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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 naïve 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

Upset tummy!



If your dog has been diagnosed with Giardia, it is infected with the one-celled protozoan parasite Giardia Lamblia. These flagellate parasites are usually contracted by drinking contaminated water or sometimes by eating contaminated feces. Giardiasis, the disease caused by Giardia, can range from asymptomatic (no visible signs of distress) to extremely acute where the dog is severely ill. Canine giardiasis should be treated since it is potentially transmissible to humans and other animals.

Giardiasis is a malabsorptive syndrome. The parasites adhere to the lining of the small intestine where they interfere with absorption of nutrients. Light cases of Giardia often go undetected and many dogs "self cure" by expelling and developing an immunity to the parasite. In heavier infections, Giardia can interfere with absorption of certain types of nutrients, especially fats and certain vitamins. Fats are not absorbed and result in excess mucus in the stools which are very pungent and diarrhetic.

The parasites interfere with normal metabolism by forming a physical barrier between the lumen of the intestine and the absorptive cells. Excess mucus results from malabsorption of fats while excess water results in the diarrhea. The intestinal lining is not usually injured so stools should not contain blood. The parasites feed on partially digested food in the lumen of the intestine. They do not compete directly with the host for food. Their metabolism is primarily anaerobic, meaning that they do not utilize oxygen in their respiration. They lack cellular organelles concerned with aerobic respiration such as mitochondria.

The active stage within the host is the trophozoite (feeding body); this is the only pathological form. The transfer stage of the parasite is the termed the cyst. Giardia forms cysts by extruding cellular food particles and other vacuoles and secreting a resistant cyst membrane around the cell. This highly resistant cyst is then passed from the host in the feces. Trophozoites may be passed but quickly die. Cysts that are passed into water can survive for an extended time, up to 1-2 months under proper conditions. Survival times on land are somewhat less. A new host becomes infected by drinking fecally contaminated water or eating the feces of an infected animal.

While food-borne transmission is rare, it has been documented for humans. Dogs may become infected by drinking out of streams, lakes or ponds containing Giardia cysts. Other sources of infection are wild animals that visit the kennel area and deposit infected feces in an area accessible to the dog. Scats of other dogs or wild animals are potential sources of infection for domestic dogs. Giardia is potentially transmissible to humans so caution is warranted.

Giardia can be difficult to detect even for professionals. It is too small to be seen by the unaided eye. A high quality microscope is needed for proper diagnosis; phase contrast microscopy is helpful. A definitive negative diagnosis should include stools collected on multiple days since cyst production tends to be cyclic with millions produced one day and few the following day. The cyst is the diagnostic stage of Giardia. Cysts tend to be approximately 9-15 micrometers in length and 4-5 um in width. Cysts are identified by size, the presence of four nuclei, axostyles and claw-hammer shaped median bodies.
The current drug of choice is metronidazole, known by the trade name FLAGYL. Although highly effective it is a known carcinogen and mutagen in mice. Quinacrine (ATABRINE) can also be used but is not as effective. Treatment is usually one tablet per day for 7-10 days, depending on the weight of the dog. Recovery is usually uneventful but a dog may become reinfected after treatment. Thus, it is important to try to isolate and eliminate the source of infection.

By Dr. James Coggins


Etiologic agent = Giardia intestinalis/duodenalis (formerly lamblia)

This is a flagellate protozoan that inhabits the intestinal tract of a wide variety of domestic and wild animals species.

Life cycle of Giardia:

Giardia trophozoites are binucleate and replicate in the small intestines by binary fission. Once they exit the jejunum, they encyst as they begin to lose moisture content. Within their cyst, they undergo one further division to produce a tetranucleate cyst. This cyst that is passed in the feces is very resistant in the environment and is the infective form for new hosts. When ingested, the tetranucleate parasite emerges from the cyst in the duodenum, rapidly divides into (2) binucleate trophozoites, and proceeds to undergo continued binary fission.

Epidemiology of Giardia:

  • Infections in humans are most commonly waterborne. Backpackers and people pursuing other outdoor activities may be infected from what appear to be pristine water sources because of human or animal fecal contamination upstream.
    • Beaver and muskrat have been reported to have Giardia carriage rates of 16% and >95%, respectively. In addition, pinnipeds in Canada have also been demonstrated to shed Giardia cysts.
    • The organism survives well in cold water and may not be inactivated by routine chlorination tablet water purification systems. Filtration of water is essential.
  • The second most common mode of infection is person-to-person, e.g. in day-care centers.
  • Infected dogs and cats can clearly serve as a source for contamination of the environment with the organism and must be considered as potential zoonotic risks. The same may be true for cattle, goats, llamas and pigs, although some isolates from these species appear to represent strains that are restricted only to livestock and have not been recovered from humans. Thus, the overall zoonotic impact of giardiasis in animals remains to be fully understood.
    • It has been estimated that 1-2% of well-cared for dogs and cats may be shedding Giardia cysts at any one time. The rate of shedding increases to ~10% in kennel or shelter environments, and infection and clinical disease are greatest in puppies and kittens, with up to 50% of puppies potentially shedding the organism at any one time.

Giardiasis clinical disease:

Following a 1-2 week incubation period, both people and dogs and cats initially suffer an acute GI'itis with diarrhea. Fever is much less common than with bacterial agents of gastroenteritis. This acute phase of disease may be followed by a CHRONIC syndrome of malabsorptive diarrhea, weight loss and abdominal pain that waxes and wanes over a period of many months.

  • The stools may initially be watery, but then typically progress to soft, semi-formed stools with steatorrhea and a rancid, foul odor.
  • Patients may also exhibit malaise, nausea, bloating and flatulence.
  • The diarrhea may continue indefinitely if not treated. In fact, because of difficulties in diagnosis, empirical treatment of Giardia is sometimes considered in cases of chronic, undiagnosed malabsorptive diarrhea.
    • The diarrhea appears to be due primarily to disruption of the enterocyte brush borders and loss of disaccharidase enzymes.
    • Actual invasion of enterocytes is rare, but the organism can colonize large portions of the small intestinal tract.
    • The host immune/inflammatory response may also be a contributing factor in the pathology of Giardia infection.

Diagnosis of Giardia infections in dogs and cats:

  • demonstration of trophozoites in direct fecal smears
    • Multiple smears over time must be done because of the intermittent nature of Giardia shedding.
  • fecal floatation (zinc sulfate solution) for cysts
    • Trophozoites will NOT be detected by floatation techniques because the floatation solution lyses the trophozoites.
  • demonstration of the organism in duodenal aspirates collected during endoscopy
  • ELISA and IFA assays for Giardia antigens in feces
    • These can be highly sensitive and specific in humans, but some studies suggest they are less so when used in domestic animals.
    • response to empirical therapy

Treatment of Giardia infections in dogs and cats:

  • Metronidazole (e.g., Flagyl) can be used in both dogs and cats (not in pregnant animals).
  • Albendazole (e.g., Valbazen) was recently found to be quite effective in dogs, and may be more efficacious than metronidazole in stopping the shedding of cysts. However, both metronidazole and albendazole have been associated with significant adverse reactions in dogs and cats:
    • Albendazole: leukopenia +/- anemia and thrombocytopenia; anorexia, lethargy; CNS signs; vomiting and diarrhea; salivation; elevated hepatic and pancreatic enzyme levels; abortion and teratogenicity
    • Metronidazole: vomiting; CNS signs
  • Fenbendazole (e.g., Panacur® or Drontal-Plus®) now appears to be the drug of choice. Used in dogs and cats at 50 mg/kg for 3 (-5) days, fenbendazole has been shown to be completely effective in eliminating experimental Giardia infections, and with only mild vomiting/diarrhea as potential side effects.

Vaccination against Giardia in dogs:

In 1999, a killed, whole-organism vaccine (GiardiaVax, Fort Dodge Animal Health) was approved for use in dogs in the U.S. and has been shown by the manufacturer to reduce the shedding of Giardia cysts after experimental infection. (9/20 vaccinates shed cysts on day 7 after infection and 0/20 vaccinates shed cysts on day 42 after infection, compared to 10/10 placebo controls on both days.)

Copyright © 2000 Christopher W. Olsen. All Rights Reserved.
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The prevalence of Giardia in dogs and cats in Perth, Western Australia.

A survey of dogs and cats in the Perth metropolitan area revealed a high prevalence of Giardia. Overall, 21% of 333 dogs and 14% of 226 cats were infected. More dogs and cats from refuges and breeding establishments were infected than household pets, although among the latter a significant number of dogs (9%) and cats (8%) was infected. Giardia did not show any breed or sex predisposition but prevalence was higher in young animals. The species of Giardia present in dogs and cats was identified as G. duodenalis, which is the same as that affecting man. The potential significance of this animal reservoir of infection to man is discussed in the light of increasing evidence that Giardia is a zoonosis.

Swan JM, Thompson RC. 1986
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The following are recommendations from the July 1995 Cornell Animal Health Newsletter for eliminating Giardia from kennels:

  • treat all non-pregnant dogs with fenbendazole for 5 days

  • disinfect kennel areas, etc., with quaternary ammonium disinfectants which are effective in inactivating Giardia cysts (takes about a minute at room temperature)

  • bath dogs with shampoo to remove all fecal matter, rinse with water

  • rinse dogs with quaternary ammonium disinfectants, then water

  • allow kennels to dry thoroughly for several days

  • retreat with fenbendazole for 5 days

  • treat any new dogs with fenbendazole for 5 days even if they test negative for Giardia because it is so hard to detect in fecal tests

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