Giardiasis is a communicable gastrointestinal disease characterized by acute diarrhea. It is caused by a parasite, Giardia lamblia, also known as Giardia intestinalis and Giardia duodenalis. Giardiasis is the most common water-borne infection of the human intestine worldwide. It is classified as a zoonosis because it can be transmitted to humans through contact with animals.

The organism that causes giardiasis, G. lamblia, is a protozoan, a single-celled organism formerly classified as a member of the animal kingdom. It is a pearshaped parasite with four flagella, which are long whip-like extensions of the cell that allow the organism to move. It was first seen under a microscope by the Dutch lens maker Antony van Leeuwenhoek in the seventeenth century. G. lamblia was found in human stool samples in 1859 by a Czech physician named Lambl but was not identified as the cause of giardiasis until the 1970s. It was given its present name in 1915 to honor Alfred Giard, a French biologist, as well as Dr. Lambl.


Older adolescents and adults are more likely to be infected with giardiasis while hiking or traveling abroad. G. lamblia is a common cause of so-called traveler's diarrhea, although it is not the only organism that causes it. Giardiasis acquired its nickname of “beaver fever” because backpackers and hikers who drink water from or swim in streams close to beaver colonies are likely to ingest G. lamblia cysts shed into the water by infected animals. The CDC reports that as many as 80% of water samples from lakes, streams, and ponds in the United States contain G. lamblia cysts.

Outbreaks of giardiasis are most likely to occur in Canada and the United States during warmer weather, particularly in summer and fall. Race does not appear to be a factor in contracting giardiasis; however, males in all age groups are about 1.2 times more likely than females to develop the disease.

Internationally, giardiasis in the industrialized countries is more common in Eastern Europe, Russia, and New Zealand than elsewhere. In the developing countries, giardiasis is most common in Nepal, Bangladesh, and parts of western Africa.


Life cycle of G. lamblia

To understand the symptoms, treatment, and prevention of giardiasis, it is helpful to understand the life cycle of G. lamblia. The parasite that causes giardiasis has a simple two-stage life cycle that does not require an intermediate host; it can be spread directly among human beings as well as from animals to humans. The cycle begins when a person swallows as few as 10 to 15 cysts of G. lamblia. The cyst is a protective shell that the organism forms around itself that enables it to survive outside a human or animal host. The cysts of G. lamblia are smooth walled and oval shaped, about 8–12 micrometers long and 5–15 micrometers wide. They are hardy and can survive for several months in cold water. They usually enter the human body through the mouth. The cysts may be transferred to the mouth directly from unwashed hands that have touched fecal matter (including animal fecal matter) containing cysts, or through having oral sex with an infected person. They may also enter the mouth through eating food or swallowing liquids contaminated by fecal matter containing G. lamblia cysts. G. lamblia is not, however, transmitted through the blood.

Once inside the body, the cysts pass through the digestive tract until they reach the small intestine. Each cyst then opens—often within five minutes after arrival—and releases two trophozoites, which are the active feeding stage of the parasite. The trophozoites multiply rapidly, reproducing every 9 to 12 hours. They may remain free within the central cavity (lumen) of the small intestine or attach themselves to the mucous tissue lining the intestine by a sucking disk located on their ventral surface. The trophozoites cause the violent diarrhea, nausea, intestinal gas, and cramping associated with giardiasis. Researchers, however, do not know the exact reason for the symptoms; some think that the parasites compete with the host for nutrients, whereas others believe that they affect the host's immune system, cause damage to the tissues lining the intestine, or block the functioning of the intestinal mucosa by their sheer numbers.

As the trophozoites are carried toward the colon, they begin to secrete proteins to form the walls of a new cyst. Within the next 24 hours, the trophozoite completes the construction of its cyst and is shed into the outside environment through the person's feces.

Risk factors

Some people are at increased risk of contracting giardiasis because of their location or lifestyle:

Some people are at increased risk of a severe case of giardiasis because they have other health problems:

Causes and symptoms


The basic cause of giardiasis is ingestion of the parasite's cysts.


About 15% of people who swallow cysts are asymptomatic. These cases are usually detected only if the person's stool is tested during a community outbreak. They are significant, however, because people carrying the cysts in their digestive tract, known as carriers, can still transmit giardiasis to others even if they do not develop symptoms of the illness. It is estimated that between 30%–60% of children in daycare centers and adults on Native American reservations are carriers of G. lamblia. Some domestic and wild animals can also be carriers, with dogs, cats, horses, and beavers being the most common animal reservoirs.

Of the patients who have symptoms, 90% develop acute diarrhea within 7 to 10 days of ingesting the cysts, and 70%–75% have abdominal cramps, bloating, vomiting, and flatulence (the passage of intestinal gas). A small percentage of patients develop symptoms within three days of swallowing the cysts, including violent diarrhea, extremely foul-smelling intestinal gas, severe vomiting, fever, and headache. Most patients lose their appetite, and 50% lose weight—an average of ten pounds in adults. Without treatment, these symptoms can last for as long as seven weeks or even longer.

Between 20%–40% of adults with giardiasis develop a temporary difficulty digesting lactose, a sugar found in milk or milk products. This condition is called secondary lactose intolerance and may last for a month or so after treatment with anti-parasite medications for giardiasis. Having lactose intolerance does not mean that the person has become reinfected or that they will have any problem with lactose absorption in the future.

There is no universal pattern to recovery from giardiasis. It is rarely fatal except in severely dehydrated and malnourished children, but it may develop into chronic forms—malabsorption syndrome in adults and failure to thrive in children. Chronic giardiasis in adults is characterized by episodes of diarrhea that come and go, alternating with periods of constipation and normal bowel movements. Other symptoms of chronic giardiasis in adults include:

The symptoms of chronic giardiasis in children include:


Most people with giardiasis can be diagnosed and treated by their primary care physician. The first step is taking a careful patient history, particularly a history of travel to developing countries or recent outdoor or wilderness hiking or camping. Diagnosis is usually done by examining stool samples under a microscope for the characteristic cysts and trophozoites of G. lamblia (both forms of the organism may appear in the stool); by tests for the antigen produced by the parasites; or by an Entero-test. The Entero-test, also called the string test, consists of a gelatin capsule containing a nylon string attached to a weight. The patient tapes one end of the string to the edge of the cheek and swallows the capsule. The string is left in place for four to six hours or overnight while the patient is fasting; it is then removed and the mucus on the string is examined for trophozoites.

A shrub native to southern Europe and western Asia that produces oblong red berries that have a sour taste. Barberry has been used as a natural treatment for giardiasis.
Beaver fever—
An informal name for giardiasis, so called because beavers are a common animal reservoir of the parasite that causes giardiasis.
A person or animal who bears or carries a disease agent in or on their body and can transmit the disease to others but is immune to the disease or has no symptoms of it. Some people can be carriers of the organism that causes giardiasis without developing diarrhea.
The protective shell formed by G. lamblia that keeps the organism alive after it has been expelled from the host's body.
A special tube-shaped instrument that allows a doctor to examine the interior of or perform surgery inside the stomach or intestines.
The passage of intestinal gas. Flatulence is a common symptom of giardiasis.
A doctor who specializes in the diagnosis and treatment of diseases of the stomach and intestines.
A sugar found in milk and milk products that requires an enzyme called lactase to digest it. Some people with giardiasis must avoid foods containing lactose for several weeks or months after they are treated for the disease because their bodies are temporarily unable to produce enough lactase to digest the milk sugar.
Malabsorption syndrome—
A condition characterized by indigestion, bloating, diarrhea, loss of appetite, and weakness, caused by poor absorption of nutrients from food as a result of giardiasis, other bowel disorders, or certain surgical procedures involving the digestive tract.
Notifiable disease—
Any disease that is required by law to be reported to government public health authorities. Giardiasis is a notifiable disease in Canada and the United States as of 2018.
Any member of a phylum of one-celled eukaryotes (organisms with cell nuclei) that are able to move but are not animals in the strict sense. The organism that causes giardiasis is a protozoan.
A term used for animals that carry parasites that cause disease in humans without falling ill themselves. Beavers, dogs, cats, cattle, and horses are common reservoirs of G. lamblia.
The passage of large amounts of fat or grease in the stool, caused by failure to absorb it during digestion. Steatorrhea is often associated with chronic giardiasis.
Traveler's diarrhea (TD)—
A nonspecific term for a formof diarrhea that frequently affects tourists abroad. TD is the most common illness affecting visitors to other countries. Some cases of TD are caused by G. lamblia, but others result from infection with various bacteria, rotaviruses, and other intestinal parasites.
The active feeding stage in the life cycle of G. lamblia. The trophozoites multiply within the small intestine and cause diarrhea and other symptoms of giardiasis.
Zoonosis (plural, zoonoses)—
Any infectious disease that can be transmitted from other animals to humans, or from humans to other animals.


Giardiasis is treated with a combination of drugs, rehydration, rest, and dietary therapy. Dietary treatment for giardiasis benefits the patient by giving the digestive tract a rest and by replacing nutrients and electrolytes lost through acute diarrhea or vomiting.


Drug treatment for giardiasis is often started before the organism is detected in stool samples when the patient's history and symptoms are consistent with Giardia infection. Giardiasis is most commonly treated with one of the following drugs, which work by causing the death of the disease organisms:

Children or adults who are carrying cysts are sometimes given anti-infective drugs even if they are not symptomatic to lower the risk of transmission to other children in a daycare center or to other family members.


Some herbalists and naturopaths recommend barberry (Berberis vulgaris) as an anti-infective agent in treating giardiasis. There is limited evidence of its effectiveness.

Nutrition/dietetic concerns

People with mild cases of giardiasis may not need any special dietary therapy after they have started taking medications to kill the parasites.

Children and adults who are dehydrated because of severe diarrhea may be given a rehydration drink (Lytren, Rehydralyte, or Pedialyte) to sip. Adults should drink 1 cup of water or rehydration drink for each large passage of watery stool. Children should be given 1/2 to 1 cup of rehydration fluid (or Pedialyte frozen pops) per hour, as dehydration is more dangerous to them than to adults. Children should not be given undiluted sports drinks, soda pop, or fruit juice, as these contain too much sugar and not enough electrolytes. If a commercial rehydration drink is not available and the diarrhea does not stop within 24 hours, the World Health Organization (WHO) formula for oral rehydration can be used. To make the WHO formula at home, combine 1 quart of boiled or purified water with 2 teaspoons table sugar, 1/2 teaspoon salt, and 1/2 teaspoon of baking soda (sodium bicarbonate).


Dietary therapy for adults recovering from giardiasis includes the following:

Patients who have developed lactose intolerance as a result of giardiasis can usually treat the problem themselves by avoiding milk and dairy products containing lactose for a full month (or longer) after the end of symptoms and then adding them to the diet in small amounts on a gradual basis. They may also wish to consider such alternative products as soy milk, almond milk, oat milk, or rice milk, or try some of the lactose-free milk, yogurt, spreads, and cheese products that are now available.



Prevention of giardiasis requires both personal and public health measures.

Personal measures

There are no vaccines that can prevent giardiasis as of 2018. The risk of contracting giardiasis, however, can be lowered by simple dietary and personal cleanliness measures:

Public health concerns

As of 2018, giardiasis is a notifiable disease in both Canada and the United States, which means that doctors are required by law to report confirmed cases to state, provincial, or national public health authorities. This notification allows public health physicians to investigate clusters of cases or local outbreaks as quickly as possible, determine the source of the parasites (contaminated water, infected food handlers, etc.), and inform the public to prevent further spread of the infection.

See also Digestive diseases ; Lactose intolerance diet ; Traveler's diarrhea .



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American Society of Tropical Medicine and Hygiene (ASTMH), One Parkview Plaza, Ste. 800, Oakbrook Terrace, IL, 60181, (847) 686-2238, Fax: (847) 686-2251,, .

American Veterinary Medical Association (AVMA), 1931 North Meacham Road, Ste. 100, Schaumburg, IL, (800) 248-2862, Fax: (847) 925-1329, , .

Centers for Disease Control and Prevention (CDC), 1600 Clifton Rd., Atlanta, GA, 30329, (800) CDC-INFO, CDC-INFO, .

Infectious Diseases Society of America (IDSA), 1300 Wilson Boulevard, Ste. 300, Arlington, VA, 22209, (703) 299-0200, Fax: (703) 299-0204, , .

World Health Organization (WHO), Avenue Appia 20, 1202 Geneva, Switzerland, CH - 1211 Geneva 27, , .

Rebecca J. Frey, PhD

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