Table of Contents


Giardiasis is a communicable gastrointestinal disease characterized by acute diarrhea. It is caused by a parasite, Giardia lamblia, also known as Giardia intes-tinalis. Giardiasis is the most common water-borne infection of the human intestine worldwide, affecting as many as 200 million people each year. According to the Centers for Disease Control and Prevention (CDC), there were 90 major community outbreaks of giardiasis in the United States between 1964 and 1984, and 34 major outbreaks since 1985.

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 pear-shaped 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 17th 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.


Dietary treatment of patients with giardiasis has been a gradual process, dependent on better understanding of the causes of the disease as well as the development of nutritionally adequate rehydration solutions and anti-infective medications. A large part of dietary therapy for giardiasis, in fact, consists of measures to prevent the spread of the disease, not just to treat the symptoms after they appear.

Life cycle of G. lamblia

In order 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. 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


Barberry—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.

Carrier—A person 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.

Cyst—The protective shell formed by G. lamblia that keeps the organism alive after it has been expelled from the host’s body.

Endoscope—A special tube-shaped instrument that allows a doctor to examine the interior of or perform surgery inside the stomach or intestines.

Flatulence—The passage of intestinal gas. Flatulence is a common symptom of giardiasis.

Gastroenterologist—A doctor who specializes in the diagnosis and treatment of diseases of the stomach and intestines.

Lactose—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.

Protozoan—Any member of a phylum of one-celled eukaryotes (organisms with nuclei) that are able to move but are not animals in the strict sense. The organism that causes giardiasis is a protozoan.

Reservoir—A term used for animals that can carry parasites that cause disease in humans without falling ill themselves. Beavers, dogs, cats, cattle, and horses are common reservoirs of G. lamblia.

Steatorrhea—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 form of 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.

Trophozoite—The active feeding stage in the life cycle of G. lamblia. It is the trophozoites that multiply within the small intestine and cause the diarrhea and other symptoms of giardiasis.

walled and oval in shape, 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 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 blood

Once inside the body, the cysts pass through the digestive tract until they reach the small intestine. Each cyst then opens—often within 5 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. It is the trophozoites that cause the violent diarrhea, nausea, intestinal gas, and cramping associated with giardiasis. As of 2007, however, researchers do not know the exact reason for the symptoms; some think that the parasites compete with the host for nutrients, while others think 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.


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 persons carrying the cysts in their digestive tract, known as carriers, can still transmit giardiasis to others even if they do not develop the symptoms of the illness. It is estimated that between 30 and 60% of children in daycare centers and adults on Native American reservations are carriers of G. lambliaSome domestic and wild animals can also be carriers of G. lamblia,dogs 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 3 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 10 pounds in adults. Without treatment, these symptoms can last for as long as 7 weeks or even longer

Between 20 and 40% of adults with giardiasis develop a temporary difficulty with digesting lactose, a sugar found in milk or milk products. This condition is called 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

There is no universal pattern to recovery from giardiasis. It is rarely fatal except in severely dehydrated and malnourished children, but 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:

  • Ongoing weight loss apart from intentional weight reduction.
  • Steatorrhea. Steatorrhea is the medical term for the passage of large amounts of fat or greasy-looking material in the stool.
  • Discomfort in the stomach or abdomen that is worse after a meal.
  • Persistent bad breath or burping that smells like sulfur.
  • Ongoing bloating, flatulence, or abdominal cramping.
  • Recurrent headaches.
  • Malaise (general feeling of sickness), fatigue, or weakness.

The symptoms of chronic giardiasis in children include:

  • Failure to grow and gain weight at a normal rate for the child’s age and sex.
  • Recurrent episodes of pale, frothy, foul-smelling diarrhea.
  • Loss of appetite.
  • Abdominal pain and vomiting.
  • Nutritional deficiencies caused by the inability to absorb nutrients in food.


In the United States and other developed countries, giardiasis is most likely to affect children, particularly children in daycare centers. About 20 to 25% of the children in daycare centers are infected with giardiasis even though they may not be symptomatic. Most of the community outbreaks in the United States since the 1980s, in fact, began in daycare centers.

Older adolescents and adults are more likely to be infected with giardiasis while hiking or traveling abroad. G. lambliais 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. lambliacysts 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. lambliacysts.

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.

Risk factors

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

  • Parents of infected small children.
  • Employees (and their family members) of daycare centers in which some of the children are not yet toilet-trained.
  • Employees (and their family members) of nursing homes or other custodial facilities.
  • People who swim in or boat on rivers, lakes, streams, or other bodies of water liable to contamination by fecal matter. The greatest risk of infection comes from accidentally swallowing a mouthful of water while swimming, diving, rafting, or water skiing.
  • People who depend on well water for their household drinking supply.

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

  • An impaired immune system.
  • Crohn’s disease, cystic fibrosis, or other diseases that weaken the intestines.
  • Malnutrition.
  • Recent surgery on the stomach or taking medications to lower stomach acid secretion (stomach acid kills G. lamblia).


Most people with giardiasis can be diagnosed and treated by their primary care physician. 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 enzyme-linked immunosorbent assay (ELISA) tests; 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 inside of the cheek and swallows the capsule. The string is left in place for 4 to 6 hours or overnight while the patient is fasting; it is then removed and the mucus on the string is examined for trophozoites.

Patients suspected of having chronic giardiasis may be referred to a gastroenterologist, who is a doctor with special training in digestive disorders. In some cases, the doctor may need to examine the patient’s small intestine through an endoscope or remove a sample of tissue from the lining of the patient’s intestine to make sure that the patient’s symptoms are caused by a parasite and not by some other disorder.


Giardiasis is most commonly treated with one of the following drugs, which cause the death of the disease organisms:

  • Metronidazole (Flagyl). The most common drug given to treat giardiasis. Adults are usually given three doses per day over a 5-day period, while children are usually given a 10-day course.
  • Furazolidone (Furoxone). Some doctors prefer to treat children with this drug because it is available in a liquid form.
  • Nitazoxanide (Alinia). This drug is also preferred for treating children because it causes fewer adverse effects in younger patients.
  • Tinidazole (Tindamax). Tinidazole is a relatively new anti-infective drug; it was approved by the Food and Drug Administration (FDA) only in 2004. It has the advantage of requiring only one dose of 2000 mg for treatment of giardiasis rather than several days of repeated doses.
  • Paramomycin (Humatin). Paramomycin is the only drug effective against G. lambliathat is considered safe to give pregnant women.

Some herbalists and naturopaths recommend barberry (Berberis vulgaris)as an anti-infective agent in treating giardiasis

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

Dietary treatment

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 have become 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:

  • Begin eating mild foods without spices on the second day. These foods include plain boiled rice, dry toast, saltines, applesauce, and bananas.
  • Avoid spicy foods, citrus fruits, caffeinated beverages, and alcohol until two days after the last symptoms of giardiasis have disappeared.
  • Avoid milk and other dairy products for three days after the last symptoms of giardiasis.

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


Anti-infective medications benefit the patient by killing the parasites responsible for giardiasis, thereby lowering the risk of developing chronic giardiasis and malabsorption syndrome.

Dietary treatment

Dietary treatment for giardiasis benefits the patient by giving the digestive tract a rest and by replacing nutrients and electrolytes lost by acute diarrhea or vomiting.


Patients taking anti-infective drugs should tell their doctor about any other medications they may be taking, including over-the-counter drugs, to lower the risk of drug interactions.

Pregnant women should not take any anti-infective medication for giardiasis except paramomycin, and then only if they are severely ill.

Anti-infective medications should be taken with food to lower the risk of nausea or heartburn. They should, however, neverbe taken with alcohol, as they will interact to cause severe nausea and vomiting. Alcoholic beverages should be avoided for at least 48 hours after the last dose of the medication.

Preventive dietary and public health measures

The risk of contracting giardiasis can be lowered by simple dietary and personal cleanliness measures:


  • What can I do to prevent giardiasis in my household?
  • What precautions would you advise before a family vacation trip?
  • Avoid drinking untreated or unpurified water when hiking or camping. Boil the water for at least 1 full minute or add commercial purification tablets before drinking.
  • When traveling in other countries known to have high rates of giardiasis, drink only bottled water and do not eat raw fruits or vegetables or drink beverages containing ice made from untreated water. The CDC maintains a frequently updated list of travelers’ advisories on its website.
  • Wash hands often, particularly after using the toilet, changing diapers, helping someone else use the toilet, or before food preparation.
  • Avoid swallowing water when swimming in lakes, rivers, or streams.
  • Protect others by avoiding swimming pools, hot tubs, the ocean, or lakes and streams while infected with G. lamblia,and for at least 2 weeks after the diarrhea stops. It is possible to pass the organism in stool and contaminate water for several weeks after the symptoms have ended.
  • Avoid exposure to fecal material during sexual activity.


The anti-infective drugs listed above cause some adverse effects in some patients, including nausea, constipation, headache, temporary discoloration of the urine, a metallic taste in the mouth, drowsiness, skin rashes or itching, dry mouth, or depression

Anti-infective medications interact with such other drugs as cimetidine, praziquantel, tricyclic antidepressants, selective serotonin reuptake inhibitors (SSRIs), lithium, phenytoin, and others. They counteract the effects of some of these medications and intensify the effects of others.

Dietary treatment

Research and general acceptance

Anti-infective medications for the treatment for giardiasis have been tested and used for several decades as of 2007. The WHO formula for oral rehydration has been used in developing nations since the 1960s in treating people dehydrated by diarrhea resulting from cholera, giardiasis, and other intestinal disorders.


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American College of Gastroenterology (ACG). 6400 Goldsboro Road, Suite 450, Bethesda, MD 20817. Telephone: (301) 263-9000. Website:

Centers for Disease Control and Prevention (CDC). 1600 Clifton Road, Atlanta, GA 30333. Telephone: (404) 639-3311. Website:

North American Society for Pediatric Gastroenterology, Hepatology and Nutrition (NASPGHAN).P.O.Box 6, Flourtown, PA 19031. Telephone: (215) 233-0808. Website:

U. S. Food and Drug Administration (FDA). 5600 Fishers Lane, Rockville, MD 20857-0001. Telephone: (888) INFO-FDA. Website:

Rebecca J. Frey, PhD