Merck Manual

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Chelsea Marie

, PhD, University of Virginia;

William A. Petri, Jr

, MD, PhD, University of Virginia School of Medicine

Reviewed/Revised Jun 2022

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, by assays for Giardia antigen, or molecular testing for parasite DNA 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.

Giardia infection is prevalent throughout the world, particularly in areas with poor sanitation. Giardiasis is the most common intestinal parasitic disease in the US. Waterborne transmission is the major source of infection (1 General reference Giardiasis is infection with the flagellated protozoan Giardia duodenalis (G. lamblia, G. intestinalis). Infection can be asymptomatic or cause symptoms ranging from intermittent... read more ), but transmission can also occur by ingestion of contaminated food or by direct person-to-person contact.

Giardia cysts remain viable in surface water and are resistant to routine levels of chlorination in drinking water. Thus, mountain streams as well as chlorinated but poorly filtered municipal water supply systems have been implicated in waterborne epidemics. Infections are also associated with childcare, especially involving diaper-aged children; close contact with family or household members who have giardiasis; ingestion of water or ice from untreated or improperly treated water from lakes, streams, or wells; backpackers, hikers, and campers who drink unsafe water or fail to practice good hand hygiene; ingesting water while swimming or playing in lakes, ponds, rivers, or streams; or exposure to feces through sexual contact.

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 of Giardiasis

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, fatigue, and anorexia. Acute giardiasis usually lasts 1 to 3 weeks. Giardiasis is often accompanied by acquired lactose-intolerance. Malabsorption Overview of Malabsorption Malabsorption is inadequate assimilation of dietary substances due to defects in digestion, absorption, or transport. Malabsorption can affect macronutrients (eg, proteins, carbohydrates, fats)... read more of fat and sugars can lead to significant weight loss in severe cases. Neither blood nor white blood cells are present in stool.

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

Diagnosis of Giardiasis

  • Enzyme immunoassay for antigen or molecular test for parasite DNA 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.

Molecular tests for parasite DNA in stool are available.

Treatment of Giardiasis

  • Tinidazole, metronidazole, or nitazoxanide

For symptomatic giardiasis, tinidazole, metronidazole, or nitazoxanide are used. Treatment failures and resistance can occur with any of them.

Tinidazole is as effective as metronidazole, but tinidazole is better tolerated and administered as a single dose as follows:

  • Adults: 2 g orally once

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

Metronidazole is given as follows:

  • Adults: 250 mg orally 3 times a day for 5 to 7 days

  • Children: 5 mg/kg orally 3 times a day for 5 to 7 days

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

Nitazoxanide is given orally for 3 days as follows:

  • Age 1 to 3 years: 100 mg 2 times a day

  • Age 4 to 11 years: 200 mg 2 times a day

  • Age > 12 years (including adults): 500 mg 2 times a day

Nitazoxanide is available in liquid form for children.

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 orally 3 times a day for 5 to 10 days) is an option if the benefit outweighs the risk.

Furazolidone, quinacrine, or albendazole are rarely used because of potential toxicity, lower efficacy, or cost.

Prevention of Giardiasis

Prevention of giardiasis requires

  • Appropriate public water treatment

  • Hygienic food preparation

  • Appropriate fecal-oral hygiene

  • Thorough handwashing after contact with feces

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.

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.

Drugs Mentioned In This Article

Drug Name Select Trade
Flagyl, Flagyl ER, Flagyl RTU, LIKMEZ, MetroCream, MetroGel, MetroGel Vaginal, MetroLotion, Noritate, NUVESSA, Nydamax, Rosadan, Rozex, Vandazole, Vitazol
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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