Definition

• 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

Systems

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

Prevalence

• 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.

Signalment

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

Causes

• 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.

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

Pathogenesis

• 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.

Signs

• 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.

Definitive

• 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.

Fecal ELISA

• SNAP Giardia Test (idexx.com) 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

Differential

• 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

Postmortem

• 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.

• 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

Prevention

• 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.

Complications 

• 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

Relative Cost

• 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: $$–$$$

Prognosis

• 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.