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Medically reviewed by Last updated on Oct 11, 2023.

What is Giardiasis?

Harvard Health Publishing

Giardiasis is an intestinal illness caused by infection with the parasite Giardia lamblia, which lives in contaminated water. Although the illness most often occurs in developing countries, giardiasis is also a common cause of waterborne illness in the United States. A person can remain infected with Giardia until the infection is diagnosed and treated. In developing areas of the world, it is common for more than 20% of a country's population to have ongoing Giardia infection. In the United States, only 1 or 2 out of every 10,000 people have Giardia in a typical year, but the infection is found in about 1 out of 3 people who have prolonged diarrhea symptoms if they have recently traveled to a developing country.

As one part of their life cycle, G. lamblia parasites change into cysts. Contagious cysts are found in the feces of infected people or animals. You can become infected with G. lamblia by:

G. lamblia can survive in cold, chlorinated water for up to two months, and outbreaks have occurred in municipal water supplies.

People at greatest risk of giardiasis include:

Children are three times more likely to develop giardiasis than adults. It is possible that the human body develops some immunity to the parasite over time.


Up to two-thirds of people infected with the organism do not have any symptoms. When symptoms occur, they can appear suddenly and be obvious or they may worsen slowly. Typically, symptoms start one to three weeks after exposure and include:

Some of the symptoms can take several months or more to begin because they are caused by gradual changes in the lining of your intestine. G. lamblia interferes with the body's ability to absorb fats, so your stools may have more fat in them during a Giardia infection. This is why your stools may float and smell foul.


Your doctor will ask you about your travel history, whether you might have had contact with contaminated water during camping or hiking, and whether your home has well water. If the patient is a child who attends day care, the doctor will ask about any recent outbreaks of diarrhea at the day care center. He or she also will review the patient's symptoms.

The diagnosis is made by testing the stool for Giardia antigen, a protein that is made by G. lamblia parasites, or by identifying G. lamblia cysts or parasites in stool samples. Multiple stool samples may need to be collected since the infection can only be detected in a portion of collected stool samples, even if infection is present. Infrequently, diagnosis may require an inspection of the intestine with a procedure called endoscopy. In this procedure, an instrument called an endoscope is inserted through your mouth into your intestine. An endoscope is a narrow flexible cord-shaped instrument that is equipped with a camera. If necessary, your doctor can use the endoscope to take a small piece of tissue from your small intestine (a biopsy) to be examined in a laboratory.

Expected Duration

The worst symptoms of giardiasis typically last for five to seven days, as long as diagnosis and treatment is not delayed. Symptoms can take as long as several months to completely go away after treatment because the intestine needs to repair itself. It is common to be intolerant to milk and other dairy products that contain lactose for the first few months after a Giardia infection. In some people who are not treated, the infection can cause repeated bouts of abdominal pain and diarrhea for a year or longer.


There is no vaccine that can prevent giardiasis. Medicine to prevent infection is not recommended. The best way to prevent infection is good travel habits and good sanitation.

Travelers should take special care to avoid food and water that could be contaminated. It is safest to eat produce that has been peeled or cooked. Cooking kills Giardia parasites and cysts.

To prevent giardiasis caused by contaminated water, drink water only from approved sources. When camping and when traveling to developing countries, drink bottled water or other beverages that have been bottled or canned. Campers can drink bottled water, treat water with iodine for eight or more hours, use a high-quality water filter or boil water for at least one minute. Travelers to developing countries should avoid drinking beverages that are served with ice.

It is always a good habit to wash your hands frequently. This can reduce your risk of Giardia infection at home and while traveling. It is especially important to wash your hands after you use the bathroom, before you eat, after you change a diaper and after you care for a sick person or animal.


If you don't receive treatment for a Giardia infection, you will probably eventually recover on your own. However, treatment is a good idea for anyone who is having symptoms. Treatment can also help if you don't have symptoms because treatment can prevent the spread of infection to others. This is especially true for children and for people who prepare or serve food.

Commonly prescribed medications used to treat Giardia infection include tinidazole (Tindamax), nitazoxanide (Alinia) and metronidazole (Flagyl).

A doctor should examine and consider treatment for sexual partners and people who have had close contact with the infected person, such as household members, even if they have no symptoms. Pregnant women generally are not treated with medications during the first trimester.

If you have giardiasis, be sure to drink plenty of fluids to prevent dehydration. Over-the-counter medications for diarrhea, such as loperamide (Imodium), may help your symptoms. Wash your hands frequently if you have giardiasis or if you are caring for a person or animal with this infection.

Treatment options

The following list of medications are in some way related to or used in the treatment of this condition.

View more treatment options

When To Call A Professional

See your doctor if you get diarrhea, especially if this diarrhea lasts for longer than several days, produces stools that float and smell foul, or if you also have abdominal cramps, bloating and fever.


In otherwise healthy people, giardiasis generally goes away completely within weeks, with or without treatment. In some cases, Giardia can be a long-term problem if it is not treated.

Additional Info

Centers for Disease Control and Prevention

National Digestive Diseases Information Clearinghouse



Learn more about Giardiasis

Treatment options

Care guides

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.