Not Found

Find information on medical topics, symptoms, drugs, procedures, news and more, written for the health care professional.


By Richard D. Pearson, MD, Emeritus Professor of Medicine, University of Virginia School of Medicine

Click here for
Patient Education

Giardiasis is infection with the flagellated protozoan Giardia duodenalis (G. lamblia, G. intestinalis). Infection can be asymptomatic or cause symptoms ranging from intermittent flatulence to chronic malabsorption. Diagnosis is by identifying the organism in fresh stool or duodenal contents or by assays of Giardia antigen in stool. Treatment is with metronidazole, tinidazole, or nitazoxanide or, during pregnancy, paromomycin.

Giardia trophozoites firmly attach to the duodenal and proximal jejunal mucosa and multiply by binary fission. Some organisms transform into environmentally resistant cysts that are spread by the fecal-oral route.

Giardiasis is the most common intestinal parasitic disease in the US. Waterborne transmission is the major source of infection (1). Transmission can also occur by ingestion of contaminated food and by direct person-to-person contact, especially in mental institutions and day care centers or between sex partners.

Giardia cysts remain viable in surface water and are resistant to routine levels of chlorination. Wild animals may also serve as reservoirs. Thus, mountain streams as well as chlorinated but poorly filtered municipal water supply systems have been implicated in waterborne epidemics.

There are 8 genetic groups (assemblages) of G. duodenalis. Two infect humans and animals; the others infect only animals. The clinical manifestations appear to vary with genotype.

General reference

  • 1. Schnell K, Collier S, Derado G, et al: Giardiasis in the United States - an epidemiologic and geospatial analysis of county-level drinking water and sanitation data, 1993-2010. J Water Health 14(2):267–279, 2016. doi: 10.2166/wh.2015.283.

Symptoms and Signs

Many cases of giardiasis are asymptomatic. However, asymptomatic people can pass infective cysts.

Symptoms of acute giardiasis usually appear 1 to 14 days (average 7 days) after infection. They are usually mild and include watery malodorous diarrhea, abdominal cramps and distention, flatulence, eructation, intermittent nausea, epigastric discomfort, and sometimes low-grade malaise and anorexia. Acute giardiasis usually lasts 1 to 3 wk. Malabsorption of fat and sugars can lead to significant weight loss in severe cases. Neither blood nor WBCs are present in stool.

A subset of infected patients develop chronic diarrhea with foul stools, abdominal distention, and malodorous flatus. Substantial weight loss may occur. Chronic giardiasis occasionally causes failure to thrive in children.


  • Enzyme immunoassay for antigen in stool

  • Microscopic examination of stool

Enzyme immunoassay to detect parasite antigen in stool is more sensitive than microscopic examination. Characteristic trophozoites or cysts in stool are diagnostic, but parasite excretion is intermittent and at low levels during chronic infections. Thus, microscopic diagnosis may require repeated stool examinations.

Sampling of the upper intestinal contents can also yield trophozoites but is seldom necessary.

Specific DNA probes exist. Testing is available at the CDC and is likely to become increasingly available at reference laboratories.


  • Tinidazole, metronidazole, or nitazoxanide

For symptomatic infections, metronidazole, tinidazole, or nitazoxanide may be used.

Metronidazole is given as follows:

  • Adults: 250 mg po tid for 5 to 7 days

  • Children: 5 mg/kg po tid for 5 to 7 days

Tinidazole is as effective as metronidazole and is given as follows:

  • Adults: 2 g po once

  • Children: 50 mg/kg [maximum 2 g] po once

Adverse effects of metronidazole include nausea and headaches. Metronidazole and tinidizole should not be given to pregnant women. Alcohol must be avoided because these drugs have a disulfiram-like effect. In terms of GI adverse effects, tinidazole is generally better tolerated than metronidazole.

Nitazoxanide is given orally for 3 days as follows:

  • Age 1 to 3 yr: 100 mg bid

  • Age 4 to 11 yr: 200 mg bid

  • Age > 12 yr (including adults): 500 mg bid

Nitazoxanide is available in liquid form for children. Resistance has been reported.

Metronidazole and tinidazole should not be given to pregnant women. The safety of nitazoxanide during pregnancy has not been assessed. If therapy cannot be delayed because of symptoms, the nonabsorbable aminoglycoside paromomycin (8 to 11 mg/kg po tid for 5 to 10 days) is an option.

Furazolidone and quinacrine are effective but are now rarely used because of potential toxicity.


Prevention requires

  • Appropriate public water treatment

  • Hygienic food preparation

  • Appropriate fecal-oral hygiene

Water can be decontaminated by boiling. Giardia cysts resist routine levels of chlorination. Disinfection with iodine-containing compounds is variably effective and depends on the turbidity and temperature of the water and duration of treatment. Some handheld filtration devices can remove Giardia cysts from contaminated water, but the efficacy of various filter systems has not been fully assessed.

Treatment of asymptomatic cyst passers can theoretically reduce the spread of infection, but whether it is cost-effective remains unclear.

Key Points

  • The major source of giardiasis is waterborne transmission, including via fresh-appearing mountain streams and poorly filtered municipal water supplies.

  • Giardia cysts resist routine levels of chlorination, and disinfection with iodine-containing compounds is variably effective.

  • Enzyme immunoassay to detect parasite antigen in stool is preferred because it is more sensitive than microscopic examination.

  • For symptomatic patients, use tinidazole, metronidazole, or nitazoxanide.