Giardialamblia, often referred to
simply as “Giardia”, the causative agent of giardiasis,
is a flagellated protozoan that was originally observed
by Van Leeuwenhoek in 1681. The genus name of this parasite
was named after French biologist Alfred Giard.
Giardiahas an interesting morphology. Giardia
exists in two forms, the trophozoite and the cyst. Trophozoites
are motile due to their four pairs of flagella. This form
is dorsoventrally flattened, piriform and has a unique internal
structure. Giardia has a large adhesive disk that comprises
the majority of the protozoan’s ventral surface. Through
the use of light microscopy, a pair of recurrent flagella
that run longitudinally within the organism can be seen.
These recurrent flagella are called axonemes. There are
two nuclei, one on each side of the axonemes. The trophozoites
measure 9-21 μm long x 5-15 μm wide x 2-4 μm
thick. This form may be found attached to the epithelium
of the duodenum and jejunum within an infected host.
The cyst form of Giardia is nonmotile. These
are oval and have a thick, refractile wall. Two nuclei
are in the recently formed cysts with four nuclei in the
mature cysts. The cysts measure 8-12 μm long x 7-10
μm wide. This form is the infective form and may be
found in the feces of infected animals.
Giardia has the ability to infect many mammals
including the dog, cat, deer mouse, ground squirrel, chinchilla,
swine, pocket mouse, ox, guinea pig, and humans.
Transmission is by the fecal-oral route. Both humans
and animals may become infected either by direct fecal ingestion
or by the ingestion of contaminated water. Freshly passed
cysts are immediately infective. The ingestion of a mere
ten or fewer Giardia cysts is enough to cause infection.
Giardia has a direct life cycle. Once the cyst
stage is ingested by a suitable host, excystation occurs
within the duodenum. It is believed that excystation occurs
as a result of exposure to the low gastric pH in addition
to contact with pancreatic enzymes such as chymotrypsin
and trypsin. During excystation, two binuclear trophozoites
arise from each quadrinuclear cyst. The trophozoite form
uses its large adhesive disk located on its ventral surface
to attach to the epithelium of the duodenum and jejunum.
The trophozoites reproduce asexually by binary fission.
Some of these trophozoitesencyst within the small intestine
and pass out in the feces. Many theories have been proposed,
but the exact mechanism by which Giardia causes diarrhea
has not been established.
There are no pathognomonic clinical signs associated
with giardiasis. The most common sign is chronic or intermittent
foul-smelling bowel diarrhea. Diarrhea is usually lightly
colored, greasy and mixed with mucus. Diarrhea is not usually
watery and does not generally contain blood. Other common
signs of giardiasis in dogs and cats include flatulence,
weight loss, listlessness, malaise and growth retardation
in immature animals. Weight loss usually occurs in the
presence of good appetite and adequate food intake. Less
commonly reported clinical signs include acute or chronic
large bowel diarrhea with excess fecal mucus, tenesmus and
The only means by which a definitive diagnosis of giardiasis
can be made is to demonstrate the actual parasitic agent.
This diagnosis is established by identification of cysts
and, less frequently, trophozoites in in fecal specimens.
Trophozoites can be visualized by direct smears of diarrheal
feces. Fecal flotation using zinc sulfate should be used
to concentrate Giardia cysts. The passage of cysts
is, to some extent, sporadic; therefore, a suspected patient
should not be considered negative for Giardia until
three consecutive negative examinations have been completed.
Lugol’s iodine solution can be used to stain both the trophozoites
and cysts, making them easier to identify. Giardia
antigens in the feces of an infected animal may be detected
via indirect and direct immunofluorescent assays using monoclonal
antibodies, and by direct fluorescent assays.
Treatment for giardiasis in humans includes quinacrine,
metronidazole or furazolidone. Metronidazole is the drug
of choice for treatment of giardiasis in dogs. Other drugs
that may be used for canine infections are tinidazole and
quinacrine. Metronidazole, febantel, fenebendazole or albendazole
may be used to treat infected cats; however, optimal and
efficacious drug treatment in cats has not been well established.
Determination of the immune response of dogs to Giardia
has yet to be determined. Because most infections are usually
self-limiting, many researchers suggest an acquired immunological
resistance to the parasite. Epidemiologic research suggests
that previous contact with Giardia may serve to increase
resistance to re-infection. Although the exact mechanism
of immunity is not completely understood, humoral immunity
is considered to be important in the elimination of Giardiatrophozoites
from the host intestine. Immunologically naďve and immunocompromised
hosts have been found to be more vulnerable and also suffer
more severe and chronic infections. Research has shown,
in experimentally infected humans and animals, that the
immunocompetent host produces specific mucosal and serum
antibodies against both cystosolic and surface Giardia
antigens. The cellular immune system does not play a direct
role in parasite clearance.
There is currently a commercially available vaccine against
Giardia in the United
States. This vaccine has
been demonstrated by researchers to be effective for prevention
of clinical signs of giardiasis and reduction of cyst shedding
in dogs and cats. Vaccination of companion and farm animals
helps not only to reduce zoonotic transmission, but also
to reduce both interspecies and intraspecies transmission.
Is Giardia a zoonotic concern? There is evidence
that suggests that direct transmission from companion animals
to humans does occur. Zoonosis is controversial regarding
Giardia, but most researchers believe that its zoonotic
potential merits adequate precaution when working with feces
of animals that may be infected.
Control of Giardia, from a public health standpoint,
should start with municipal drinking water. The prevalence
of Giardia in humans within industrialized countries
is 2-5%. The prevalence of Giardia in humans within
developing countries is 20-30%. As many as 95% of human
travelers to St. Petersburg,
have shown signs of giardiasis. In children that attend
day care centers, the prevalence of Giardia has been
found to be as high as 35%. Filtration can be quite effective
for removing Giardia cysts from water. Since this
parasite may be found in lakes, streams, and ponds, both
hikers and backpackers must be warned to boil or filter
drinking water prior to ingestion.
Giardia is a potential health concern for both
man and animals alike. Correct measures should always be
employed in order to properly diagnose, control and treat
giardiasis. Much work has been done in the area of Giardia
research, but there is still much to be done. Preventing
and controlling giardiasis will require the joint efforts
of both the human medical and veterinary medical professions.
-by Craig Hunt, Class of 2002
-edited by Randy White, DVM, PhD, ADDL Pathologist
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Lieb, M.S., Zajac, A.M., 1999. Giardiasis in dogs and cats.
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