What Is It?

Giardiasis is an intestinal illness caused by infection with the parasite Giardia lamblia, which lives in contaminated water. Although the illness most frequently occurs in developing countries, giardiasis is also one of the most common causes of waterborne illness in the United States. In developing countries, more than 20 percent of the population may be affected, while in the United States are, the disease affects between 1 percent and 6 percent of the population.

G. lamblia parasites are found in the feces of infected people. A person can become infected with G. lamblia by:

  • Drinking water that has been contaminated with the parasite (usually by contact with sewage)
  • Eating fruits or vegetables that have been washed in contaminated water
  • Touching feces from an infected person
  • Touching objects soiled with feces
  • Having direct contact with an infected person or animal

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 in day-care centers and their families
  • Day-care workers
  • Travelers to developing countries
  • Campers who drink unprocessed water
  • Homosexual men (because of anal sex)

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.


When giardiasis is widespread in a particular population, such as in a developing country, up to two-thirds of people infected with the organism do not have any symptoms. When symptoms occur, they typically start one to three weeks after exposure and include:

  • The sudden onset of watery diarrhea
  • Abdominal cramping
  • Bloating
  • Nausea
  • Gas

Because G. lamblia interferes with the body’s ability to absorb fats, the stools have a greater fat content, causing them to float and be foul-smelling. Less common symptoms include vomiting, low-grade fever and loss of appetite. If the infection lasts a long time, it can result in weight loss and fatigue.


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 inquire 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 chemical produced by G. lamblia parasites, or by identifying G. lamblia parasites in stool samples. Multiple stool samples may be collected. Another method of diagnosis is the Entero-Test, in which a sample of contents from the upper part of the small intestine (duodenum) is collected and examined in a laboratory. In rare cases, diagnosis may require a biopsy. In this test, a small piece of tissue is taken from the small intestine to be examined in a laboratory. The tissue would be obtained using an endoscope, a flexible tube that is inserted through the mouth.

Expected Duration

Symptoms of giardiasis typically last for five to seven days, although they sometimes persist for as long as four to six weeks. In some cases, the infection enters a chronic (long-lasting) phase, in which the infected person has repeated bouts of abdominal pain and diarrhea.


There is no vaccine or medication that can prevent giardiasis. The first line of defense is frequent hand washing, particularly after using the bathroom, before eating, after changing diapers or after caring for a sick person or animal.

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 either treat water chemically or use water that has been boiled for at least one minute. Travelers should avoid using ice in beverages, and should refrain from eating raw fruits and vegetables that cannot be peeled.


Although most people will recover from giardiasis without treatment, medications such as metronidazole (Flagyl) , quinacrine hydrochloride (Mepacrine) or furazolidone (Furoxone) may be used to speed recovery time. Sexual partners and people who have had close contact with the infected person, such as household members, also may need to be examined and treated, even if they have no symptoms. Pregnant women generally are not treated with medications, particularly in the first trimester.

If you have giardiasis, be sure to drink plenty of fluids to prevent dehydration. Check with your doctor before taking any over-the-counter medications for diarrhea, because these drugs can interfere with your body’s ability to eliminate the Giardia parasite. Wash your hands frequently if you are caring for a person or animal infected with giardiasis.

When To Call A Professional

Call your doctor if you suddenly get diarrhea, especially if this diarrhea produces stools that float and are foul-smelling and if you also have abdominal cramps, bloating and fever.


In otherwise healthy people, giardiasis generally goes away completely within weeks. However, in some cases, the infection persists and the infected person has occasional bouts of abdominal pain and diarrhea.

Johns Hopkins patient information

Last revised:

Diseases and Conditions Center

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All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.