Giardiasis
(Giardia
Lamblia)
Medical Author: Jay W. Marks, MD
Medical Editor: Dennis Lee, MD
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.
Next: Who is at risk for giardiasis? »
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