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Home > Water University > Water Contaminants > Giardia Lamblia Cysts

Giardia Lamblia Cysts


This is a fact sheet about a contaminant that may be found in some public or private drinking water supplies. It may cause health problems if found in amounts greater than the health standard set by the United States Environmental Protection Agency (EPA).

What is Giardia?

Giardia (je-ar’de-ah) are protozoan parasites which occur in a trophozoite and an oval-shaped cyst form. Cysts excreted in the feces of an infected host move passively through the environment. If cysts are ingested, infection may be transmitted to another vertebrate host. The trophozoite causes infection. Excystation to the trophozoite form is initiated in the stomach and completed in the small intestine. The trophozoites divide, attach to the small intestine, and then detach for unknown reasons. During the encystment process, they become rounded and elaborate a cyst wall that protects the cyst as it is excreted and carried through water and other media.

Simply put Giardia lamblia is a parasite that enters lakes and rivers through sewage and animal waste. It causes gastrointestinal illness (e.g. diarrhea, vomiting, cramps).

Where has Giardia been found?

Numerous species of Giardia have been found in a variety of mammals, birds, reptiles, amphibians, and fishes. Giardia has also been detected in beaver, muskrats, wading birds, voles, mice, shrews, gerbils, rats, deer, native marsupials, Australian brushtail possums, ringed seals, and llamas. There is no general agreement on the criteria to define species; host specificity, body size and shape, internal structures, and biochemical, molecular, and genetic techniques have all been used. Scientists and physicians describe the specie(s) responsible for human infections as G. lamblia, G. duodenalis, or G. intestinalis.

The wide occurrence of cysts in humans and animals suggests that soil can be contaminated with Giardia through fecal deposition and sewage disposal practices. Municipal waste waters likely always contain Giardia cysts. Giardia is distributed worldwide in lakes, ponds, rivers, and streams. It is even found in high quality water sources with no municipal wastewater discharges. All surface waters probably contain Giardia, and whether cysts are detected depends largely on the methods used to collect and analyze water samples.

In North America, higher levels in water are often reported in the late summer, fall and early winter. Generally, there is no correlation of cyst levels in water with coliform bacteria. When Giardia cysts are detected in environmental samples, information about viability, infectivity, or species is not usually available.

Reported Giardia levels have ranged from 10,000 to 100,000 cysts/L in untreated sewage, 10 to 100 cysts/L in treated sewage, and 10 or few cysts/L in surface water sources and tap water. Cysts have also been detected in cisterns and in wells contaminated by surface water or sewage. Levels are generally higher in water sources influenced by agriculture (e.g., cattle or dairy farming) or municipal and residential wastewater discharges. Contamination levels may fluctuate due to storms, agricultural practices, and the operation of wastewater facilities.

Giardia has been detected on stainless steel and Formica® surfaces in day care centers. Limited information is available on the levels of cysts in foods; improvements are needed in both sampling and analytical methods. There are no published reports on the occurrence of Giardia in air.

How long can Giardia cysts survive in the environment?

The survival of Giardia cysts in the environment is significantly affected by temperature; survivability decreases as the temperature increases. A small fraction of cysts can withstand a single freeze-thaw cycle. Cysts can survive for 2 to 3 months in water temperatures of less than 10/ C, and at 21/ C, cysts have remained viable for almost one month. Cysts are killed in 10 minutes at a water temperature of 54/ C. Raising the water temperature to boiling immediately kills cysts.

How infective are Giardia cysts?

Giardia cysts are highly infective. As few as ten human-source Giardia cysts produced infection in a clinical study of male volunteers. The incubation period (time interval between ingestion and the first appearance of symptoms) can range from 3 to 25 days.

Can Giardia be transmitted between animal species?

Giardia from some animals exhibit an apparent high degree of host specificity, but other isolates may infect more than one host. The role of animals in causing human infection is not clear, but evidence suggests that the beaver and possibly the muskrat is a source of infection for humans.

How prevalent is Giardia infection in humans?

Giardiasis is the most commonly reported intestinal protozoan infection worldwide; an estimated 200 million people are infected each year. In the United States, G. lamblia is the most frequently identified parasite in stool specimens submitted for parasitological evaluation.

Giardia infection tends to be more common in children than adults. Depending on the geographical area, studies have found from 1-68% of children to be infected. In many developing countries, most children under five years of age have been infected at least once. In two studies in the United States, 7% of children aged 1 to 3 years and 11% of infants and toddlers tested for admission to day-care centers were found to be infected.

How prevalent is Giardia infection in domestic and wild animals?

Giardia is a common protozoan parasite of farm animals, especially calves and lambs. Dogs are frequently found infected; cats less frequently. In different areas of the United States, 7-16% of beavers were found to be infected; 95% of muskrats were found to be infected.

What are the health effects of Giardia infection?

Giardia infection may be acquired without producing any symptoms, and this is often the case for children. In symptomatic patients, acute diarrhea is the predominate feature. In some instances, diarrhea may be transient and mild, passing without notice; in others diarrhea can be chronic. Other symptoms may include abdominal cramps, bloating, flatulence, steatorrhea (daily losses of fat in feces greater than 7 grams), weight loss, and occasionally vomiting. Stools may be pale, greasy, and malodorous. Weight loss may be significant. In some patients, symptoms last for only 3 or 4 days, while in others symptoms can last for months or years.

Rarely does Giardia infection cause death, but each year 4,600 persons with giardiasis are estimated to be hospitalized in the United States. Hospitalized cases are primarily children under five years of age, and dehydration is the most frequent co-diagnosis.

A potentially serious consequence is nutritional insufficiency which may result in impaired growth and development of infants and children. Other reported associations with giardiasis in children include malabsorption of iron, allergic reactions, inflamation of the synovial membranes of major joints, and nonprogressive retinal changes; these all require additional study.

Is treatment available for giardiasis?

As with all diarrheas, fluid replacement is important. Anti-giardial agents can be important in the management of individual cases but may not prevent reinfection of children in day-care centers or areas where exposures are frequent. Drugs have different effectiveness in their ability to clear Giardia, and side-effects should be considered, especially for pregnant women.

Who is at risk?

Giardia is frequently spread directly from person to person, especially among young children in day-care centers, nurseries, or institutions and among persons living in areas with poor sanitation and hygiene. Although 7-54% of children attending daycare centers in the United States may be infected, infections are primarily without symptoms and do not result in adverse growth effects. An estimated 5-20% of household contacts and 9-35% of care-center staff also may be infected.

Studies have not found that pets are an importance source of infection. Several small foodborne outbreaks have been associated with ice and foods contaminated by food service workers, but restaurant-associated transmission of Giardia does not appear to be a significant problem. High attack rates have been reported in travelers to endemic areas.

Giardia can be an important cause of endemic and epidemic waterborne illness. In the United States, increased risks have been found in populations where surface water sources are not filtered, persons who use shallow well water systems, persons who drink contaminated water while picnicking, camping, and hiking, and persons who accidently ingest water during swimming and other water recreational activities. Poorly maintained wading and swimming pools and heavily used swimming areas at lakes and ponds pose an increased risk, especially if they are used by diaper-age toddlers or other persons prone to fecal accidents.

What causes waterborne outbreaks?

Since 1971, Giardia has been the most commonly identified pathogen in waterborne outbreaks reported in the United States. More than 130 waterborne outbreaks have been reported in 27 states; both residents and travelers have been affected. Outbreak statistics emphasize the need for filtration of surface water, optimization of the filtration process, frequent monitoring of treatment effectiveness, and better protection and treatment for ground water.

How effective is water treatment?

When operated under appropriate conditions, commonly used filtration technologies can effectively remove Giardia cysts from water. The highest removal by granular filters is achieved when coagulation is optimized. Care must be exercised when selecting membranes; those that can remove Giardia cysts may not be effective for other protozoa, like Cryptosporidium, that are smaller in size. Commonly used water disinfectants can effectively inactivate Giardia cysts depending on the disinfectant concentration and contact time. Cysts are relatively more resistant to disinfectants than bacteria and viruses, and high doses and lengthy contact times may be needed. This may result in high levels of disinfection byproducts which are regulated by the EPA.

What is being done to reduce waterborne risks?

EPA’s Surface Water Treatment Rule (SWTR) requires that public water systems filter, except in rare circumstances, and disinfect surface water and groundwater that is directly impacted by surface water; 99.9% of Giardia must be removed or killed.

How important is waterborne transmission of giardiasis?

A risk assessment has estimated that in the United States as many as 250 infections per 10,000 people may occur each year from exposures to Giardia in drinking water. Although the limitations of this risk assessment are recognized, this estimate suggests that more stringent water treatment requirements may be needed. The EPA is currently collecting occurrence information about Giardia in water systems throughout the country. When this information becomes available, waterborne risks can be estimated again using this and other newly developed risk assessment models.

Whom should I contact if I suspect an outbreak is occurring?

If you or members of your family are diagnosed with giardiasis and suspect that your neighbors, fellow travelers, or children’s friends may also be infected, you should discuss this with your physician or a public health worker in your local or state health department. Most health departments require that physicians and laboratories report giardiasis cases to them. Health department epidemiologists investigate disease clusters and increased reports of disease to determine if they are caused by contaminated water or food or other sources. Health departments may ask the Centers for Disease Control and Prevention (CDC) in Atlanta to assist in an outbreak investigation. The EPA can assist the CDC in the investigation of suspected waterborne outbreaks.

Whom should I contact if I am concerned about my drinking water?

If you suspect your water system is contaminated, you should contact your water utility and ask about the effectiveness of their treatment. State agencies can also provide information about public water systems and their water quality. In some states, the health department will have regulatory jurisdiction; in others, a department of environmental quality or natural resources will have this responsibility. Your health department or county agriculture extension office can provide assistance and advice about the contamination and water treatment of nonpublic or individual water systems.

Home and personal water treatment systems should be carefully selected. If your home water supply is subject to contamination with Giardia, you should select a system that can remove or kill 99.9% of Giardia and Cryptosporidium and 99.99% of waterborne enteric viruses and bacteria. Independent testing groups, like NSF International, evaluate the effectiveness of water treatment devices. Heating water to at least 70o C for 10 minutes or boiling water for one minute at sea level (three minutes at high altitudes) is also acceptable.

Note: The previous fact sheet is part of a larger publication adapted from U.S. EPA publication: EPA National Primary Drinking Water Regulations.


What is giardiasis?

Giardiasis (GEE-are-DYE-uh-sis) is a diarrheal illness caused by a one-celled, microscopic parasite, Giardia intestinalis (also known as Giardia lamblia). Once an animal or person has been infected with Giardia intestinalis, the parasite lives in the intestine and is passed in the stool. Because the parasite is protected by an outer shell, it can survive outside the body and in the environment for long periods of time.

During the past 2 decades, Giardiainfection has become recognized as one of the most common causes of waterborne disease (found in both drinking and recreational water) in humans in the United States . Giardia are found worldwide and within every region of the United States.

How do you get giardiasis and how is it spread?

The Giardia parasite lives in the intestine of infected humans or animals. Millions of germs can be released in a bowel movement from an infected human or animal. Giardia is found in soil, food, water, or surfaces that have been contaminated with the feces from infected humans or animals. You can become infected after accidentally swallowing the parasite; you cannot become infected through contact with blood. Giardia can be spread by:

Accidentally putting something into your mouth or swallowing something that has come into contact with feces of a person or animal infected with Giardia.

Swallowing recreational water contaminated with Giardia. Recreational water includes water in swimming pools, hot tubs, jacuzzis, fountains, lakes, rivers, springs, ponds, or streams that can be contaminated with sewage or feces from humans or animals.

Eating uncooked food contaminated with Giardia.

Accidentally swallowing Giardia picked up from surfaces (such as bathroom fixtures, changing tables, diaper pails, or toys) contaminated with feces from an infected person.

What are the symptoms of giardiasis?

Giardia infection can cause a variety of intestinal symptoms, which include:
Diarrhea
Gas or flatulence
Greasy stools that tend to float
Stomach cramps
Upset stomach or nausea.

These symptoms may lead to weight loss and dehydration. Some people with giardiasis have no symptoms at all.

How long after infection do symptoms appear?

Symptoms of giardiasis normally begin 1 to 2 weeks (average 7 days) after becoming infected.

How long will symptoms last?

In otherwise healthy persons, symptoms of giardiasis may last 2 to 6 weeks. Occasionally, symptoms last longer.

Who is most likely to get giardiasis?

Anyone can get giardiasis. Persons more likely to become infected include
Children who attend day care centers, including diaper-aged children
Child care workers
Parents of infected children
International travelers
People who swallow water from contaminated sources
Backpackers, hikers, and campers who drink unfiltered, untreated water
Swimmers who swallow water while swimming in lakes, rivers, ponds, and streams
People who drink from shallow wells
Contaminated water includes water that has not been boiled, filtered, or disinfected with chemicals. Several community-wide outbreaks of giardiasis have been linked to drinking municipal water or recreational water contaminated with Giardia .

What should I do if I think I may have giardiasis?

See your health care provider.

How is a Giardia infection diagnosed?

Your health care provider will likely ask you to submit stool samples to check for the parasite. Because Giardia can be difficult to diagnose, your provider may ask you to submit several stool specimens over several days.

What is the treatment for giardiasis?

Several prescription drugs are available to treat Giardia . Although Giardia can infect all people, young children and pregnant women may be more susceptible to dehydration resulting from diarrhea and should, therefore, drink plenty of fluids while ill.

My child does not have diarrhea, but was recently diagnosed as having giardiasis. My health care provider says treatment is not necessary. Is this true?

Treatment is not necessary when the child has no symptoms. However, there are a few exceptions. If your child does not have diarrhea, but is having nausea, fatigue (very tired), weight loss, or a poor appetite, you and your health care provider may wish to consider treatment. If your child attends a day care center where an outbreak is continuing to occur despite efforts to control it, screening and treating children who have no obvious symptoms may be a good idea. The same is true if several family members are ill, or if a family member is pregnant and therefore not able to take the most effective anti- Giardia medications.

If I have been diagnosed with giardiasis, should I worry about spreading the infection to others?

Yes, a Giardia infection can be very contagious. Follow these guidelines to avoid spreading giardiasis to others:
Wash your hands with soap and water after using the toilet, changing diapers, and before eating or preparing food. Do not swim in recreational water (pools, hot tubs, lakes or rivers, the ocean, etc.) if you have Giardia and for at least 2 weeks after diarrhea stops. You can pass Giardia in your stool and contaminate water for several weeks after your symptoms have ended. This has resulted in outbreaks of Giardia among recreational water users. Avoid fecal exposure during sexual activity.

How can I prevent a Giardia infection?

Practice good hygiene.
Wash hands thoroughly with soap and water.
Wash hands after using the toilet and before handling or eating food (especially for persons with diarrhea).
Wash hands after every diaper change, especially if you work with diaper-aged children, even if you are wearing gloves.
Protect others by not swimming if you are experiencing diarrhea (essential for children in diapers).

Avoid water that might be contaminated.

For information on recreational water-related illnesses, visit CDC's Healthy Swimming website.

1. Do not swallow recreational water.
2. Do not drink untreated water from shallow wells, lakes, rivers, springs, ponds, and streams.
3. Do not drink untreated water during community-wide outbreaks of disease caused by contaminated drinking water.
4. Do not use untreated ice or drinking water when traveling in countries where the water supply might be unsafe.

In the United States , nationally distributed brands of bottled or canned carbonated soft drinks are safe to drink. Commercially packaged non-carbonated soft drinks and fruit juices that do not require refrigeration until after they are opened (those that are stored unrefrigerated on grocery shelves) also are safe.

If you are unable to avoid using or drinking water that might be contaminated, then you can make the water safe to drink by doing one of the following: Heat the water to a rolling boil for at least 1 minute.
OR
Use a filter that has an absolute pore size of at least 1 micron or one that has been NSF rated for "cyst removal."

If you cannot heat the water to a rolling boil or use a recommended filter, then try chemically treating the water by chlorination or iodination. Using chemicals may be less effective than boiling or filtering because the amount of chemical required to make the water safe is highly dependent on the temperature, pH, and cloudiness of the water.

Avoid food that might be contaminated.

Wash and/or peel all raw vegetables and fruits before eating.
Use safe, uncontaminated water to wash all food that is to be eaten raw.
Avoid eating uncooked foods when traveling in countries with minimal water treatment and sanitation systems.

If my water comes from a well, should I have my well water tested?

It depends. You should consider having your well water tested if you can answer “yes” to any of the following questions:

Are members of your family or others who use your well water becoming ill?
If yes, your well may be the source of infection.
Is your well located at the bottom of a hill or is it considered shallow?
If so, runoff from rain or flood water may be draining directly into your well causing contamination.
Is your well in a rural area where animals graze?
Well water can become contaminated with feces if animal waste seepage contaminates the ground
water. This can occur if your well has cracked casings, is poorly constructed, or is too shallow.

Tests used to specifically identify Giardia are often expensive, difficult, and usually require hundreds of gallons of water to be pumped through a filter. If you answered “yes” to the above questions, consider generally testing your well for fecal contamination by testing it for the presence of coliforms or E. coli instead of Giardia . Although tests for fecal coliforms or E. coli do not specifically tell you whether Giardia is present, these tests will show whether your well water has been contaminated by fecal matter.

These tests are only useful if your well is not routinely disinfected with chlorine, since chlorine kills fecal coliforms and E. coli . If the tests are positive, it is possible that the water may also be contaminated with Giardia or other harmful bacteria and viruses. Contact your county health department, your county cooperative extension service, or a local laboratory to find out who offers water testing in your area. If the fecal coliform test comes back positive, indicating that your well is fecally contaminated, stop drinking the well water and contact your local water authority for instructions on how to disinfect your well.

Note: The previous fact sheet is part of a larger publication adapted from U.S. CDC publication: CDC Division of Parasitic Diseases.

This fact sheet is for information only and is not meant to be used for self-diagnosis or as a substitute for consultation with a health care provider. If you have any questions about the disease described above or think that you may have a parasitic infection, consult a health care provider.