Conditions & Treatments


Giardiasis is an infectious diarrheal disease caused by the parasite Giardia lamblia, which can be transmitted through oral-fecal contact and by water contaminated by feces.


What is giardiasis?

Giardiasis is an infectious diarrheal disease caused by the parasite Giardia intestinalis (also known as Giardia lamblia or Giardia duodenalis). This parasite can be transmitted through oral-fecal contact and by water contaminated by feces. Travelers are cautioned against drinking any untreated water.

Giardia intestinalis is one of the most common intestinal parasites in the world. It is most prevalent in developing countries where infections are associated with poor sanitary conditions, poor water quality control, and overcrowding. However, giardia is also a common cause of parasitic infection in the U.S. Hikers and campers who drink water from streams and other potentially contaminated sources are often infected. 

Giardia intestinalis is a major cause of waterborne outbreaks of diarrhea in the United States. It occurs primarily in areas where water supplies have become contaminated with feces from humans or, possibly, from animals (for example, cats, dogs, cattle, deer, and beavers). Outbreaks have occurred throughout the country.

What causes giardiasis?

The Giardia intestinalis parasite lives in two stages:

  • Trophozoites (the active form inside the body)

  • Cysts (the resting stage that enables the parasite to survive outside the body)

Infection begins when the cysts are taken in through food or water. Stomach acid activates the cysts and the trophozoites are released. They become attached to the lining of the small intestine and reproduce. Cysts form in the lower intestines and are then passed in the feces.

The parasite may be directly transmitted from person-to-person by contact with infected feces, or indirectly, through consuming food and water contaminated with cyst-containing feces.

What are symptoms of giardiasis?

The following are the most common symptoms of giardiasis. However, each individual may experience symptoms differently. Symptoms may include:

  • Explosive, watery, foul-smelling stools

  • Greasy stools that tend to float

  • Bloating

  • Nausea

  • Loss of appetite

  • Abdominal (belly) pain

  • Excessive gas

  • Fatigue

The time between infection and the onset of acute symptoms usually is from one to two weeks. Some infected people have very mild symptoms or no symptoms at all. The symptoms of giardiasis are a lot like those of other gastrointestinal diseases. Consult a doctor for diagnosis.

How can giardiasis be prevented?

Giardiasis can be prevented by practicing good personal hygiene, as well as proper hygiene when caring for those who may be infected with the parasite. When visiting in an area where giardiasis is known to exist:

  • Drink only boiled water or bottled water or drinks

  • Avoid ice and beverages made from tap water

  • Do not eat locally-grown uncooked or unpeeled fruits and vegetables

How is giardiasis diagnosed?

Positive diagnosis of giardiasis is made by microscopic identification of the parasite in stool samples. If you think you may have giardiasis, contact your health care provider for advice.

What is the treatment for giardiasis?

Giardiasis may be treated with prescription medications. Specific treatment for giardiasis will be determined by your doctor based on:

  • Your overall health and medical history

  • Extent of the disease

  • Your tolerance for specific medications, procedures, or therapies

  • Expectations for the course of the disease

  • Your opinion or preference

Several medications can be used to treat Giardia infection. Effective treatments include metronidazole, tinidazole, and nitazoxanide.

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Massachusetts General Hospital understands that a variety of factors influence patients' health care decisions. That's just one reason why we're dedicated to ensuring patients understand their diagnosis and treatment options. Because a single option might not serve all patients, we offer a wide range of coordinated treatments and related services across the hospital. Patients should consult with their primary care doctor or other qualified health care provider for medical advice and diagnosis information.

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