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


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Gastrointestinal system

Waterborne diseases

What Is Giardiasis?

Giardiasis (je-ar-DY-a-sis) is a common infection caused by the Giardia lamhlia (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 is found 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, it is estimated to affect up to 20 percent of the population, with toddlers in diapers at busy day care centers at particular risk.

* parasites are creatures that live in and feed on the bodies of other organisms. The animal or plant harboring the parasite is called its host.

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 bodys ability to absorb fats and carbohydrates from ingested foods (malabsorption), giardiasis often produces foul-smelling, oily stools that float. Symptoms also may 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.

Giardia lamblia protozoa.
Giardia lamblia protozoa.

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.

How Do Doctors Treat Giardiasis?

Several drugs are available for treatment of giardiasis, and some of them work well in 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. But when camping or traveling, it is important 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


The U.S. National Institute of Allergy and Infectious Diseases posts a fact sheet about giardiasis at its website.

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