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Intestinal Giardiasis
23 - Giardia lamblia

(photo from Provincial Lab website,  www.provlab.ab.ca)

  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 hematochezia.

  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, Russia 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


Ortega, Y.R., Adam, R.D., 1997. Giardia: overview and update.  ClinInfDis 25:545-550.

Connaughton, D, 1989.  Giardiasis-zoonosis or not?  JAVMA 194: 4,447-449,451.

Kirkpatrick, C.E., 1982.  Giardiasis.  Compendium on Continuing Education for the Practicing Veterinarian 4:367-379.

Brightman, A.H., 1976.  A review of five clinical cases of Giardiasis in cats.  JAAHA 12:4, 492-497.

Lieb, M.S., Zajac, A.M., 1999.  Giardiasis in dogs and cats.  Vet Med 94: 9, 793-802.

Olson, M.E., Morck, D.W., 2000.  Giardia Vaccination.  Parasitology Today 16:5,213-217


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