United States
Environmental Protection Agency
Office of Water
4304
September 2000
GIARDIA: DRINKING WATER FACT SHEET
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.
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 brush-
tail 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.
Where has Giardia been found?
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
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fluctuate due to storms, agricultural practices,
and the operation of wastewater facilities.
Giardia has been detected on stainless
steel and FormicaŽ 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 muskratis 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,
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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 non-
progressive 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 day-
care centers in the United States maybe
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.
Giardiasis can also be transmitted by some
sexual activities, particularly among male
homosexuals who practice oral-anal sex.
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
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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 by-
products 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 groundwaterthat 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
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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 non-
public 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 waterbome
enteric viruses and bacteria. Independent
testing groups, likeNSF International,
evaluate the effectiveness of water treatment
devices. Heating water to at least 70° C for 10
minutes or boiling water for one minute at sea
level (three minutes at high altitudes) is also
acceptable.
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