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What is giardiasis?

Giardiasis (say “jee-ar-DYE-uh-sus”) is an infection of the intestines caused by the parasite Giardia lamblia.

The illness, also called giardia, is most often a problem in undeveloped countries where tap water is not safe.

How can you become infected with giardia?

You may become infected with giardia if you eat food or drink water that is tainted with human or animal waste. In Canada and the United States, you can get giardia by drinking untreated water from wells, streams, rivers, and lakes. This is true even in mountain lakes and streams where the water may seem very pure.

You can get giardia from someone else through:

  • Close contact with someone who is infected.
  • Working in daycare centres for young children. For example, if you change a diaper and do not wash your hands afterward, anything or anyone you touch could get infected. You could even get the illness yourself if you touch your mouth or eat food that you've touched. Children in daycare centres are also more likely to get infected.
  • Working or living in nursing homes or other care centres where people may have poor bowel control and poor hygiene.
  • Some types of sexual contact, such as anal-oral contact.

What are the symptoms?

Giardia can cause diarrhea, stomach cramps, gas, and nausea. You may feel sick once and then get better. Or your symptoms may come and go for some time. Some children with giardiasis do not grow or gain weight normally. Sometimes giardiasis does not cause any symptoms.

After a person is exposed to the parasite, it usually takes 7 to 10 days for the infection to develop, but it can take from 3 to 25 days or longer. You can pass the infection to others during the entire time you are infected. You may be infected for months, even if you don't have symptoms.

How is it treated?

Your doctor may prescribe medicine to kill the parasite. Treatment also lowers the chance that you will pass giardia to others. Taking all the medicine is important, so the infection does not come back.

You may be tested for giardiasis even though you don't have any symptoms. For example, this could happen during an outbreak at a daycare centre. If tests show you are infected, doctors recommend that you get treatment even if you don't have symptoms. This is because a small number of people who are not treated get a long-term infection.

If you have diarrhea, try eating small amounts of bland food until you feel better. This gives your bowel a rest. But you need to take frequent sips of clear fluids like rehydration drinks to avoid dehydration. This is especially important for children, because they can become dehydrated quickly.

Can giardiasis be prevented?

There are some things you can do to avoid giardiasis.

  • Do not drink untreated or unpurified water. If you are camping or hiking, boil or purify water from lakes and streams before you drink it.
  • When you travel in high-risk areas, drink bottled water and avoid raw fruits and vegetables. Do not drink beverages containing ice cubes.
  • Wash your hands often to prevent getting giardiasis from an infected person. This is very important not only after you change diapers, use the toilet, or help someone else use the toilet but also before you prepare food.

Frequently Asked Questions

Learning about giardiasis:

Being diagnosed:

Getting treatment:

Ongoing concerns:


Giardiasis (also called giardia) is caused by infection with the single-celled parasite Giardia lamblia, also known as Giardia intestinalis or Giardia duodenalis.

The parasites live and reproduce in human or animal intestines. When they are in the intestines, the parasites attach to the inside of the intestinal wall, where they can disrupt the normal function of the intestines and compete for important nutrients. This leads to the symptoms of giardiasis.

  • Disruption of the intestinal surface can reduce the body's ability to absorb nutrients from food passing through the intestines. If not absorbed, nutrients remain in the intestinal tract, where they may contribute to the diarrhea, cramps, and gas associated with giardiasis.
  • The parasite may cause food to move more quickly through the intestines, which can contribute to diarrhea.

The stools (feces) of infected animals and humans contain the parasite. Infection with Giardia lamblia occurs in two ways:

  • Direct hand-to-mouth transfer of the parasite from the stool of an infected person. This may result from poor personal hygiene, such as not washing hands. The parasite can also be directly transmitted during oral-anal or oral-genital contact with an infected person.
  • Indirect transfer of the parasite by drinking water or eating food that has come in contact with feces containing the parasite.

The incubation period averages 7 to 10 days but may range from 3 to 25 days or longer. You can spread giardiasis as long as the parasite is in your feces, which can last for months. Not all infected people develop symptoms, but a person who does not have symptoms can still spread the infection.


Many children and adults infected with Giardia lamblia have no symptoms of giardiasis, even though they are infected and the parasite is present in their feces. These people can pass the infection to others.

The most common symptoms of giardia include:

  • Diarrhea, usually lasting 7 to 10 days. It can last longer but can also get better within 2 to 4 days. There first may be a large amount of watery diarrhea followed later by greasy, foul-smelling stools that may float. This is a sign that fat is not being digested properly and instead is being passed from the body in the stool.
  • Abdominal cramps or tenderness.
  • Nausea and loss of appetite.
  • Passing more gas or having more bloating than usual.
  • Fatigue.

People often do not seek treatment for these symptoms. In most cases, the symptoms gradually get better on their own. But if giardiasis is not treated during the first episode of symptoms, the illness may become a long-term (chronic) problem.

People with chronic giardiasis have bouts of diarrhea that come and go along with the other common symptoms of giardiasis. These episodes alternate between periods of constipation and normal bowel movements, and they last only a few days at a time. Because of this, many people do not seek treatment. Other symptoms of chronic giardiasis include:

  • Loose, soft, greasy stools (not always watery or liquid). Sometimes the stools may be foul-smelling or foamy and are often passed in small amounts.
  • Discomfort in the abdomen or pit of the stomach that is often worse after a meal.
  • Belly cramps, bloating, or pain.
  • Passing more gas than usual.
  • Persistent bad breath or belching that sometimes smells of sulfur.
  • Occasional headaches.
  • Weight loss.
  • General feeling of discomfort or illness (malaise), weakness, or fatigue.

Symptoms in children

Young children with giardiasis may show no symptoms other than a failure to grow and gain weight normally. If a child does have other symptoms, they may include:

  • Pale, foamy, foul-smelling diarrhea.
  • Weight loss or lack of appetite.
  • Belly pain, nausea, or vomiting.
  • Nutritional deficiencies caused by not being able to absorb certain nutrients.

Breast-feeding is thought to help protect infants from giardia infection. Breast milk contains substances such as antibodies that may kill the parasite.1 And babies may have less chance to get the giardia parasite while breast-feeding.

The symptoms of giardiasis are common to numerous other intestinal infections. The duration of the symptoms and a test to detect the parasite may be the only ways to distinguish between these diseases.

What Happens

Symptoms of giardiasis usually appear 7 to 10 days after infection, though it may take 25 days or longer. Symptoms can last 7 weeks or more.

  • In children or adults who are otherwise healthy, the symptoms of giardiasis may eventually go away, even without treatment.
  • Long-term diarrhea may result in weight loss, dehydration, or nutritional problems, because food is not being properly absorbed by the body. These conditions usually can be corrected with treatment.

Many people who are infected with Giardia lamblia do not have symptoms. But it is possible to infect others even if no symptoms are present, because the parasite still exists in the stool (feces).

About 20 to 40 people out of 100 people with giardiasis have temporary difficulty digesting milk and milk products (lactase deficiency, or lactose intolerance).1 This problem can last for up to 1 month after treatment for giardiasis. This does not mean that the treatment did not work or that the person has been infected again.

Babies or toddlers who have giardiasis may stop eating or growing normally until the condition is cured. Normal growth may not start right away but should begin again within a few weeks or months.

What Increases Your Risk

You are at risk for getting giardiasis if:

  • You work in a daycare centre for young children, especially if many of the children are not yet toilet trained.
  • You live in a household with young children.
  • You swim in waters such as rivers, lakes, or streams, especially during the summer and fall months when the risk of exposure is greatest. The main risk is from swallowing water while swimming.
  • You drink untreated or inadequately treated water, especially from rivers, lakes, or streams.
  • You live or work in a nursing home or assisted living centre or are a family member of an employee.
  • You have oral-anal or oral-genital contact with an infected person.

Things that increase your chances of having symptoms after infection include:

When To Call a Doctor

If you suspect giardiasis, call your doctor immediately if:

  • Diarrhea is black or bloody. Note: Medicines containing bismuth (such as Pepto-Bismol) can turn stools black, but this is not harmful.
  • Signs of severe dehydration appear. These include:
    • Little or no urine (or wet diapers) for 12 or more hours, usually with one or more of the other signs of dehydration listed below.
    • Doughy skin that doesn't bounce back when pinched.
    • Difficult or slow response or confusion.
    • Sunken eyes.
    • Sunken soft spot (fontanel) on an infant's head.
    • Low blood pressure.
    • Dizziness, especially if you feel dizzy when you stand up or change position suddenly.

Call within a day if you have:

  • Belly pain along with diarrhea, and the pain is not relieved by passing stools or gas.
  • Diarrhea along with a fever of 38°C (100°F) or higher with chills, vomiting, or fainting (not just light-headedness).
  • Severe diarrhea (many watery stools in a person who also seems very sick) that lasts longer than:
    • 2 days in an adult.
    • 1 to 2 days in a child younger than 4.
    • 8 hours in an infant 3 to 6 months old.
    • 4 hours in an infant younger than 3 months.

Call a doctor if:

  • Diarrhea develops and persists after you have swallowed untreated water.
  • Mild to moderate diarrhea (a few stools that are looser than normal without other signs of illness) lasts longer than 4 to 7 days in a child.
  • Diarrhea continues without obvious cause for more than 1 to 2 weeks.
  • An infant or child is not developing or gaining weight as expected (failure to thrive).

Watchful Waiting

Most cases of diarrhea are caused by a viral infection and improve with a few days of home care. But if diarrhea is severe or lasts longer than a week or so, or if the above symptoms are present, call your doctor for advice.

Who To See

Your family doctor, general practitioner, or your child's pediatrician can diagnose and treat giardiasis. Referral to a gastroenterologist or infectious disease specialist usually is not necessary unless symptoms are persistent or severe.

To prepare for your appointment, see the topic Making the Most of Your Appointment.

Examinations and Tests

Diagnosis of giardiasis is usually based on:

  • Medical history and physical examination. If your history and physical examination strongly suggest the symptoms are due to giardiasis, some doctors may prescribe a single course of treatment before doing further tests.
  • Stool analysis (also known as ova and parasite test).
  • Antigen tests that detect proteins (antigens) from the Giardia parasite.

Fluid from the upper part of the small intestine (duodenal fluid) may also be looked at under a microscope to look for the parasite or to do an antigen test. A duodenal fluid sample is collected by endoscopy.

Other tests to help detect giardiasis or similar conditions may sometimes be done if you have long-term symptoms, especially if problems with malabsorption are occurring. These tests are used to examine the small intestine (upper GI series) or to collect a sample (biopsy) of the upper small intestine to identify what is causing the infection.

Early Detection

When outbreaks of giardiasis occur in nursing homes or daycare centres, it may be appropriate to check those who may be infected. People who are infected but don't have symptoms can pass the infection to others.

Treatment Overview

Sometimes the symptoms of giardiasis go away without treatment. But many people who have symptoms will choose treatment to relieve their discomfort and avoid the spread of the illness. Doctors advise people infected with Giardia to get treated even if they don’t have symptoms. This prevents spread of the disease and reduces your risk of a chronic infection from the parasite.

Giardiasis is treated with antiprotozoal medicines (such as metronidazole) that kill the parasite. The medicine you take will depend on your age, whether you have been treated for giardiasis before, and whether you are pregnant.

If you have giardiasis, you should be treated if:

  • You work in a nursing home or child care centre.
  • Your job involves food preparation, handling, or serving.
  • You have health problems in addition to giardiasis. Even a short episode of diarrhea may cause dehydration or poor absorption of nutrients from the intestine, which may make other health problems worse.
  • You have bouts of diarrhea that occur off and on for several months or more (chronic giardiasis). Many cases do get better eventually. But treatment can relieve the discomfort of symptoms and may reduce the risk of spreading the infection to others.

What To Think About

If you do not have symptoms, the decision to get tested or treated depends on your risk of spreading the parasite. You don't need to treat an infection that is not causing symptoms, but most doctors do recommend treatment. The medicines may not be effective for everyone. When you need to decide whether to treat a giardiasis infection, you may want to think about the cost of treatment as well as the potential negative side effects of a medicine.

  • If the risk of infecting others is high (such as for people who work in food services) or the likelihood of shared exposure is high (such as among family members who drink untreated water from the same source), testing and treatment should be done.
  • If the risk of infecting others is low, you may want to wait and see whether symptoms develop.
  • If you become reinfected with giardia after receiving treatment, you may be getting infected by others who are spreading the parasite or you may have a persistent infection.


Practice good hygiene in daycare centres, retirement homes, and at home to prevent the spread of giardiasis infection. Wash your hands often. And avoid contact with the stool (feces) of an infected person.

Do not use untreated water in areas where the giardia parasite may be present. To prevent the spread of giardiasis from contaminated water, boil the water before you use it. Also, be sure swimming pools are properly treated and have proper filtration systems.

Prevent infection when travelling to areas where the water may contain Giardia. Use only bottled water, avoid raw fruits and vegetables, and do not drink beverages that contain ice cubes.

Home Treatment

If you think that you have giardiasis and you have diarrhea, prompt medical treatment can ease your symptoms. Supportive care at home will also make you more comfortable while you are being treated.

Self-care for diarrhea

The following will help you deal with a bout of diarrhea and avoid dehydration:

  • Rest your stomach. Drink only clear liquids until you begin to feel better. But if your diarrhea lasts longer than 24 hours, take frequent sips of a rehydration drink (such as Pedialyte) to prevent dehydration. Try to drink a cup of water or rehydration drink (slowly) for each large, loose stool.
  • Since having diarrhea can sometimes help you recover sooner from a problem (especially if it is due to a viral infection), avoid antidiarrheal medicines for the first 6 hours. Use them after that time only if cramping and discomfort continue and there are no other signs of illness, such as fever.
  • Begin eating bland foods, such as rice, dry toast or crackers, bananas, and applesauce the next day or sooner, depending on how you feel. Avoid spicy foods, other fruits, alcohol, and coffee for 2 days and dairy products for 3 days after all symptoms have stopped.
  • Children with diarrhea, especially those younger than 4 years old, need special attention to avoid dehydration. Try giving smaller portions of food and liquids at frequent intervals. For older children, give 120 mL (0.5 cup) to 240 mL (1 cup) of rehydration drink each hour. And offer mild foods described above. Pedialyte Freezer Pops may also be helpful.
  • Continue an infant's regular breast milk or formula feeding as much as possible. If you are feeding formula, it may help to switch to a soy milk formula for a short period of time.
  • Supplement feedings with small sips or spoonfuls of a rehydration drink every few minutes. Do not give undiluted sports drinks, soda pop, or fruit juice. These contain too much sugar and not enough electrolytes, which are important nutrients lost through diarrhea.

Digestive problems after treatment for giardiasis

Some people with symptoms of giardiasis will have temporary difficulty digesting milk and milk products (lactase deficiency). This can cause symptoms similar to those of giardia infection, such as diarrhea, a lot of gas, and cramping. The problem can last for up to 1 month after treatment for giardiasis.

If you have this problem while being treated for giardiasis, avoid milk and milk products for at least 1 month. Then gradually add them back into your diet as your body can handle them.

Babies or toddlers who had giardiasis may stop eating or fail to grow as expected. It can take a few weeks for them to get back to normal after treatment. In general, giardia infection does not cause any permanent problems for children or adults.


Medicines cure giardiasis in adults 80% to 95% of the time.1 But some people may not respond to the first course of medicine and may need to repeat the treatment with the same or another medicine.

If symptoms and medical history strongly suggest a diagnosis of giardiasis, some doctors may prescribe medicine before doing further testing. But this decision depends on whether family members, for example, were exposed at the same time or whether there is a risk of spreading the infection to others.

Medication Choices

Antiprotozoals (such as metronidazole)
Paromomycin (alternate medicine for pregnant women)

Paromomycin may be used during pregnancy. But most doctors prefer not to prescribe medicines during the first 3 months of pregnancy.

What To Think About

If the first course of medicine does not cure the infection, the treatment may be repeated with the same medicine, another one, or a combination of medicines.


There is no surgical treatment for giardiasis.

Other Treatment

There is no other proven treatment for giardiasis.

Other Places To Get Help

Online Resource

Healthy Environments and Consumer Safety Branch (HECSB)
Health Canada
Web Address:

The Healthy Environments and Consumer Safety Branch (HECSB) works to maintain and improve Canadians' health by promoting healthy and safe living, working, and recreational environments and by reducing the harm caused by tobacco, alcohol, controlled substances, environmental contaminants, and unsafe consumer and industrial products.


Centers for Disease Control and Prevention (CDC)
1600 Clifton Road
Atlanta, GA 30333
TDD: 800-232-6348
Web Address:

The CDC Healthy Swimming Web site provides tips and fact sheets to help people reduce the chances of getting an illness from swimming in recreational waters such as lakes, rivers, swimming pools, and oceans. CDC's Healthy Swimming program also provides resources to raise awareness about recreational water illnesses (RWIs) and how to prevent them by practicing "Healthy Swimming" behaviors.

Centers for Disease Control and Prevention (CDC): Division of Parasitic Diseases
1600 Clifton Road
Atlanta, GA  30333
Phone: 1-800-CDC-INFO (1-800-232-4636)
TDD: 1-888-232-6348
Web Address:

The Division of Parasitic Diseases is a branch of the U.S. Centers for Disease Control and Prevention (CDC). Its mission is to prevent and control parasitic diseases throughout the world. Its Web site provides information and updates on parasitic diseases.

Centers for Disease Control and Prevention, Travelers' Health
1600 Clifton Road
Atlanta, GA  30333
Phone: 1-800-CDC-INFO (1-800-232-4636)
TDD: 1-888-232-6348
Web Address:

The CDC's Travelers' Health Web site provides health information for the traveler. The Web site provides information on immunizations that are needed for travel to various areas of the world. It also provides information for safe travel, including traveling with children and with people who have special needs. Information about current outbreaks of disease in the world is also provided. The CDC is the leading federal agency for protecting U.S. citizens' health and safety by providing credible health information and health promotion.

KidsHealth for Parents, Children, and Teens
4600 Touchton Road East, Building 200
Suite 500
Jacksonville, FL  32246
Phone: (904) 232-4100
Fax: (904) 232-4125
Web Address:

This Web site is sponsored by Nemours Foundation. It has a wide range of information about children's health, from allergies and diseases to normal growth and development (birth to adolescence). This Web site offers separate areas for kids, teens, and parents, each providing age-appropriate information that the child or parent can understand. You can sign up to get weekly e-mails about your area of interest.



  1. Hill DR (2005). Giardia lamblia. In GL Mandell et al., eds., Principles and Practice of Infectious Disease, 6th ed., vol. 2, pp. 3198–3205. Philadelphia: Elsevier.

Other Works Consulted

  • Adachi JA, et al. (2007). Infectious diarrhea from wilderness and foreign travel. In PS Auerbach, ed., Wilderness Medicine, 5th ed., pp. 1418–1444. Philadelphia: Mosby Elsevier.
  • American Academy of Pediatrics (2006). Giardia intestinalis infections (giardiasis). In LK Pickering, ed., Red Book: 2006 Report of the Committee on Infectious Diseases, 27th ed., pp. 296–301. Elk Grove Village, IL: American Academy of Pediatrics.
  • Huston CD (2006). Intestinal protozoa. In M Feldman et al., eds., Gastrointestinal and Liver Disease, vol. 2, pp. 2413–2433. Philadelphia: Saunders Elsevier.
  • Yoder JS, Beach MJ (2007). Giardiasis surveillance, United States, 2003–2005. MMWR, 56(SS07): 11–18. [Erratum in MMWR, 56(43): 1141.]


By Healthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD - Internal Medicine
Specialist Medical Reviewer W. David Colby IV, MSc, MD, FRCPC - Infectious Disease
Last Revised February 25, 2011

This information does not replace the advice of a doctor. Healthwise, Incorporated disclaims any warranty or liability for your use of this information.