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 Oct 2022

Giardiasis is an infection of the small intestine caused by the single-celled protozoan parasite Giardia. The main symptoms are abdominal cramping and diarrhea.

  • People may have abdominal cramping, gas, belching, diarrhea, and nausea and feel tired.

  • People acquire the infection by drinking water or eating food that is contaminated with stool containing Giardia or by coming into contact with stool from an infected person.

  • Doctors diagnose the infection by testing or examining a stool sample.

  • Boiling water kills Giardia protozoa and is the safest way for hikers to ensure that water from streams and lakes is safe to drink.

  • Infected people are treated with an antiparasitic drug, such as tinidazole, metronidazole, or nitazoxanide.

Giardiasis is a protozoan infection Overview of Parasitic Infections that occurs worldwide and is the most common parasitic infection of the intestine in the United States. Giardia protozoa can form an outer shell (called a cyst). It enables them to survive outside the body for long periods of time (for example, in lakes and streams) and makes them less likely to be killed by chlorine (for example, in swimming pools). These cysts are passed in stool and can cause infection.

Giardia protozoa are a common contaminant of fresh water, including many lakes and streams—even ones that appear clean. Poorly filtered municipal water supply systems contribute to some outbreaks. Most people acquire the infection from drinking contaminated water. But people can acquire the infection if they eat contaminated food or have contact with stool from an infected person, which can occur between children or sex partners.

Giardiasis is more common among

  • Children in day care centers

  • People who practice oral-anal sex

  • People who have traveled to countries with poor sanitation

  • Backpackers and hikers who drink untreated water from streams and lakes

  • People who swim in contaminated pools or lakes

Giardia can live in wild animals.

Did You Know...

  • Giardiasis may be present in lakes and streams that appear fresh and clean.

Symptoms of Giardiasis

Some infected people have no symptoms, but these people can pass Giardia cysts in their stool and can thus infect others. Symptoms, when they occur, appear about 1 to 2 weeks after infection.

Symptoms of giardiasis typically include abdominal cramps, gas (flatulence), belching, and watery, foul-smelling diarrhea. Nausea may come and go. People may feel tired and vaguely uncomfortable and lose their appetite. If untreated, the diarrhea may persist for weeks. People with giardiasis often develop lactose intolerance Lactose Intolerance Lactose intolerance is the inability to digest the sugar lactose because of a lack of the digestive enzyme lactase, leading to diarrhea and abdominal cramping. Lactose intolerance is caused... read more (inability to digest lactose, the sugar in milk), which can result in diarrhea, gas, and bloating.

A few people with giardiasis develop diarrhea that persists longer. These people may not absorb enough nutrients from food (called malabsorption Overview of Malabsorption Malabsorption syndrome refers to a number of disorders in which nutrients from food are not absorbed properly in the small intestine. Certain disorders, infections, and surgical procedures can... read more ), resulting in significant weight loss.

Diagnosis of Giardiasis

  • Stool tests

The symptoms often suggest giardiasis.

The easiest way to make the diagnosis of giardiasis is by testing the stool for proteins (antigens) released by Giardia lamblia or for its DNA.

Microscopic examination of stool samples may also detect the parasite. However, because people who have been infected for a long time tend to excrete the parasites at unpredictable intervals, repeated microscopic examinations of stool are often needed.

If these tests do not identify what is causing the intestinal symptoms, doctors may use a flexible viewing tube (endoscope Endoscopy Endoscopy is an examination of internal structures using a flexible viewing tube (endoscope). In addition to examinations, doctors can use endoscopy to do biopsies and give treatment. Endoscopes... read more ) to examine the upper part of digestive tract, including the first part of the small intestine (duodenum). Doctors may use this procedure to obtain a sample of the contents of the small intestine for examination.

Prevention of Giardiasis

Giardiasis prevention requires

  • Appropriate treatment of public water (including drinking water and water in swimming pools)

  • Use of good hygiene when preparing food

  • Good personal hygiene (for example, washing hands thoroughly after using the toilet)

  • Avoiding contact with stool during sex

Boiling water kills the parasite and is the safest way for hikers to ensure that water from streams and lakes is safe to drink.

Water from streams and lakes can sometimes be disinfected using chlorine-containing compounds or iodine. This method is less reliable because its effectiveness varies depending on how cloudy or muddy the water is (turbidity) and what the water's temperature is. The amount of chlorine routinely used in drinking water may be insufficient to kill the cysts.

Water filters that use reverse osmosis or have the words "tested and certified by NSF/ANSI Standard No. 53 or No. 58 for cyst removal/reduction" can remove cysts of Giardia and other protozoa as well as bacteria from water, but other filter systems may not be effective.

Treatment of Giardiasis

  • Tinidazole, metronidazole, or nitazoxanide

Infected people who have symptoms can be treated with tinidazole, metronidazole, or nitazoxanide, taken by mouth.

Tinidazole, taken in a single dose, has fewer side effects than metronidazole, which is taken three times a day for 5 to 7 days. Drinking alcohol within a few days of taking tinidazole or metronidazole may cause nausea, vomiting, flushing, and headaches. Nitazoxanide is available in liquid form, which is useful for children, and as tablets. It is taken twice a day for 3 days. It has few side effects.

Pregnant women should not take metronidazole or tinidazole. The safety of nitazoxanide during pregnancy has not been assessed. Consequently, the treatment of pregnant women is delayed if possible until after pregnancy. If symptoms are severe and treatment cannot be delayed, paromomycin can be used.

Select Medical Literature

  • 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

Drugs Mentioned In This Article

Generic Name Select Brand Names
Flagyl, Flagyl ER, Flagyl RTU, LIKMEZ, MetroCream, MetroGel, MetroGel Vaginal, MetroLotion, Noritate, NUVESSA, Nydamax, Rosadan, Rozex, Vandazole, Vitazol
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