Giardiasis, also known as beaver fever, is a common intestinal infection spread by eating contaminated food, drinking contaminated water, or through direct contact with the organism that causes the disease, Giardia lamblia (also known as Giardia intestinalis). Giardiasis is found throughout the world and is a common cause of traveller's diarrhea. In the United States it is a growing problem, especially among children in childcare centers. Giardia was first discovered by Antonie van Leeuwenhoek in 1681 in his own diarrheal stool. The organism was further described by Vilem Dusan Lambl in 1859 and by Alfred Giard in 1895. It was not known that it was a pathogen until the 1970s.


Giardia is one of the most common intestinal parasites in the world, infecting as much as 20% of the entire population of the earth. It is common in overcrowded developing countries with poor sanitation and a lack of clean water. Recent tests have found Giardia in 7% of all stool samples tested nationwide, indicating that this disease is much more widespread than was originally believed. According to 2003–2005 data from the U.S. Centers for Disease Control and Prevention, the greatest number of reported cases occurred among children aged 1–4 and 5–9 years and adults aged 35–44 years. The disease is not only found in humans, but also in wild and domestic animals, including beavers, cattle, deer, and sheep. Giardia also infects cats, dogs, and birds.

Giardiasis is becoming a growing problem in the United States, where it affects three times more children than adults. In recent years, giardiasis outbreaks have been common among people in schools or daycare centers and at catered events and large public picnic areas. Children can easily pass on the infection by touching contaminated toys, changing tables, utensils, or their own feces, and then touching other people. Twenty-five percent of family members with infected children become infected themselves. As the ingestion of as few as ten Giardia cysts can cause the disease, infection can spread quickly through a daycare center or an institution for the developmentally disabled. The ingestion of more than 25 cysts results in 100% of persons becoming infected.

Unfiltered streams or lakes that may be contaminated by human or animal wastes are a common source of infection. Outbreaks can occur among campers and hikers who drink untreated water from mountain streams. Although 20 million Americans drink unfiltered city water from streams or rivers, giardiasis outbreaks from tainted city water have been rare. Most of these outbreaks have occurred not due to the absence of filters, but because of malfunctions in city water treatment plants, such as a temporary drop in chlorine levels. It is also possible to become infected in a public swimming pool, since Giardia lamblia can survive in chlorinated water for about 15 minutes. During that time, it is possible for an individual to swallow contaminated pool water and become infected.


From 1965 to 1996, Giardia was implicated in 133 waterborne outbreaks, with over 28,000 cases of giadiasis were reported in 27 states; 108 outbreaks were associated with the consumption of contaminated drinking water from public water systems, 10 outbreaks were associated with individual and nonpotable water sources, and 15 outbreaks were associated with recreational water. However, no deaths were associated with these waterborne outbreaks. In 2008, it was reported that of the 46 U.S. states reporting giardiasis, the mean number of cases per 100,000 persons ranged from 0.1 to 23.5 cases. Because not all outbreaks are recognized or reported, these statistics should be considered an underestimate of the occurrence of outbreaks and incidence of waterborne giardiasis.

Most U.S. waterborne outbreaks occur in the western mountain regions, where many people camp and backpack. However, waterborne outbreaks have occurred in every part of the U.S. Most infections occur during the months of July through October, the summer recreational water sports and camping season.

Causes and symptoms

Giardiasis is spread by food or water contaminated by the Giardia protozoan organism found in the human intestinal tract and feces. When Giardia cysts are ingested, the stomach acid degrades the cysts and releases the active parasites (known as trophozoites) into the body. Once within the body, the parasites cling to the lining of the small intestine, reproduce, and are swept into the fecal stream. As the liquid content of the bowel dries up, the parasites form cysts, which are then passed in the feces. Once excreted, the cysts can survive in water for more than three months. The parasite is spread further by direct fecal-oral contamination, such as can occur if food is prepared without adequate hand-washing, or by ingesting the cysts in water or food.

Giardiasis is usually not fatal, and about two-thirds of infected people exhibit no symptoms but do excrete the cysts. Symptoms will not occur until between one and two weeks after infection. When present, symptoms include explosive, watery diarrhea that can last for a week or more and, in chronic cases, may persist for months. Because the infection interferes with the body's ability to absorb fats from the intestinal tract, the stool is filled with fat. Other symptoms include foul-smelling feces, stomach pains, gas and bloating, loss of appetite, nausea and vomiting. In cases in which the infection becomes chronic, which particularly may occur in persons who lack the immunoglobulin A antibody, the disease may last for months or years, with symptoms including poor digestion, problems digesting milk, intermittent diarrhea, fatigue, weakness, and significant weight loss.


Diagnosis can be difficult because it can be easy to overlook the presence of the Giardia cysts during a routine inspection of a stool specimen. In the past, the condition has been diagnosed by examining three stool samples for the presence of the parasites. However, because the organism is shed in some stool samples and not others, the infection may not be discovered using this method.

Another method is the entero-test, which uses a gelatin capsule with an attached thread. One end is attached to the inner part of the patient's cheek, and the capsule is swallowed. Later, the thread is withdrawn and shaken in saline to release the parasites, which can be detected microscopically.

Another more accurate method of diagnosing the condition is the enzyme-linked immunosorbent assay (ELISA) that detects cysts and antigen in stool, and is approximately 90% accurate. While slightly more expensive, it only needs to be done once and is therefore less expensive overall.


Acute giardiasis can usually be allowed to run its natural course and tends to clear up on its own. Antibiotics are helpful, however, in easing symptoms and preventing the spread of infection. Effective treatments include metronidazole, tinidazole, and nitazoxanide. Alternatives to these medications include paromomycin, quinacrine, and furazolidone. Healthy carriers with no symptoms do not need antibiotic treatment. If treatment should fail, the patient should wait two weeks and repeat the drug course. Anyone with an impaired immune system (immunocompromised), such as a person with AIDS, may need to be treated with a combination of medications.

Public health role and response

In the U.S., giardiasis is a nationally notifiable disease, which means that health care providers and laboratories that diagnose cases of laboratoryconfirmed giardiasis are required to report those cases to their local or state health departments, which in turn report the cases to the Centers for Disease Control.

It is especially important to inform local, state, and federal health authorities about cases of giardiasis affecting multiple people that are related to water, food, or person-to-person transmission so that appropriate public health responses can be taken to help control the spread of this disease.

A typical public health response to reported case(s) of giardiasis might include:

If there is an outbreak of giardiasis in a community (which can be calculated by determining if the current number of cases is greater than 2x the median number of the previous 5 years (disease action threshold), the public health response would include developing and implementing intensive control methods for the community facilities responsible for the outbreak and providing the public with educational information on methods to avoid spreading the disease.


Giardiasis is rarely fatal, except in cases of extreme dehydration, which can occur in infants or malnourished children, and when treated promptly, antibiotics usually cure the infection. While most people respond quickly to treatment, some have lingering symptoms and suffer with diarrhea and cramps for long periods, losing weight and not growing well. Those most at-risk for a course like this are the elderly, people with a weakened immune system, malnourished children, and anyone with low stomach acid.


The best way to avoid giardiasis is to avoid drinking untreated surface water, especially from mountain streams. The condition also can be minimized by practicing the following preventive measures:

A specific protein produced by the immune system in response to a specific foreign protein or particle called an antigen.
A substance (usually a protein) identified as foreign by the body's immune system, triggering the release of antibodies as part of the body's defense mechanism.
Enzyme-linked immunosorbent assay (ELISA)—
A laboratory technique used to detect specific antigens or antibodies. It can be used to diagnose giardiasis.
Giardia lamblia
A type of protozoa with a whiplike tail that infects the human intestinal tract, causing giardiasis. The protozoa will not spread to other parts of the body.
A state in which the immune system is suppressed or not functioning properly.
Prevalence rate—
The proportion of a population found to have a condition, such as a disease or a risk factor (e.g., smoking or seat-belt use). It is calculated by dividing the number of people found to have the condition by the total number of people studied, and is expressed as a ratio, as a percentage or as the number of cases per 10,000 or 100,000 people.

Children with severe diarrhea (and others who are unable to control their bowel habits) should be kept at home until the stool returns to normal. Additionally, tips for swimmers include:


See also Centers for Disease Control and Prevention ; Parasites ; Sanitation .



Ortega-Pierres, M.G., Caccio, S., Fayer, R., Mank, T., Smith, H., and Thompson, R.C.A. Giardia and Cryptosporidium Oxfordshire, UK: CABI Publishing, 2009.


Centers for Disease Control. .

Giardiasis Investigation Guideline. Kansas Department of Health.

International Society of Travel Medicine. .


Centers for Disease Control and Prevention (CDC), 1600 Clifton Road, Atlanta, GA, 30333, (800) 232-4636,, .

Judith L. Sims

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