Medical Author: Jay W. Marks, MD
Medical Editor: Dennis Lee, MD

• What is giardiasis?
• Who is at risk for giardiasis?
• What signs and symptoms does giardiasis cause?
• How is giardiasis diagnosed?
• How is giardiasis treated?
• How can giardiasis be prevented?

What is giardiasis?

Giardiasis (gee-ar-die-a-sis with a soft "G") is an infection of the small intestine that is caused by the parasite, Giardia intestinalis, also known as Giardia lamblia. It is the most common cause of parasitic gastrointestinal disease, and it is estimated that up to 2.5 million cases of giardiasis occur each year in the US. Up to 20% of the world's population is chronically infected with Giardia lamblia.

Giardia lamblia exists in two forms, an active form called a trophozoite, and an inactive form called a cyst. The active trophozoite attaches to the lining of the small intestine with a “sucker” and is responsible for causing the signs and symptoms of giardiasis. The trophozoite cannot live long outside of the body, therefore it cannot spread the infection to others. The inactive cyst, on the other hand, can exist for prolonged periods outside the body. When it is ingested, stomach acid activates the cyst, and the cyst develops into the disease-causing trophozoite. It takes ingestion of only ten cysts to cause infection. Trophozoites are important not only because they cause disease, they also produce cysts that exit the body in feces and spread infection to others.

Cysts of Giardia are present in the feces of infected persons. Thus, the infection can be spread from person to person by contamination of food with feces, or by direct fecal-oral contamination. Cysts also survive in water, for example in fresh water lakes and streams. As a result, giardiasis is the most common cause of water-borne, parasitic illness in the United States. Domestic mammals (for example, dogs, cats, calves) and wild mammals (for example, beavers) can become infected with Giardia; however, it is not clear how often domestic or wild mammals transmit giardiasis to humans. Giardiasis also has occurred as outbreaks from recreational water sources such as swimming pools, water parks, and hot tubs, most likely because of an infected user rather than a source of water that was contaminated.

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