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.
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
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
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
Can Giardia be transmitted between animal
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
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
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
What are the health effects of Giardia
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
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
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
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
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
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:
Gas or flatulence
Greasy stools that tend to float
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
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.
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.