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http://www.cfsph.iastate.edu/Factsheets/pdfs/giardiasis.pdf
Giardiasis
Giardia Enteritis,
Lambliasis,
Beaver Fever
Last Updated: December 2012
Importance
Giardiasis, a gastrointestinal disease characterized by acute or chronic diarrhea, is
caused by protozoan parasites in the genus Giardia. Giardia duodenalis is the major
species found in mammals, and the only species known to cause illness in humans. This
organism is carried in the intestinal tract of many animals and people, with clinical signs
developing in some individuals, but many others remaining asymptomatic. In addition
to diarrhea, the presence of G. duodenalis can result in malabsorption; some studies
have implicated this organism in decreased growth in some infected children and
possibly decreased productivity in young livestock. Outbreaks are occasionally reported
in people, as the result of mass exposure to contaminated water or food, or direct contact
with infected individuals (e.g., in child care centers).
People are considered to be the most important reservoir hosts for human
giardiasis. The predominant genetic types of G. duodenalis usually differ in humans
and domesticated animals (livestock and pets), and zoonotic transmission is currently
thought to be of minor significance in causing human illness. Nevertheless, there is
evidence that certain isolates may sometimes be shared, and some genetic types of G.
duodenalis (assemblages A and B) should be considered potentially zoonotic.
Etiology
The protozoan genus Giardia (Family Giardiidae, order Giardiida) contains at
least six species that infect animals and/or humans. In most mammals, giardiasis is
caused by Giardia duodenalis, which is also called G. intestinalis. Both names are in
current use, although the validity of the name G. intestinalis depends on the
interpretation of the International Code of Zoological Nomenclature. Two older
names for the organism, Giardia lamblia and Lamblia intestinalis, are no longer
considered to be taxonomically valid. Nevertheless, the term G. lamblia can still be
found sometimes in the human clinical literature. Additional species in animals
include G. agilis in amphibians, G. ardeae and G. psittaci in birds, G. muris in
rodents and G. microti in muskrats and voles. G. varani, which infects reptiles, is also
thought to be a distinct species. Other species of Giardia probably also exist in
animals, including fish. None of these species, other than G. duodenalis, is known to
affect people.
G. duodenalis has been divided into at least 7 genetic assemblages, A through G,
which might be distinct enough to be considered species. Assemblages A and B have
broad host specificity. Almost all isolates from humans belong to these two
assemblages. They also occur in many species of animals. Assemblages C, D, E, F
and G appear to have narrow host ranges. Assemblages C and D are found mainly in
dogs, assemblage E in artiodactyls, assemblage F in cats and assemblage G in rodents.
Proposed species names for the assemblages are G. canis for assemblages C and D,
G. bovis for assemblage E, G. cati for assemblage F and G. simondi for assemblage
G. An additional proposal is to reserve the name G. duodenalis for assemblage A, and
rename assemblage B G. enterica. These names have not yet been accepted, and there
are some objections to their validity; however, they may be encountered in some
articles. Novel G. duodenalis genotypes include a proposed assemblage H, reported in
seals and a gull; another isolate from southern brown bandicoots (Isoodon obesulus)
in Western Australia, and two genotypes of G. duodenalis identified in house mice on
an island in Australia.
Subassemblages (also called subgroups) have been recognized within some
assemblages. Three subassemblages - AI, AII and AIII – have been defined, to date,
in assemblage A. Subassemblage AII is usually found in people, while subassemblage
AI mainly occurs in livestock and pets. However, this division is not absolute;
subassemblage AI has been isolated occasionally from people, and AII from animals.
Subassemblage AIII has been detected in hoofed wild animals. As of 2012, it has not
been found in humans. It is more difficult to define subassemblages in assemblage B,
which is genetically diverse. Two subassemblages, BIII and BIV, were described by
allozyme electrophoretic studies, but DNA sequence analyses do not support these 2
groups. Host-specific subassemblages have not yet been identified in assemblages C
through G.
© 2012
page 1 of 13
Giardiasis
Challenges in classifying G. duodenalis into
assemblages
Molecular tools (e.g., PCR) are used to classify G.
duodenalis into assemblages and subassemblages. Most
studies use tests based on one or more of 4 genetic loci:
SSU rRNA and the triosephosphate isomerase (tpi),
glutamate dehydrogenase (gdh) and β-giardin genes. Many
studies examine only a single locus, often SSU rRNA.
However, the use of a different gene, or even a different set
of PCR primers, can occasionally assign the same isolate to
a different assemblage.
Multilocus genotyping (MLG) studies use more than
one gene to classify the organism. Although this is more
accurate than using a single gene, some isolates still cannot
be unequivocally assigned to a single assemblage. One
possible explanation for this phenomenon is that the
individual is infected with two or more organisms that
belong to different assemblages. A second possibility is that
these samples contain recombinant Giardia organisms with
sequences from more than one assemblage type. Although
Giardia species mainly reproduce by asexual means, there
is recent evidence that recombination may also occur. PCR
of single Giardia cysts might be used to distinguish these
two possibilities in a sample; however, this technique is still
in its infancy.
As a result, it is possible for two studies to assign an
isolate to different assemblage types, only one of which
may be potentially zoonotic. Furthermore, the interpretation
of discrepant results in MLG studies can vary. While some
authors assume that the animal is infected with two or more
assemblage types (one of which may belong to a zoonotic
assemblage), others are more cautious in assigning such
isolates.
Zoonotic potential of G. duodenalis in animals
Humans are thought to be the main reservoir for G.
duodenalis infections in people. Whether infections in
animals are zoonotic, and to what extent, is still uncertain
and controversial. Cross-transmission studies can be
difficult to interpret, and studies examining whether contact
with animals is a risk factor for giardiasis have not provided
a definitive answer. Genetic analyses that compare G.
duodenalis isolates from people and animals may shed light
on this question. Such studies have found that people are
often, though not always, infected with different G.
duodenalis assemblages or subassemblages than livestock
and pets. For example, assemblages A and B, which cause
giardiasis in people, can be found in livestock, but
assemblage E is usually more common. A few MLG studies
have also been published. One reported that 2 of 6
multilocus assemblage A genotypes in G. duodenalis
databases were potentially zoonotic. One of these 2
genotypes contained a few human isolates, although most
were from animals. The other included one isolate from a
cat, while the rest were from humans. The remaining 4
genotypes seemed to be limited to either animals or people.
Last Updated: December 2012
© 2012
Another MLG study found that 3 of 7 assemblage A
genotypes had zoonotic potential. Only one study examined
assemblage B. It reported that approximately 4-7% of
assemblage B isolates were potentially zoonotic. Several
recent reviews conclude that zoonotic transmission from
domesticated animals is possible but unproven, and likely
to be of minor importance compared to person-to-person
transmission. However, they also stress that additional welldesigned epidemiological studies that use genetic tools,
especially MLG, are needed before the extent of zoonotic
giardiasis can be fully evaluated. Thus, G. duodenalis
assemblage A and assemblage B isolates in animals should
be considered to be potentially zoonotic at this time.
Likewise, assemblage A and B isolates in people may have
the ability to infect animals.
In contrast to pets and livestock, assemblage A (and
sometimes assemblage B) is common in nonhuman
primates, marsupials, marine mammals and terrestrial and
aquatic wildlife. The role of wild animals in maintaining
and transmitting G. duodenalis to people is uncertain.
Beavers have been implicated in zoonotic transmission,
based on epidemiological investigations of waterborne
outbreaks, and reports of giardiasis in hikers and campers.
However, this conclusion was based on circumstantial
evidence, such as the recovery of Giardia cysts from
beavers in areas with contaminated water. The possibility
that the water was initially contaminated by humans or
other animals could not be ruled out. It is possible that
people or domesticated animals transmit G. duodenalis to
wild animals, which may then maintain and/or amplify the
organism.
Geographic Distribution
G. duodenalis occurs worldwide, and is particularly
common in warm climates.
Transmission and Life Cycle
Giardia spp. have two stages, cysts and trophozoites.
The infection is acquired by ingesting cysts, which are
excreted in the feces. As few as 10-25 cysts are sufficient to
establish an infection in some humans. Some livestock may
be infected by as few as 1–10 cysts. Giardia cysts can be
transmitted directly between hosts, or on various fomites
including contaminated water and food. Animals can also
be re-infected from their fur when they groom.
Trophozoites are released from the ingested cysts in the
small intestine, where the trophozoites then multiply. Many
of the dividing trophozoites are carried toward the colon,
and encyst along the way, in response to bile salts and other
stimuli. Cysts can appear in the feces from 3 days to 3
weeks after infection, depending on the host species.
Excretion usually begins around the same time the first
symptoms (if any) appear. In both people and animals,
infections may last from a few days to several months, and
cyst shedding is usually intermittent. Shedding has been
reported to increase during the periparturient period in
page 2 of 13
Giardiasis
sheep, goats, pigs and cattle. Most sources consider Giardia
cysts to be immediately infectious when they are excreted
in the feces (although there is evidence that some cysts
might become infectious after a maturation period of up to
7 days).
Giardia cysts can survive for long periods in the
environment under cool, moist conditions. They are
susceptible to desiccation and direct sunlight, and are
destroyed more quickly under hot and dry conditions. In
various experiments, cyst were shown to survive in tap or
lake water for approximately two months at 0-8°C, in tap
water for 2 weeks at 20-28°C; and in lake water for 1 month
at 17-20°C. Cysts remained viable in river water for nearly
3 months at 0-4°C, and 1 month at 20-28°C, while they
survived in seawater for more than 2 months at 4°C. In soil
held at 4°C, almost 90% of cysts were still viable after 49
days; however, infectivity was lost within 7 days at 25°C.
Cysts also survived for one week in solid cattle manure at
4ºC, and for as long as 18 days in human feces.
Giardia trophozoites may also be found in the feces of
some animals or people with diarrhea. These trophozoites
survive only briefly in the environment, and are thought to
be of little or no epidemiological significance.
Disinfection
G. duodenalis cysts on surfaces are susceptible to 5%
sodium hypochlorite at a 1:30 dilution, as well as to some
other disinfectants including most quaternary ammonium
solutions. In laboratories, 6% H2O2 can also be used to
disinfect surfaces or decontaminate spills. Leaving
disinfectants on contaminated surfaces for 5-20 minutes
before rinsing helps ensure inactivation. Giardia cysts are
susceptible to ultraviolet (UV) light,, as well as to heat
(e.g., steam) and desiccation. Freezing reduces the number
of cysts, although some may survive.
In water, G. duodenalis cysts can be killed by a rolling
boil maintained for one minute, or by filtration through an
absolute pore size of at least one micron (e.g., a filter that
has been National Safety Foundation rated for cyst
removal). Giardia cysts are relatively resistant to
chlorination, particularly if the water is cold, and, the
amount of chlorine used routinely in drinking water is not
sufficient to kill G. duodenalis. Treatment conditions
reported to inactivate cysts in 5°C water include 4 mg/L
chlorine for 60 minutes, at pH 6-8; 8 mg/L chlorine for 10
minutes, at pH 6 or 7; and 8 mg/L chlorine for 30 minutes,
at pH 8. At 25°C, cysts in pH 6 water are inactivated after
10 minutes of exposure to 1.5 mg/L of chlorine. Likewise,
the effectiveness of iodination depends on the temperature,
pH and turbidity of the water, as well as contact time with
the chemical. Ozone and UV light can also inactivate cysts
in water.
Composting manure can reduce or eliminate Giardia
cysts. Cyst numbers and viability were also significantly
reduced in slurry waste from cattle held for 90 days.
Last Updated: December 2012
© 2012
Giardia trophozoites are killed much more readily than
cysts; treatments that inactivate the cysts, as well as milder
methods, are expected to be effective.
Infections in Humans
Incubation Period
The incubation period in humans is 1 to 45 days; in
most cases, the symptoms appear in 1-2 weeks.
Clinical Signs
Many human infections with G. duodenalis are
asymptomatic, with affected individuals either clearing the
infection spontaneously or shedding cysts subclinically.
Symptomatic cases are characterized by mild to severe
gastrointestinal signs. Some patients develop acute or
chronic diarrhea, with loose, foul-smelling stools. The feces
may have a greasy appearance, but blood is rarely seen.
Other gastrointestinal signs may also be present, either with
or without diarrhea. They can include abdominal cramps or
pain, bloating, flatulence, nausea, anorexia and vomiting.
Weight loss or dehydration can result, and fatigue may
accompany the illness. Low-grade fever may be seen
occasionally at the beginning of the illness, although some
sources report that this is uncommon. Anemia, weight loss
and anorexia have been reported as the most prominent
signs in some elderly individuals. Most infections are selflimited, and last for a few days to a few weeks; however,
cases may occasionally persist for months or even years. In
chronic giardiasis, episodes of diarrhea or loose stools can
occur continuously, intermittently, sporadically or
recurrently. The stools may be normal between bouts of
diarrhea. Constipation is also possible. Abdominal
discomfort can be continuous in chronic cases, persisting
even when diarrhea is absent. Chronic infections may lead
to malabsorption syndromes that can include disaccharidase
deficiency and vitamin deficiencies, as well as severe
weight loss and debilitation. Chronic infections occur in
both immunodeficient and immunocompetent individuals.
Clinical giardiasis is reported to be more frequent in
patients with immunoglobulin deficiency states, and most
of these patients have chronic diarrhea. This population
includes
people
with
congenital
(genetic)
hypogammaglobulinemia, as well as those with acquired
conditions associated with protein-calorie malnutrition,
lymphoma and other syndromes. The symptoms in HIVinfected individuals appear to be similar to those
immunocompetent persons, and G. duodenalis infections
are often asymptomatic. However, as CD4+ lymphocyte
counts decrease and immunosuppression increases, clinical
signs are more likely to occur.
Disaccharide intolerance, mainly in the form of lactose
deficiency, is the most common complication of giardiasis.
It can last for several weeks after the organism is cleared
either spontaneously or by treatment. The symptoms
resemble those caused by the organism, and may include
page 3 of 13
Giardiasis
abdominal cramps, bloating and diarrhea. Deficiencies of
fat-soluble vitamins and vitamin B12 are also possible, but
less common. A few studies have suggested that G.
duodenalis infections might cause growth retardation in
some populations (e.g., in Bedouin children living in the
desert). In contrast, there were no apparent detrimental
effects, including gastrointestinal signs or decreased
growth, in healthy, well-nourished children in day care
facilities. A few studies have suggested that G. duodenalis
infections might be correlated with an increased incidence
of irritable bowel syndrome or other syndromes.
Occasional case reports have also attributed extraintestinal
signs, including urticaria, pruritus, bronchospasm, reactive
arthritis and other allergic signs, to giardiasis. Most sources
consider extraintestinal signs to be rare, although one
recent survey found that self-reported rashes and ocular,
joint and urinary signs were relatively common in
symptomatic patients.
Giardiasis is rarely fatal; however, deaths can be
caused by extreme dehydration, mainly in infants or
malnourished children.
Communicability
G. duodenalis can be transmitted from person to person
by fecal contamination. Both symptomatic and
asymptomatic individuals can excrete cysts. Cysts are shed
in the feces, often intermittently, during the entire period of
infection. G. duodenalis from people might be able to infect
animals.
Diagnostic Tests
Giardiasis can be diagnosed by direct observation of
the trophozoites or cysts in the feces. Either stained
preparations (e.g., preserved with polyvinyl alcohol or 10%
formalin) or unstained wet mounts can be used. Because
they are small and can resemble other fecal components,
Giardia cysts and trophozoites can sometimes be difficult
to identify by morphology alone.
Direct smears or fecal wet mounts can be used to look
for trophozoites. This stage usually observed only in fresh,
watery stools. The flagellated trophozoite is 9-21 μm long
by 5-15 μm wide, and has a “tear drop” shape, with two
nuclei at the anterior end and tumbling motility. Cysts can
be found in formed as well as unformed stools. G.
duodenalis cysts are approximately 8-15 µm long and 7-10
μm wide, and oval, with four nuclei. Various flotation or
sedimentation processes can be used to concentrate the
cysts. Repeated sampling may be necessary when there are
few organisms. Because shedding occurs intermittently;
cysts are more likely to be found if specimens from 3
different days are examined.
If chronic giardiasis is suspected, but repeated stool
examinations are negative, the intestinal contents can be
examined directly for trophozoites. One technique is the
“string test” (Entero-test), in which the patient swallows a
gelatin capsule on a string, and the string is later retrieved
Last Updated: December 2012
© 2012
and examined for trophozoites. Aspiration of duodenal
contents has also been used.
Infections can also be diagnosed by enzyme-linked
immunosorbent
assays
(ELISAs)
and
immunochromatographic tests to detect G. duodenalis
antigens in the feces, as well as by directimmunofluorescence. Antigen shedding may persist for
weeks after elimination of the parasite. Rapid tests such as
ELISAs should supplement but not replace routine ova and
parasite examination by microscopy, as the latter test can
also diagnose other diseases. PCR assays can detect
Giardia in clinical samples. Genetic characterization of
isolates at the assemblage level is usually employed only in
epidemiological studies and research.
Serology has been used in epidemiologic investigations
and other research. Giardia can be cultured in vitro, but this
technique is used only in research.
Treatment
Giardiasis can be treated with a number of drugs, such
as nitroimidazole derivatives, benzimidazole compounds or
acridine dyes. Metronidazole or tinidazole are used most
often in humans, but other drugs (e.g., furazolidone or
paromomycin) may be recommended in some cases. Some
drugs are either not available or not recommended in some
countries. Supportive care, such as fluid and electrolyte
management, may also be necessary. Symptoms can recur
for a variety of reasons, such as drug resistant organisms,
reinfection or post-Giardia lactose intolerance. In some
cases, a lactose-free diet may be needed for several months.
Asymptomatic carriers do not usually need treatment,
but they may be treated to reduce transmission of the
organism. Whether or not treatment is recommended can
vary with the situation and risk of reinfection.
Prevention
Drinking water treatment plants reduce the number of
Giardia using conventional water treatment processes (e.g.,
filtration), followed by chemical or physical disinfection.
Some countries have established regulations for the level of
Giardia cyst removal in municipal drinking water (e.g., at
least 3-log cyst removal or inactivation is required in the
U.S.). Chlorination is commonly employed for the
disinfection stage, but ozone is used in some plants. Other
alternatives being studied include chlorine dioxide and UV
irradiation. The use of chemical disinfectants, including
chlorine and ozone, is limited by the production of toxic byproducts from reactions with compounds in the water.
Because G. duodenalis is widespread in the
environment, untreated water from lakes, rivers, springs or
shallow wells should not be drunk. In countries where the
municipal water supply may not be safe (or during an
outbreak involving Giardia-contaminated drinking water),
untreated drinking water or ice should also be avoided.
Methods that can be used to treat potentially contaminated
water include heating the water to a rolling boil for at least
page 4 of 13
Giardiasis
one minute, or filtering the water through a filter that has an
absolute pore size of at least one micron (or has been
National Safety Foundation rated for cyst removal, e.g.,
NSF Standard 53 or NSF Standard 58). Chlorination or
iodination may also destroy the cysts, but these methods are
less reliable and depend on the temperature, pH and
turbidity of the water, as well as contact time with the
chemical.
Good hygiene, such as hand washing, reduces the risk
of acquiring G. duodenalis, or transmitting it to others.
Prevention efforts should be especially aimed at preventing
contact between feces (human or animal) or contaminated
items (including soil, untreated water and fomites) and
food, potable water or other items that may be ingested or
contact the mouth. Fecal contact with skin or mucous
membranes should be avoided during sexual activity.
People with giardiasis should not swim in recreational
water for at least two weeks after the symptoms end.
Both soil and irrigation water can contain Giardia, and
cysts have been detected on crops. Vegetables and fruits
should be washed before eating them. In higher risk
situations, they should also be peeled if they will be eaten
raw, as washing may not remove all organisms. G.
duodenalis has been detected in filter-feeding aquatic
invertebrates such as clams, mussels, oysters and cockles,
which appear to concentrate the organism from the water as
they feed; however, the epidemiological significance of this
source is still uncertain. In addition, Giardia cysts have
been found on raw retail meat.
Routine screening of healthy pets is not recommended,
as infections are common and the organism is difficult to
eliminate permanently. The risk of transmitting the
infection to people is also thought to be low. Diagnostic
testing is recommended only for pets with clinical signs or
when there is a specific zoonotic concern. Environmental
modifications, such as restricting access to streams and
other surface waters, and providing alternative sources of
drinking water can decrease fecal contamination of
watersheds by livestock. Composting and other measures
can decrease or eliminate Giardia cysts in livestock
manure.
Morbidity and Mortality
Infections with G. duodenalis are very common.
Individuals at increased risk include those who drink
contaminated drinking water or swallow surface water
during recreational activities, children (especially in day
care facilities), travelers to regions where human infections
are prevalent, men who have sex with men, and contacts of
infected individuals. Giardiasis can occur sporadically in
individuals, as well as in outbreaks. Many outbreaks have
been linked to contaminated water, including unsafe
drinking water, recreational water such as ponds, and
occasionally other types of water (e.g., contaminated
shower water at a camp). Municipal drinking water is
occasionally involved, even in developed countries, from
Last Updated: December 2012
© 2012
deficiencies in water treatment process such as insufficient
barriers, and inadequate or poorly operated treatment.
Food-related outbreaks have been reported less frequently,
and often involve contamination by a food handler. Personto-person transmission can propagate outbreaks, especially
among young children.
Factors such as the level of hygiene, and the
availability of clean water and toilets, influence the general
risk of infection in an area. Studies from developed
countries (mainly conducted in asymptomatic children)
have reported overall infection rates of <1% to 6% in
various European countries and the U.S., 1% to 7% in
Saudi Arabia, 2.5% in South Korea, 2-8% in Australia and
8% in New Zealand. Localized high infection rates have,
however, been reported in some groups in these locations,
including people living under unhygienic conditions.
Infection rates reported in various groups of
immunocompromised patients varied from 3% to 14%.
Infections are more prevalent in developing countries.
In surveys mainly of children, the infection rate was
typically 8-30% in Africa, Asia, South America, Cuba,
Mexico and Nicaragua, with a few studies reporting higher
or lower rates. Among adults, the prevalence was reported
to be 12% in Morocco; 25% among pregnant women in
Minatitlan, Mexico; and 5-14% among African refugees
and new immigrants to the U.S., the Netherlands and Spain.
Depending on the population, the level of immunity
and other factors, approximately 20-40% of G. duodenalis
infections are estimated to become symptomatic. Although
most cases are probably not detected, the incidence of
clinical giardiasis (reported cases) was 5.5 to 70 cases per
100,000 population in New Zealand and various developed
countries in North America and Europe.
Symptomatic cases in healthy people usually resolve
spontaneously within a few weeks. Some studies have
reported that assemblage A is more virulent than
assemblage B, while other studies found the opposite. It is
possible that the virulence of the assemblage depends on
the organisms already circulating in the population, and the
level of immunity to those organisms. Chronic infections
have been reported to occur in less than 4% of patients.
Infections in Animals
Species Affected
Several species of Giardia infect mammals,
marsupials, birds, reptiles, amphibians and fish.
Giardia duodenalis in mammals and marsupials
G. duodenalis infections have been reported in many
species of domesticated mammals, including cattle, water
buffalo, sheep, goats, South American camelids, pigs,
horses and other livestock; dogs, cats and ferrets; and small
mammals such as chinchillas (Chinchilla lanigera), guinea
pigs, rats and rabbits. This organism also occurs in wildlife
and captive wild animals including terrestrial mammals,
page 5 of 13
Giardiasis
freshwater aquatic mammals (e.g., beavers and muskrats),
marine mammals (seals, whales, dolphins and porpoises)
and a wide variety of marsupials.
Many published studies describe G. duodenalis
assemblage types found in animals. Given the difficulties in
assigning some isolates to an assemblage, it is possible that
some organisms (especially those found rarely) were
misidentified.
G. duodenalis assemblage A has been reported in
domesticated livestock including cattle, water buffalo,
farmed bison (Bison bison), yaks, sheep, goats, pigs, horses
and alpacas; and in companion animals including dogs,
cats, pet ferrets and chinchillas. In these species, it is
usually (though not always) less common than speciesspecific assemblage types (e.g., assemblage E in livestock,
C and D in dogs, or F in cats). In contrast, assemblage A is
the predominant isolate, together with assemblage B, in
wild and captive nonhuman primates. It was also the most
prevalent genotype in a study of captive and wild
Australian marsupials. Assemblage A seems to be common
in wild mammals, where it has been documented in a wide
variety of species such as beaver, cervids, muskoxen
(Ovibos moschatus) and marine mammals.
G. duodenalis assemblage B is the predominant
isolate, together with assemblage A, in wild and captive
nonhuman primates. It can also infect many other species
such as cattle, sheep, pigs, horses, dogs, cats, rabbits,
guinea pigs, chinchillas, wild and captive mammals and
marsupials. However, this assemblage type seems to be
uncommon in most mammals. In many studies, it was
detected in only 0-2% of the population. On rare occasions,
assemblage B is reported to be the most common genotype
in a population. For example, one study reported that 92%
of the isolates from pooled manure samples on 10
Canadian pig farms belonged to this assemblage. Some
studies have reported that assemblage B might occur
frequently in beaver, muskrats and seals.
G. duodenalis assemblages C and D are mainly found in
dogs and other canids including foxes, wild dogs/dingos,
coyotes and wolves, and seem to be adapted to these species.
One study reported that assemblages C and D were rare in
captive and wild African painted dogs (Lycaon pictus), and
most isolates belonged to assemblages A and B. Assemblages
C and D have also been detected in seals and kangaroos. Rare
infections were documented in other domesticated and wild
species, including a goat and a few cats.
G. duodenalis assemblage E is usually the predominant
type in domesticated, cloven-hoofed species. It has been
reported from cattle, yaks, water buffalo, farmed bison,
sheep, goats, alpacas and pigs. Assemblage E also occurs in
horses. Surprisingly, most studies report that this assemblage
type is rare in wild ungulates, although one study from
Australia found that it was present in a significant number of
the Giardia-positive samples from wild deer. Infections have
been reported occasionally in other species including wild
Last Updated: December 2012
© 2012
dogs/dingos and foxes in Australia, as well as a few cats,
chinchillas and a wild mouse.
G. duodenalis assemblage F has been detected mainly
in cats, and appears to be adapted to this species. It has been
reported rarely in a few other species including dogs, pigs
and rodents.
G. duodenalis assemblage G is seen mainly in rodents
including rats.
G. duodenalis assemblage H has been reported from
seals, as well as a gull from the same ecosystem.
Other species of Giardia in mammals
G. muris has been found in rodents, including mice and
rats. G. microti occurs in voles and muskrats. Possible
infections with this organism were reported in mice, a dog,
a captive cheetah (Acinonyx jubatus) and a captive leopard
(Panthera pardus japonensis).
Birds
At least two species of Giardia seem to be adapted to
birds. G. psittaci occurs in psittacine birds. G. ardeae has
been reported from various species of herons and egrets, and
a white stork (Ciconia ciconia). A closely related organism,
which may be a strain of G. ardae, was detected in a strawnecked ibis (Threskiornis spinicollis). G. duodenalis is also
reported occasionally in birds, but it is still uncertain whether
these birds are infected or acting as paratenic (transport)
hosts. In a study of pet birds and zoo birds from 14 orders
and 63 species, G. duodenalis assemblage A was found only
in members of the Psittaciformes. Other studies detected G.
duodenalis assemblages A and B in wild gulls (Larus spp.).
There are very few studies of Giardia in domesticated
poultry. One found G. duodenalis in one of 11 domesticated
geese (Anser anser f. domestica), and another reported G.
duodenalis assemblage B in domesticated ostriches (Struthio
camelus). The classification of Giardia spp. found in some
orders of birds (e.g., Charadriiformes, Passeriformes and
Piciformes) is still uncertain.
Reptiles and Amphibians
G. agilis occurs in amphibians. A proposed new
species, G. varani, was reported in reptiles.
Fish
Fish are probably infected with unique species of
Giardia, but G. duodenalis has also been detected. In an
Australian survey, G. duodenalis assemblages A, B and E
were found in cultured fingerlings (barramundi, black
bream, mulloway and snapper) at hatcheries, and in wild
mullet, as well as in a single minnow from a freshwater
environment. Sequences resembling G. microti were
reported in one cultured barramundi fingerling. The
intestines of many fish contained large numbers of Giardia
trophozoites and cysts, suggesting that they might have
been infected rather than acting as transport hosts. In
another study, assemblage A was documented in a mako
shark (Isurus paucus).
page 6 of 13
Giardiasis
Incubation Period
Communicability
In some experimentally infected animals, clinical signs
were reported to occur around the time cyst excretion
begins. The prepatent period is 5 to 16 days in dogs and
cats. It was reported to be 6 to 21 days in early studies in
ruminants, probably due to the method of detection, but
more recent research suggests 3 to 10 days.
G. duodenalis cysts shed in the feces are infectious.
Both symptomatic and asymptomatically infected animals
excrete cysts, for days to months. The duration of shedding
was found to be at least 100 days in some dairy calves, up
to 25 weeks in beef calves and more than 10 weeks in
lambs and goats.
Clinical Signs
Diagnostic Tests
Most infections are asymptomatic, particularly in adult
animals. Acute, chronic or intermittent diarrhea or soft
stools may be seen in some dogs and cats. The stools are
typically light-colored and mucoid. They are often
malodorous, and may contain undigested fat, but blood is
rare. Vomiting occurs occasionally, but fever is not usually
present. Infected animals may also have an unthrifty
appearance and occasionally lose weight or fail to gain
weight. The diarrhea is self-limited in most
immunocompetent dogs and cats.
Giardia is a potential cause of diarrhea and decreased
productivity in livestock, especially calves and other young
ruminants, but it is still uncertain whether this occurs to any
significant extent. Most studies have been done in the field,
and are complicated by the possibility of concurrent
infections or infestations with other organisms, rapid
reinfection with Giardia after elimination of the organism,
and other factors. Two studies in experimentally infected
ruminants suggest that some infections can become
symptomatic. Experimentally infected specific pathogen–free
(SPF) lambs developed diarrhea, took longer to reach
slaughter weight and had lower carcass weight compared to
uninfected lambs. Diarrhea, decreased appetite and
depression were reported in experimentally infected goats.
Diarrhea that does not respond to antibiotic or coccidiostatic
treatment, especially in young ruminants, might raise a
suspicion of giardiasis. Pasty to fluid feces with a mucoid
appearance would be expected. Alone, Giardia does not
seem to cause severe or watery diarrhea in ruminants.
Clinical giardiasis appears to be uncommon in horses.
In one recent study, the shedding of Giardia was correlated
with mild diarrhea in young foals.
Giardia spp. infections may also cause diarrhea in
some birds. Forms of diarrhea described by various sources
include voluminous, aerated “popcorn” feces, soft green
stools and mucoid, malodorous diarrhea. Dry, flaky skin,
which may progress to pruritus, feather pulling, (especially
from the axillae and inner thigh), and alopecia have been
attributed to giardiasis in pet birds. Weight loss, depression,
decreased feed and water consumption, dehydration and
deaths have been documented in some case reports. Giardia
has not been proven to cause some of these clinical signs in
birds, and some cases may have been exacerbated by coinfections.
The diagnosis of giardiasis is based on the detection of
Giardia spp., in conjunction with the clinical presentation
and exclusion of other causes. Cysts can also be present in
asymptomatic animals, or as an incidental finding in
animals with other diseases.
Giardia spp. can be found in feces by direct
microscopic examination, using either stained preparations
(e.g., Lugol’s solution) or unstained wet mounts. An
advantage of this method is that it can also detect other
parasites that may cause diarrhea. Because they are small
and can resemble other fecal components, Giardia cysts
and trophozoites can sometimes be difficult to identify.
Direct smears or fecal wet mounts can be used to look
for trophozoites. Samples should be taken from the surface
of the feces, where organisms are more common. The
flagellated trophozoite, 9-21 μm long by 5-15 μm wide,
has a “tear drop” shape, with two nuclei at the anterior end
and tumbling motility. Trophozoites are usually detected
only in very fresh samples from animals with diarrhea.
Living organisms will probably not be found after several
hours or in refrigerated samples.
More often, diagnosis is based on detection of the
cysts, which are present in formed stools as well as
diarrheic feces. Giardia cysts are oval, approximately 8-15
µm long and 7-10 μm wide, with 4 nuclei. Cysts can be
concentrated by passive fecal flotation or centrifugal fecal
flotation. Zinc sulfate preserves the morphology better than
sugar solutions, which can distort the cyst. Shedding can be
intermittent; 3 samples from different days are usually
recommended to rule out giardiasis with a high probability.
In livestock, multiple samples can also be taken from
several animals within the same housing facility. Young
animals should be sampled, if possible, as they are most
likely to excrete cysts.
Direct immunofluorescence, available in veterinary
diagnostic laboratories, can be used to visualize the
organism. Some researchers consider this test to be the gold
standard for the diagnosis of giardiasis in dogs and cats.
Morphology and fluorescence can both be examined,
reducing the risk of false positives.
Giardiasis can also be diagnosed by detecting antigens
in the feces, using various ELISA or rapid solid-phase
qualitative immunochromatography assays. These assays
have not been validated for all species. Most of the tests are
performed by diagnostic laboratories, but at least one inhouse ELISA test kit is available in some countries.
Last Updated: December 2012
© 2012
page 7 of 13
Giardiasis
Antigen shedding may persist for weeks after elimination of
the parasite.
PCR assays are performed in some university research
and service laboratories, but they are not widely available in
commercial diagnostic laboratories. PCR is used mainly to
identify G. duodenalis assemblage types. Culture of
Giardia spp. is employed only in research.
There is continuing controversy about the relative
merits of the various testing methods, but experts generally
agree that a combination of multiple tests on multiple
samples is optimal. The Companion Animal Parasite
Council currently recommends that, if a direct smear and
fecal centrifugal flotation do not detect Giardia or are not
definitive in symptomatic dogs and cats, a Giardia antigen
assay should be added. Diagnostic testing is recommended
only for pets with clinical signs or when there is a specific
zoonotic concern. Routine screening of healthy pets for this
organism is not recommended.
Treatment
Pets
In dogs and cats, treatment is usually recommended
only to end the clinical signs if the animal is symptomatic.
The infection may or may not be eliminated. Commonly
used drugs include metronidazole, fenbendazole and a
combination product that contains praziquantel, pyrantel
and febantel. The latter two products are registered in some
countries for use against G. duodenalis in dogs, but there is
currently no FDA-approved drug for treating animal
giardiasis in the U.S. Other drugs have also been used;
however, there is limited information about some drugs,
and others are no longer recommended for use in dogs and
cats, due to the potential for serious side effects (e.g.,
myelosuppression has been reported with albendazole).
Some infections are self-limiting.
Treated animals are often reinfected from the
environment (where cysts are widespread) or other animals,
or even from the animal’s own fur. Bathing the animal is
often recommended on the last day of treatment. It may be
helpful to dry the coat with warm air (especially in the
perineal region) after the bath. Cleaning and disinfection, as
well as a dry environment, can help eliminate cysts from
some fomites.
Treatment for subclinically infected pets is
controversial. Healthy cats and dogs with normal stools are
not considered to present significant health risks for
humans. In addition, the infection may persist despite
treatment, and animals are readily reinfected from the
environment. There are also concerns about side effects
from the drugs used to treat giardiasis, For these reasons,
most sources do not currently advise routine treatment if the
animal is not symptomatic. Treatment may, however, be
considered in individual circumstances (e.g., a request from
an owner who is immunocompromised). It can be
particularly challenging to eradicate Giardia from an
Last Updated: December 2012
© 2012
animal population where it has become established. In this
situation, all infected animals in the household or facility
must be treated concurrently, and environmental sources of
contamination addressed. Disposable litterboxes can be
used while cats are being treated, and new litterboxes and
scoops purchased once G. duodenalis is no longer present.
Livestock
Whether treatment is required or advisable in livestock
is unknown. No drug is currently licensed to treat giardiasis
in these animals, and while there are some studies in calves,
there is little or no information on treatment efficacy in
sheep, goats and pigs. One review considered that
benzimidazole compounds (e.g., fenbendazole) or
paromomycin were currently the most suitable drugs to
treat giardiasis in calves and other livestock. Metronidazole
and dimetridazole have also been used in calves; however,
the nitroimidazoles are no longer approved for use in
livestock in some countries. Concurrent cleaning and
disinfection of the environment is expected to increase the
effectiveness of treatment by reducing the parasite burden.
Most treated calves begin shedding cysts again within 2–3
weeks, probably after reinfection from the environment.
Prevention
Prevention is considered to be impractical in livestock,
and difficult in most species, because the organisms are so
prevalent in the environment. Some measures may,
however, reduce exposure or decrease the parasite burden,
which might also decrease the potential for clinical signs.
Keeping pets indoors can reduce exposure to Giardia
sources such as soil, unsafe water and feces from other
animals and wildlife. Water treatments similar to those used
for humans (e.g., boiling or filtering) can destroy the
organism in unsafe water supplies. Indoor housing is
expected to favor transmission of G. duodenalis between
livestock, but housing calves individually and avoidance of
crowding are expected to reduce transmission. Rodent
control and decreased contact with wild birds might lower
the risk of infections in captive birds and other species
Regular cleaning, prompt removal of feces, and
frequent changes of bedding materials (where applicable)
can limit contamination of animal environments. Hard
surfaces can be disinfected or steam cleaned after cleansing,
and should be left to dry, as the cysts are susceptible to
desiccation. Livestock facilities should be cleaned and dried
between introductions of animals. In birds, the use of wirefloors in cages can reduce access to feces. Raised food and
water containers in bird cages are less likely to become
contaminated by droppings. Ensuring that newborn
mammals receive adequate colostrum can help build
resistance to disease.
Giardia vaccines were available for dogs and cats at
one time. These vaccines were not proven to prevent
infections, and were categorized as “generally not
recommended” in the 2006 American Animal Hospital
page 8 of 13
Giardiasis
Association vaccine guidelines and the American
Association of Feline Practitioners vaccine guidelines. They
have now been discontinued by the manufacturers.
Vaccines are not available for other species.
Widespread screening of healthy pets is not
recommended, as infections are common, the organism is
difficult to eliminate permanently, and the risk of
transmitting the infection to people is thought to be low.
Morbidity and Mortality
Many Giardia spp. infections in animals are
asymptomatic. Symptomatic infections are not usually life
threatening, and are often self-limiting.
Reported prevalence rates for G. duodenalis vary
widely, depending on the composition of the study
population, location and other factors. Although all ages
can be infected, both infection and disease occur more often
in young animals. Crowding increases the risk of
transmission, while frequent cleaning and disinfection can
decrease exposure. The sensitivity of the test used for
surveillance is also a factor: lower rates are generally
reported by surveys that use fecal flotation/ microscopy
compared to those using ELISAs.
Dogs and cats
In dogs and cats, animals under the age of 6 months are
reported to have the highest infection rates. One study
reported that the prevalence was also relatively high in cats
between the ages of 6 months and a year. Animals in
shelters, breeding facilities, kennels and catteries are more
likely to carry G. duodenalis, and one study found an
increased prevalence among dogs that visit dog parks.
Some studies found that this organism was more likely to
be present in dogs and cats with diarrhea, while others
detected no significant difference in prevalence between
animals with diarrheic and normal feces.
A large multi-country European study, performed
mainly in dogs with gastrointestinal signs, found that the
overall prevalence by antigen-capture ELISA was 25% in
all dogs, and 43% in puppies under the age of 6 months. A
nationwide U.S. survey of pet dogs, using microscopy,
reported that the prevalence was 4% overall, 13% in
puppies less than 6 months of age, and less than 1% in dogs
more than 3 years of age. Other studies of various healthy
and/or symptomatic canine populations reported infection
rates of 1% to 36% in Europe, <1% to 16% in the U.S.
and Canada, 1% to 37% in South and Central America,
<1% to 57% in Asia, and 9% in Australia. Foci with
especially high prevalence (e.g., 61-64%) have been
reported among dogs in some communities where dogs are
allowed to roam and hygiene standards are very low.
A multi-country European study, mainly of cats with
gastrointestinal signs, reported a prevalence of 20% by
antigen-capture ELISA, while a small number of
additional studies from Europe found infection rates of
1% to 37% in various feline populations. Infection rates
Last Updated: December 2012
© 2012
reported from other countries were <1% to 44% in the
U.S., <1% to 4% in Canada, 6% in Brazil, 19% in Chile,
6.5% in Colombia, 0% to 40% in Japan, 1% in Iran and
2% in Australia.
Ferrets and small mammals
There is limited information about Giardia spp. in
ferrets and small mammals such as rabbits and guinea pigs
or other pet rodents. One diagnostic laboratory in Germany
found that Giardia was present in 13% of fecal samples
submitted from ferrets in 2009-2010, compared to 3% in
2002-2004. In another study, 36% of the chinchillas in a
Brazilian breeding facility were shedding Giardia spp.
cysts.
Birds
A limited number of surveys found Giardia spp. in 2%
to 16% of pet and zoo birds.
Livestock
Feed and management practices affect the prevalence
of G. duodenalis in livestock. Transmission is expected to
be higher in intensive management systems, where young
animals are housed together in close contact.
In cattle, published infection rates range from 2% to
57% in various European countries, 9% to 73% in Canada
and the U.S., 4% to 50% in Asia (Taiwan, Vietnam and
Malaysia), 14% to 58% in Australia, 5% to 49% in New
Zealand and 8% to 10% in Uganda. The prevalence is
highest in calves under the age of 6 months (however,
calves under a month of age are not usually infected).
Reported infection rates in sheep were 1-42% in Europe,
25-56% in North America and 9-44% in Australia. In
goats, the prevalence was 4-53% in studies from Belgium,
Spain, Brazil and Uganda. One Italian study found that
26% of water buffalo were infected. In the published
surveys, G. duodenalis was found on 10% to 100% of the
studied farms. When Giardia has been diagnosed on a farm,
nearly all animals are expected to be infected at some point
in their lives; the cumulative incidence is reported to be
100% in cattle and goats, and nearly 100% in sheep.
Several studies of pigs in Australia, Asia, Europe and
North America reported infection rates of < 1% to 31%. In
Denmark, G. duodenalis was found in 38% of weaners,
3% of piglets and 4% of sows.
Horses
In studies from Italy, Germany, the Czech Republic,
Brazil, the U.S. and Canada, the prevalence in horses
varied from less than 1% to 37%. Foals were infected
more often than adult horses.
Zoo animals
Giardia spp. were detected in 29% of asymptomatic
mammals at the zoo of Zagreb, Croatia. The prevalence was
50% among the Artiodactyla, 57% in Carnivora, 40%
among primates, and 60% in Rodentia. At the Antwerp Zoo
page 9 of 13
Giardiasis
in Belgium, infections were found in 10% of captive wild
ruminants under the age of 6 months. Giardia spp. were
also detected in 13% of captive marsupials in Australia.
Fish
There is very limited information on Giardia spp. in
fish. In Australia, a PCR-based survey detected Giardia
spp. in approximately 8% of the cultured fingerlings
(barramundi, black bream, mulloway and snapper) from
hatcheries; 3% of wild marine fish (all infected fish were
mullet); and <1% of wild freshwater fish (a single minnow).
Post Mortem Lesions
Click to view images
Gross lesions are not usually found, other than
evidence of dehydration (especially in birds). Microscopic
lesions consist of villous atrophy and cuboidal enterocytes.
Internet Resources
Centers for Disease Control and Prevention (CDC)
http://www.cdc.gov/parasites/giardia/
Companion Animal Parasite Council
http://www.capcvet.org
Public Health Agency of Canada. Pathogen Safety Data
Sheets
http://www.phac-aspc.gc.ca/lab-bio/res/psds-ftss/indexeng.php
The Merck Manual
http://www.merckmanuals.com/professional/index.html
The Merck Veterinary Manual
http://www.merckvetmanual.com/mvm/index.jsp?cfile
=htm/bi/mvm_bkindex.htm
U.S. FDA Foodborne Pathogenic Microorganisms and
Natural Toxins Handbook (Bad Bug Book)
pdf.usaid.gov/pdf_docs/PNADO152.pdf
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