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Giardia Infections Factsheet

Giardiasis is an infectious diarrheal disease. It is caused by a parasite that is most commonly transmitted through oral-fecal contact and by water contaminated by feces.


Giardia lamblia, the parasite causing giardiasis, lives in two stages: trophozoites and cysts. Trophozoites are the active form of the parasite inside the body. Cysts represent the resting stage that enables the parasite to survive outside the body.

Infection begins with the ingestion of the cysts. The acid in the stomach activates the cysts, which in turn release the trophozoites. The parasites attach to the lining of the small intestine, reproduce, and are swept down the intestine in the fecal stream. Cysts form in the lower intestines and are then passed in the feces.

The parasite is transmitted from person to person through contact with infected feces, either directly (such as between children or between sexual partners through anal sex) or indirectly, via food and water contaminated with cyst-containing feces.


Giardia, one of the most common intestinal parasites worldwide, infects up to 20 percent of the world's population. The disease is most prevalent in developing countries, where infections are associated with poor sanitary conditions, poor water quality control, and overcrowding.

In the United States, Giardia is a major cause of waterborne outbreaks of diarrhea. Giardiasis occurs primarily in mountainous areas where water supplies have become contaminated with feces from humans or possibly from animals, such as beavers. Because outbreaks have occurred from Washington state to New York state, campers and backpackers should avoid drinking any untreated water from mountain streams.

Giardiasis affects three times more children than adults, particularly diapered children and toddlers being toilet-trained. Families with young children who attend day-care centers are at greater risk of developing giardiasis than is the general population.


The usual interval between infection and the onset of acute symptoms ranges from one to two weeks. In most instances, the individual will experience sudden explosive, watery, foul-smelling diarrhea; excessive gas; abdominal pain; bloating; nausea; tiredness; and loss of appetite. Upper gastrointestinal symptoms such as vomiting may predominate. Fever is unusual. Many infected people, however, have very mild symptoms or no symptoms at all. Blood or mucus in the stool rarely occurs. Occasionally, the illness may last for months, or even years, causing recurrent mild or moderate symptoms such as impaired digestion, especially lactose intolerance; intermittent diarrhea; tiredness and weakness; and significant weight loss.


Giardiasis is sometimes overlooked because the symptoms are similar to other gastrointestinal diseases, and because many physicians are unfamiliar with parasitic diseases. Stool specimens should be collected and examined over a period of at least three days. It is sometimes necessary to collect samples for four to five weeks. A positive diagnosis of giardiasis can be made by microscopic identification of the parasite in stool specimens. New tests that detect antigens (proteins) to Giardia in feces are especially useful for screening children in day-care settings, and for testing adults after treatment. Because of the high accuracy of the antigen tests, it is now rarely necessary to have an intestinal biopsy of the jejunum (part of the small intestine).


Metronidazole, although not approved by the Food and Drug Administration for routine treatment of giardiasis, often is used to treat adults and children who are mildly affected or asymptomatic. Because of the potentially adverse effects on a fetus, pregnant women are advised not to take metronidazole during the first trimester. Paromomycin is an alternative and safer drug. Sometimes, however, a pregnant woman with mild symptoms can be treated after the first trimester or after delivery. If relapses occur, re-treatment with the same drug is usually effective. Furazolidone generally is given to infants and to children under 5 years old, since it comes in a liquid form.


To prevent the spread of giardiasis in day-care centers or other areas where there is close contact, good personal hygiene should be maintained. Day-care staff should not change diapers in food preparation areas. They should dispose of diapers in a sanitary manner, and they should wash their hands after each diaper change.

Presently, there is no drug available to prevent giardiasis. The following precautions should be taken if visiting an area where Giardia may exist: drink only boiled or filtered water, or bottled carbonated drinks; avoid ice and beverages made from tap water; and do not eat uncooked or unpeeled fruits and vegetables grown locally.

National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH) Bethesda, MD 20892, October 1998.

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