ISSN 2398-2942      

Giardia duodenalis


Synonym(s): Giardia canis, Giardia duodenalis Assemblages C&D, Giardia intestinalis, Giardia lamblia




  • Within the Excavata. phylum Metamonada, and the order Diplozoa.
  • The species G. duodenalis (syn.Giardia lamblia,Giardia intestinalis) is broken into 8 groups called assemblages based on various molecular genetic markers. These Assemblages are designated by letters:
    • The ones most commonly found in dogs are Assemblages C and D.
    • The one most commonly found in cats is Assemblage F.
    • Occasionally, dogs are also found infected with Assemblage A that has been reported from many different animals and humans, Assemblage B that is for the most part restricted to human hosts, and the feline Assemblage F which may simply be a case of coprophagy.
    • Assemblage B is found mainly in people.
    • Assemblage A has been reported from a number of different animal types and is the only Assemblage that appears to be readily shared by hosts in disparate mammalian groups.
  • There are other distinct species recognized in other hosts: Giardia microti and Giardia muris in rodents; Giardia psittaci and Giardia ardeae in birds; and Giardia agilis in amphibians.

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Clinical Effects



  • Intestinal tract: surface of villi of duodenal and jejunal intervillar spaces Giardia 01.
  • Resistant stage Giardia: cysts.
  • Environment: in water or moist material.
  • Infective stage to new host.
  • Dogs, humans, and other species: dogs may on occasion be passing cysts of Assemblages (A and B) that could show varying degrees of cross-infectivity.


  • Giardia lifecycle Lifecycle Giardia duodenalis - diagram.
  • Trophozoite.
  • Cysts are passed 1-2 weeks after infection.


  • Direct feco-oral contamination: ingestion of cysts in feces or on objects contaminated with feces.
  • Water-borne: cysts survive several weeks in cool water but, while water-borne epidemics occur in humans, their relevance to dogs is far less clear and presumed less common than feco-oral transmission.

Pathological effects

  • Polymorphonuclear leukocyte infiltration in lamina propria.
  • Increase in intra-epithelial lymphocytes.
  • Intestinal secretory antibodies (IgA, IgG) important in immune protection which develops following infection, but protection is not absolute.
  • Steatorrhea: correlated with decrease in pancreatic lipase (mechanism?).
  • Decreased disaccharidases and brush border enzymes.
  • Malabsorption: due to decreased absorptive area and increased immature cells, villous atrophy and increased crypt mitotic activity:
    • Immune response and inflammation may contribute to villous atrophy.
    • Considerable shortening of microvilli.
  • Vacuolization of epithelial cells.

Other Host Effects

  • Giardia use their sucking disk to attach, detach and re-attach themselves to the brush border of the epithelial cells Giardia 01.
  • Trophozoites may carpet the intestinal villi.
  • Rim of adhesive disk displaces and distorts microvilli leaving 'footprint' shapes of the disk in the brush border Giardia 01.


Control via animal

  • Treatment of infected dogs: not 100% effective.
  • Separation of infected dogs.
    Asymptomatic carriers common and direct feco-oral contamination hard to break, and water-borne transmission a possibility.

Control via chemotherapies

  • No licensed claim for use of chemotherapeutic agents against giardiosis in dogs.
    Treatment efficacy is less than 100%; probability of reinfection high.
  • The concern as to the potential of any infection being a possible source of a zoonotic agent has led to the proposal that all cases, with or without signs, should be treated. The trouble is some dogs will not clear after treatment, and some may not clear after several treatments. Also, there is a good chance that the Assemblage present is a canine associated Assemblage that is not a zoonotic agent. Therefore, if a case does not clear after treatment, there may be need to consider the magnitude of the risk posed by the specific circumstances and a decision made as to whether further treatment is actually warrranted on an individual basis.
  • Fenbendazole Fenbendazole 50 mg/kg every day for 3 days. Probably most effective therapy licensed for use in dogs.
  • Metronidazole Metronidazole 25 mg/kg BID for 5 days.
    Drug resistance is known in human strains.
  • Ronidazole (not licensed for dogs) 30-50 mg/kg BID for 7 days.
  • Albendazole Albendazole (not licensed for dogs) 25 mg/kg BID for 2-5 days.
    Albendazole is associated with serious side-effects (bone marrow suppression) and should not be used.

Control via environment

  • Desiccation.
  • Disinfection.
  • Desiccation is lethal to cysts and rate of death is increased at temperatures greater than 30°C.
  • Steam cleaning kills cysts.
  • Chemicals:
    • Wash animals and treat before return to cleaned area:
      • a) Quaternary ammonium compounds: effective within 1 min on cleaned surfaces.
      • b) Sodium hypochlorite: cysts in water are sensitive to chlorine treatment but destruction is very dependent on concentration, time, pH, temperature.
        Cysts are relatively resistant to chemicals.


  • The vaccine for canine Giardia is no longer available.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Fiechter R, Deplazes P, Schnyder M (2012) Control of Giardia infections with ronidazole and intensive hygiene management in a dog kennel. Vet Parasitol 187 (1-2), 93-98 PubMed.
  • Bowman D D & Lucio-Forster A (2010) Cryptosporidiosis and giardiasis in dogs and cats: veterinary and public health importance. Exp Parasitol 124 (1), 121-127 PubMed.
  • Rishniw M, Liotta J, Bellosa M et al (2010) Comparison of 4 Giardia diagnostic tests in diagnosis of naturally acquired canine chronic subclinical giardiasis. J Vet Inter Med 24 (2), 293-297 PubMed.
  • Barr S C, Bowman D D, Frongillo M F et al (1998) Efficacy of a drug combination of praziquantel, pyrantel pamoate, and febantel against giardiasis in dogs. Am J Vet Res 59 (9), 1134-1136 PubMed.
  • Barr S C, Bowman, D D, Heller R L (1994) Efficacy of fenbendazole against giardiasis in dogs. Am J Vet Res 55 (7), 988-990 PubMed.
  • Barr S C, Bowman D D, Heller R L et al (1993) Efficacy of albendazole against giardiasis in dogs. Am J Vet Res 54 (6), 926-928 PubMed.

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