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Giardia is the genus (and common) name of a protozoan parasite in the phylum Sarcomastigophora. It was first described in 1681 by Antoni van Leeuwenhoek (called "The Father of Microbiology"), who discovered it in his own stool. The most common species is Giardia intestinalis (also called lamblia ), which is a fairly common parasite found in humans. The disease it causes is called giardiasis.

The trophozoite (feeding) stage is easily recognized by its pear-shaped, bilaterally-symmetrical form with two internal nuclei and four pairs of external flagella; the thin-walled cyst (infective) stage is oval. Both stages are found in the upper part of the small intestine in the mucosal lining. The anterior region of the ventral surface of the troph stage is modified into a sucking disc used to attach to the host's abdominal epithelial tissue. Each troph attaches to one epithelial cell. In extreme cases, nearly every cell will be covered, causing severe symptoms. Infection usually occurs through drinking contaminated water. Symptoms include diarrhea, flatulence (gas), abdominal cramps, fatigue, weight loss, anorexia, and/or nausea and may last for more than five days. Diagnosis is usually done by detecting cysts or trophs of this parasite in fecal specimens.

Giardia has a worldwide distribution. It is more common in warm, tropical regions than in cold regions. Hosts include frogs , cats, dogs, beaver, muskrat, horses, and humans. Children as well as adults can be affected, although it is more common in children. It is highly contagious. Normal infection rate in the United States ranges from 1.5 to 20%. In one case involving scuba divers from the New York City police and fire fighters, 2255% were found to be infected, presumably after they accidentally drank contaminated water in the local rivers while diving. In another case, an epidemic of giardiasis occurred in Aspen, Colorado, in 1965 during the popular ski season and 120 people were infected. Higher infection rates are common in some areas of the world, including Iran and countries in Sub-Saharan Africa.

Giardia can typically withstand sophisticated forms of sewage treatment , including filtration and chlorination . It is therefore hard to eradicate and may potentially increase in polluted lakes and rivers. For this reason, health officials should make concerted efforts to prevent contaminated feces from infected animals (including humans) from entering lakes used for drinking water.

The most effective treatment for giardiasis is the drug Atabrine (quinacrine hydrochloride). Adult dosage is 0.1 g taken after meals three times each day. Side effects are rare and minimal.

See also Cholera; Coliform bacteria

[John Korstad ]



Markell, E. K., M. Voge, and D. T. John. Medical Parasitology. 7th ed. Philadelphia: W. B. Saunders, 1992.

Schmidt, G. D., and L. S. Roberts. Foundations of Parasitology. 4th ed. St. Louis: Times Mirror/Mosby, 1989.

U.S. Department of Health and Human Services. Health Information for International Travel. Washington, DC: U.S. Government Printing Office, 1991.