

A newer test, an antigen detection test, also produces many false negatives, but yields false-positives only rarely. It is also cheaper than previous tests and consequently has become the diagnostic tool of choice.

Because the likelihood of false-negatives is high with any test, your physician may still prescribe antiparasitic medications even if your test results are negative, basing the decision on your history and symptoms. The drugs are extremely effective, though they have a wide range of side effects.