What is Giardia?

• Giardiasis (gee-ar-die-a-sis with a soft "G") is an infection of the small intestine that is caused by the parasite, Giardia duodenalis, also known as Giardia lamblia and Giardia intestinalis.
• It is the most common cause of parasitic gastrointestinal disease; it is estimated that 20,000 cases of giardiasis occur each year in the U.S., and there is a 20% to 40% prevalence in the world's population.
• 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 the symptoms of giardiasis, but also because they produce the cysts that exit the body in the feces and spread the infection to others.
• Cysts of Giardia are present in the feces of infected people. Thus, the infection is 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 freshwater lakes and streams. As a result, giardiasis is the most common cause of water-borne, parasitic illness in the U.S.
• 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.

Who is at risk for giardiasis?

Giardiasis occurs where there is inadequate sanitation or inadequate treatment of drinking water. Giardiasis is one of the causes of "traveler's diarrhea" that occurs during travel to less-developed countries, for example, the Soviet Union, Mexico, Southeast Asia, and western South America. Giardiasis is a common cause of outbreaks of diarrhea in day-care centers because of the high probability of fecal-oral contamination from children; the children, their families, and daycare center workers, all are at risk for infection. In fact, children are three times more likely to develop giardiasis than adults. Hikers exploring back-country areas who drink from contaminated freshwater lakes also are at risk for developing giardiasis. Individuals who practice anal/oral sex also may become infected.