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Giardiasis in Dogs and Cats: An Overview

Valeria Scorza, MV, MS, PhD, Colorado State University

Infectious Disease

February 2013
Peer Reviewed

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Giardiasis in Dogs and Cats: An Overview

Giardiasis is a protozoal infection that can affect a wide variety of vertebrates, including cats, dogs, and humans.



  • Based on genetic studies, Giardia duodenalis is a species complex composed of 8 assemblages (A–H).
    • Some assemblages are host specific, but others can be harbored by several species and may be considered potentially zoonotic.1
  • Dogs are most commonly infected by host-specific assemblages C and D, while cats harbor host-specific assemblage F.
  • Cats and dogs also harbor zoonotic assemblages A-I, A-II, A-III, A-IV, and B.2


  • The infection can be asymptomatic or produce GI signs (ie, diarrhea, malabsorption, weight loss).


  • Prevalence is 5% in healthy cats and dogs, and 15% in clinically ill animals.3
  • Prevalence is greater in young cats and dogs than in mature ones.

Geographic Distribution

  • Giardiasis is diagnosed worldwide.


  • Giardiasis can occur in humans, domestic animals, livestock, and wild animals.
  • There is no breed or sex predilection.


  • Transmission occurs via the fecal–oral route by direct or indirect ingestion of contaminated water, food, or fomites.
  • Infection with Giardia spp occurs in 2 stages: the trophozoite and the cyst.
    • The cyst (See Figure 1, Giardia cysts seen on (1A) fecal flotation at 100× magnification and (1B) immunofluorescent antibody assay at 400× magnification) is responsible for transmission and can survive several months outside the host in wet, cold conditions; it can dehydrate in dry and hot conditions.
    • The prepatent period ranges from 5 days–16 days in cats and 5 days–12 days in dogs.
    • Peak periods of cyst shedding can occur on days 2–7.


Clinician's Brief

Risk Factors


  • Immunosuppressed animals and animals living in crowded environments are at highest risk for exhibiting GI disease.3
  • Younger animals are more likely to show clinical signs.3


  • The mechanism of infection can involve:
    • Production of toxins
    •  Disruption of normal flora
    •  Induction of inflammatory bowel disease
    • Inhibition of normal enterocyte enzymatic function
    • Blunting of microvilli
    • Induction of motility disorders
    • Induction of intestinal epithelial cell apoptosis4
  • As a result, diarrhea can be caused by a combination of intestinal malabsorption and hypersecretion of electrolytes.


  • Most dogs and cats that shed Giardia organisms are asymptomatic.
  • In clinically affected animals, diarrhea can be mucoid, pale, and soft and have a strong odor; steatorrhea may be present.
    • Mild to moderate discomfort from abdominal inflammation can occur with diarrhea.
    • Immunosuppressive disease or co-infection with other pathogens can exacerbate clinical signs.5
    • Whether different assemblages cause different signs remains unknown.


Clinician's Brief



  • In cats and dogs with diarrhea, the Companion Animal Parasite Council (CAPC) guidelines recommend testing by direct smear, fecal flotation with centrifugation, and a specific fecal ELISA.

Related Article:Fecal Examination Techniques

Direct Smear

  • Trophozoites are rarely seen in solid feces but can be observed in a small sample of fresh diarrhea when mixed with a drop of 0.9% saline solution on a microscope with a coverslip.
  • At 100× magnification, active falling leaf motion of trophozoites can be observed.
    • Trophozoites can appear very active within a small area of the slide and can be easily kept in the field of view.
    • At 400× magnification, structural characteristics can be observed.

Fecal Flotation with Centrifugation

  • If Giardia trophozoites are not detected in direct smears, cysts should be examined by fecal flotation using Sheather sugar (1.25 SG) and zinc sulfate (1.18 SG).
  • Zinc sulfate is considered the flotation medium of choice for Giardia cyst detection.


  • SNAP Giardia Test ( is the only commercially available ELISA for detecting Giardia spp in cats and dogs.
  • Some laboratories use ELISA plate assays that have been internally validated to detect Giardia spp in cats and dogs.
  • To rule out Giardia infection, ≥3 samples should be examined within 1 week.

Related Article: Idexx Snap: Giardia for Dogs & Cats


  • In dogs and cats, noninfectious and other infectious causes of small intestinal diarrhea, IBD, exocrine pancreatic insufficiency, malabsorption syndromes, and neoplastic intestinal disease need to be ruled out.6
  • In cats, Tritrichomonas foetus infection must be ruled out.

Laboratory Findings

  • In general, results of CBC, serum biochemistry panel, and urinalysis will be within reference ranges.
  • Any detected abnormalities are likely from dehydration and electrolyte losses associated with diarrhea.

Other Diagnostics

  • Immunofluorescent antibody (IFA) assay can detect Cryptosporidium spp.
    • IFA assay is more sensitive than fecal flotation (compared with that of performing single fecal ELISA).
  • A fluorescence microscope is needed to read the slide.
  • PCR testing currently is only indicated for determining G duodenalis assemblages.
    • Multilocus (amplification of more than 1 gene) analysis has been recommended.7


  • Giardiasis is nonfatal.
  • In experimentally infected gerbils, histopathologic findings include reduced jejunal villus height and increased jejunal crypt depth.
  • Slight to moderate infiltration of inflammatory cells was noted in the lamina propria and was typically more severe in the duodenum.
  • Inflammatory cells included plasma cells, lymphocytes, macrophages, and mast cells.


Inpatient or Outpatient   

  • In general, cats and dogs with giardiasis are treated as outpatients.
  • If marked diarrhea occurs and fluid therapy is indicated, animals may require hospitalization.
  • If vomiting and small bowel diarrhea are primary clinical signs, highly digestible bland diets are indicated.
  • If large bowel diarrhea is principal clinical sign, high-fiber diets should be used.
  • Primary focus of treatment is resolution of diarrhea.
  • Treatment of asymptomatic dogs that shed Giardia cysts and of cats in general has been controversial.

Client Education

  • Because healthy pets are not considered significant health risks for humans, elimination of infection is secondary.
  • CAPC guidelines suggest that asymptomatic dogs and cats may not need treatment.

Contraindications, Precautions, & Interactions

  • GI and neurologic toxicity after chronic therapy or acute high doses of metronidazole has been reported in some dogs and kittens.9,12
  • Metronidazole USP induces salivation and inappetence in some cats, but metronidazole benzoate is well tolerated.
  • Neurotoxicity has been observed after administration of ronidazole.
  • Albendazole can cause bone marrow suppression in cats and is not currently prescribed for use in cats or dogs.13
  • Paromomycin can cause deafness and renal failure in some cats.9

Medications Drugs/Fluids

  • No drugs are approved in the United States for treatment of giardiasis in cats and dogs, but several drugs are commonly used (See Table in PDF).
        • In Florida, cats and dogs with acute giardiasis must be reported to the state department of health.8
  • Metronidazole (USP or benzoate) may be indicated if clinical findings suggest concurrent Clostridium perfringens overgrowth.
  • Ipronidazole has been shown effective for treatment of giardiasis in 2 dogs.9
  • Six dogs treated with ronidazole and bathed twice with a chlorhexidine shampoo, along with disinfection of enclosures with 4-chlorine-M-cresol, became negative for Giardia cysts and antigen within 26 days.10
  • Fenbendazole can be used, particularly when concurrent infection with cestodes or nematodes is suspected.
    • When fenbendazole was used for treatment of cats concurrently infected with Giardia and Cryptosporidium parvum, only 4 of 8 cats stopped shedding Giardia cysts.11
  • Combination of febantel, pyrantel, and praziquantel using different protocols was generally successful when treating giardiasis in dogs and cats.8,9


Patient Monitoring

  • Fecal flotation can be used to evaluate treatment success.
  • In persistent infections, combination therapy with a second drug from an alternate class is indicated (eg, fenbendazole plus metronidazole).
    • At Colorado State University laboratory, paromomycin or nitazoxanide was used in some cats and dogs with resistant giardiasis, but data were not controlled.14
    • Azithromycin has been used successfully in dogs with resistant giardiasis, but additional studies are needed.15


  • Environmental disinfection is recommended.
  • Feces should be removed daily and contaminated surfaces disinfected by steam cleaning or use of quaternary ammonium compounds (1-minute contact time).
  • Infected animals should be bathed with shampoos to remove fecal debris and cysts.


  • In animals with persistent diarrhea from Giardia infection, underlying disorders should be considered:11,14
    • Inflammatory bowel disease
    • Bacterial overgrowth
    • Coinfection with other organisms (ie, Cryptosporidium spp, T foetus)
    • Exocrine pancreatic insufficiency
    • Immunodeficiency

In General

Relative Cost

Cost Key
$ = up to $100    
$$ = $101–$250    
$$$ = $251–$500
$$$$ = $501–$1000
$$$$$ = more than $1000
  • Treatment and follow-up care for uncomplicated cases of diarrhea: $
  • Diagnostic workup for chronic cases of diarrhea and presence of Giardia cysts in fecal flotation: $$–$$$


  • In most cases, diarrhea will resolve after treatment.

Future Considerations

  • The role that cats and dogs play in the transmission of giardiasis to humans should be investigated further.

Public Health Considerations

  • Most cats and dogs harbor species-specific assemblages of G duodenalis and are not considered a significant human health risk, although the presence of zoonotic assemblages in dogs, cats, and humans in the same household has been described.1,16 
  • Healthy cats and dogs should be screened for infection q6–12mo.
    • It has been suggested that if Giardia cysts are detected in an asymptomatic animal, the animal should not be treated. 

References and Author Information

For global readers, a calculator to convert laboratory values, dosages, and other measurements to SI units can be found here.

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