Giardiasis is an infection of the small intestine by the Giardia lamblia para-site. It is spread from person to person or by contact with contaminated water or food. Its major symptom is diarrhea * .

What Is Giardiasis?

Giardiasis (je-ar-DY--a-sis) is a common infection caused by the Giardia lamblia (je-AR-de-a LAM-bli-a) protozoan, which is a one-celled organism that lives as a parasite * . Giardia contamination can occur in any water source, from clear mountain streams to poorly filtered city water supplies. The most common carriers of giardiasis are dogs, beavers, and humans. Giardiasis is easily passed from person to person through poor hygiene.

Giardiasis occurs worldwide in both developed and developing countries and in both temperate and tropical climates. In developing nations, infection rates from 20 to 50 percent may occur. In the United States, the disease is estimated to affect up to 20 percent of the population, with toddlers in diapers at busy day care centers at particular risk. Giardiasis is the most commonly diagnosed parasitic infection in the United States, and although most cases are relatively mild or asymptomatic, it nonetheless results in thousands of hospitalizations each year.

What Are the Signs and Symptoms of Giardiasis?

It is estimated that more than 50 percent of people with giardiasis have no symptoms or only mild symptoms. When symptoms of giardiasis do occur, they may start gradually or suddenly, usually within one to three weeks after exposure to the parasite. The illness usually begins with frequent watery diarrhea without blood or mucus. Because giardiasis affects the body's ability to absorb fats and carbohydrates from ingested foods (malabsorption), giardiasis often produces foul-smelling, oily stools that float. Symptoms may also include abdominal * cramps, a swollen or large abdomen, excessive gas, nausea and vomiting, loss of appetite, and sometimes a low-grade fever. Persistent symptoms can lead to weight loss and dehydration * . Severe symptoms may require hospital care.

How Do Doctors Diagnose Giardiasis?

Giardiasis is diagnosed by examination of stool samples under a microscope. Doctors look for evidence of trophozoites (tro-fo-ZO-ites), which are active Giardia protozoa inside the body, or for evidence of cysts, which are Giardia surrounded by a protective wall, the form of the protozoan during the resting stage of its life cycle. Detecting Giardia is difficult, so doctors often need to repeat the stool sample tests several times before they can confirm or rule out Giardia infection. Diagnostic tests may sometimes take as long as four or five weeks.

Giardia lamblia protozoa.

Giardia lamblia protozoa.
Illustration by Frank Forney. © 2016 Cengage Learning®.

How Do Doctors Treat Giardiasis?

Several drugs are used to treat giardiasis, and some of them work well with a single dose. Sometimes a second round of drug treatment is required. There is some controversy among doctors about whether to treat people who carry the parasite but who do not have symptoms of giardiasis. Treatment is sometimes considered because these individuals may transmit the infection to others and may eventually show symptoms themselves.

How Do People Prevent Giardiasis?

There is no vaccine or prophylactic (disease-preventing) drug for giardiasis. Preventing giardiasis depends on maintenance of safe supplies of drinking water, the sanitary disposal of human and animal waste, washing of fruits and vegetables before eating or cooking, and proper hygiene, which includes thorough washing of hands after going to the bathroom and before eating.

The Giardia protozoan can be filtered from water but is otherwise difficult to destroy. It can survive in cold water for as long as two months, and it is resistant to chlorine levels used to purify municipal water supplies. When municipal water supplies have been approved by local health departments, the water may be considered safe to drink. However, when people are camping or traveling, they need to make sure that drinking water, cooking water, and ice come only from safe sources. Clear cold mountain streams may look safe and inviting, but they may also carry the Giardia parasite.

See also Diarrhea • Gastroenteritis


Books and Articles

Beer, Karlyn D., et. al. “Outbreaks Associated with Environmental and Undetermined Water Exposures—United States, 2011–2012.” Morbidity and Mortality Weekly Report 64 (2015): 849–851. (accessed October 21, 2015).

Friedman, Sam. “September Is Worst Month for ‘Beaver Fever.’” News Miner, September 13, 2015. (accessed October 21, 2015).


Centers for Disease Control and Prevention. “Facts about Giardia and Swimming Pools.” (accessed October 21, 2015).

MedlinePlus. “Giardia Infection.” U.S. National Library of Medicine, National Institutes of Health. (accessed October 21, 2015).

U.S. Environmental Protection Agency. “Giardia: Drinking Water Fact Sheet.” (accessed October 21, 2015).


Centers for Disease Control and Prevention. 1600 Clifton Road, Atlanta, GA, 30333. Toll-free: 800-311-3435. Website: (accessed July 14, 2015).

* diarrhea (di-ah-RE-a) refers to frequent, watery stools (bowel movements).

* parasite (PAIR-uh-sites) is an organism such as a protozoan (one-celled animals), worm, or insect that must live on or inside a human or other organism to survive. An animal or plant harboring a parasite is called its host. A parasite lives at the expense of the host and may cause illness.

* abdominal (ab-DAH-mih-nul) refers to the area of the body below the ribs and above the hips that contains the stomach, intestines, and other organs.

* dehydration (dee-hi-DRAY-shun) is a condition in which the body is depleted of water, usually caused by excessive and unreplaced loss of body fluids, such as through sweating, vomiting, or diarrhea.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)