Giardia

Synopsis

CAPC Recommends

• Parasite testing should follow general guidelines.
• Treat all symptomatic dogs with recommended medication and protocol. See treatment section for more information on appropriate protocols.
• Follow up fecal flotation testing may be done 24-48 hours after the completion of therapy if clinical signs have not resolved.
• In cases where repeated infection is apparent, treatment combined with bathing and proper feces removal from the environment should be considered to prevent reinfection.

Stages

• Trophozoite—motile stage in small intestine
• Cysts—resistant stage for environmental transmission

Giardia cyst 400X

Giardia cyst and Ancylostoma

Giardia cyst and trophozoite 400X

Cyst of Giardia sp. (iodine stain)

Trophozoite of Giardia sp. (iodine stain)

Disease

• Infections with Giardia are often asymptomatic, but if clinical signs are present they can include: diarrhea, which may be acute, intermittent, or chronic, and can be accompanied by flatulence, vomiting or mucous in the stool. In severe cases lethargy, inappetence, and weight loss may also occur. However, most infections are subclinical.

Prevalence

• Giardia duodenalis infection is common in dogs.
• Regional differences in Giardia prevalence exist, but infections in dogs with clinical signs averaged 15.6% in dogs in the US. Overall, the most common intestinal parasite in dogs was Giardia (8.1%), and in urban parks Giardia spp. was present in 24.7% of dogs attending parks in Calgary, Canada.
• A nationwide survey conducted in 2023 in the United States found a 5.72% prevalence of Giardia infections in fecal samples submitted to a commercial laboratory.
• Animals in shelters, breeding facilities, and kennels are more likely to carry Giardia duodenalis, and studies have found an increased prevalence among dogs that visit dog parks.

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Host Associations and Transmission Between Hosts

• Giardia exists in different "assemblages," which vary in their infectivity for animals and humans. Dogs have mainly Assemblages AI, C and D. However, some studies have identified human Assemblages of Giardia in canine fecal samples.
• The total number of Giardia strains and host-infectivity ranges is unknown.
• Transmission occurs upon ingestion of cysts shed by animals or humans. Cysts are immediately infective when passed in feces.
• Cysts are acquired from fecal-contaminated water, food, or fomites or through self-grooming.
• There are reports of dog adapted assemblages infecting cats as well as feline adapted assemblages infecting dogs.
• Human infections are primarily acquired from other humans; transmission from dogs to humans appears to be rare.

Molecular Characterization

Each assemblage (A-H) is capable of infecting certain species, and some assemblages are more commonly seen than others.

Some sub-assemblages have been shown to be zoonotic. Assemblage testing is commercially available.

Below is a summary of each assemblage and the primary species that it infects.

Assemblage A-I

• Species affected: Humans and animals (cats, dogs, livestock, deer, muskrats, beavers, voles, guinea pigs, ferrets)

Assemblage A-II

• Species affected: Humans (more common than A-I)

Assemblage A-III and A-IV

• Species affected: Exclusively animals

Assemblage B

• Species affected: Humans and animals (livestock, chinchillas, beavers, marmosets, rodents)

Assemblage C and D

• Species affected: Dogs and wild canids

Assemblage E

• Species affected: Alpacas, cattle, goats, pigs, sheep

Assemblage F

• Species affected: Cats

Assemblage G

• Species affected: Rats, mice

Assemblage H

Species affected: Seals

Site of Infection and Pathogenesis

• Trophozoites attach to the surface of enterocytes in the small intestine, usually in the proximal portion or may float freely in the intestinal lumen.
• Attachment causes damage to enterocytes, resulting in functional changes and blunting of intestinal villi, which leads to maldigestion, malabsorption, and diarrhea.
• Dogs may be asymptomatic or display acute or chronic diarrhea with or without weight loss. Additional signs may include abdominal pain, lethargy, or hyporexia. Feces in clinically affected animals are often malodorous and may contain mucus.
• There are no intracellular stages.
• There are no infections of other tissues, except very rare cases of ectopic infection following intestinal perforation attributable to other causes.

Diagnosis

• Giardiasis is commonly misdiagnosed or underdiagnosed because of intermittent shedding and difficulty identifying cysts and trophozoites. Yeast may be mistaken for Giardia cysts due to their similar size and shape; however, yeast often show evidence of budding and do not have the internal structures seen in Giardia (i.e., median bodies, two to four nuclei).
• Trophozoites are usually 12 to 18 µm by 10 to 12 µm in size. They are motile, flagellated organisms that are teardrop or pear-shaped. Trophozoites are bilaterally symmetrical, have a large ventral adhesive disc, and have two nuclei, each with a large endosome. They also have a pair of transverse, dark-staining median bodies.
• Cysts are transparent, ellipsoidal, nonmotile, and contain two to four nuclei along with long and short curved rods. They are 8 to 12 µm by 7 to 10 µm in size and possess a thick refractile wall.
• Dogs may have subclinical infections and show no signs of disease. Treatment approaches may vary.
• Various tests are used, including direct smear (with or without a fixative stain), fecal flotation via centrifugation, centrifugation with AI assisted reading, fecal antigen ELISA, direct fluorescent antibody assay (IFA), and PCR.
• CAPC recommends testing symptomatic (intermittently or consistently diarrheic) dogs with a combination of direct smear, fecal flotation with centrifugation, centrifugation with AI assisted reading, and a sensitive, specific fecal ELISA or by PCR optimized for use in companion animals. Repeat testing performed over several (usually alternating) days may be necessary to identify infection.
• Direct smear:
  • Direct smear is used primarily for detection of trophozoites in diarrheic stools.
  • Use a small sample of fresh, unrefrigerated feces (preferably less than 30 minutes old).
  • Mix sample into two to three drops of saline (not water) on a glass slide to make a fine suspension and add a coverslip (a 22 by 40 mm coverslip works well).
  • A Lugol’s iodine stain may be added to aid in identification.
• Fecal flotation with centrifugation techniques:
  • This method is used primarily for detection of cysts in solid or semisolid stools.
  • Click here to see Fecal Examination Procedures.
  • ZnSO₄ is preferred, as sugar solution will collapse the Giardia cysts, albeit in a characteristic way.
  • If desired, a Lugol’s iodine stain may be added to aid identification at 40x.
  • This is the recommended test for healthy, non-symptomatic patients entering boarding, grooming or daycare.
• Fecal Antigen ELISA:
  • Giardia antigen ELISA assays are approved and commercially available for patient-side testing in dogs and cats. Many diagnostic laboratories also use various ELISA microtiter well assays that have been internally validated for the detection of giardiasis in dogs and cats.
• Commercially available fixative stains (e.g. Proto-fix™) are also useful for microscopic diagnosis.
• Polymerase Chain Reaction (PCR) assays can also be used to amplify Giardia species DNA in feces and are available in reference laboratories. Additionally, some PCR tests may determine whether the Giardia assemblage is potentially zoonotic.
• Patient status and lifestyle are important considerations when interpreting Giardia positive diagnostic results.
  • Symptomatic patients should be treated.
  • Non-symptomatic patients with zoonotic assemblages should be treated.
  • Non-symptomatic patients with non-zoonotic assemblages may not require treatment.
  • Patients living in a household with vulnerable or immunocompromised humans or pets should be treated.
  • Ultimately, it is the practitioner's responsibility to use their best judgement in consultation with the owner in deciding treatment options.
• Commercial laboratories publish algorithms specific to their diagnostic tests and may be useful.

Treatment

• Insufficient evidence is available for definitive recommendations in each clinical scenario; however, the majority opinion of the CAPC Board is that asymptomatic dogs may not require treatment.
• In healthy, non-symptomatic patients with a non-zoonotic Giardia assemblage, treatment may not be necessary.
• Factors to consider in the decision to treat include contact with vulnerable or immunocompromised humans or dogs, lifestyle of the dog, and prior treatment history. See Tysnes et al., 2014 for additional information.
• In symptomatic patients with non-zoonotic Giardia assemblage, a high fiber diet with probiotics may be sufficient.
• If pharmacological treatment is indicated, CAPC recommends the following:
  • Metronidazole oral suspension is FDA approved for use in dogs (25 mg/kg, BID, 5 days).
  • Metronidazole (10-25 mg/kg BID 5-8 days) is the most commonly used therapy.
  • Fenbendazole (50 mg/kg SID for 3 to 5 days) has been shown to be effective in eliminating Giardia infection in dogs. Fenbendazole is approved for Giardia treatment in dogs in Europe.
  • A combination of febantel, pyrantel pamoate, and praziquantel (DrontalPlus) has been shown to be effective in treating Giardia in dogs when administered daily for 3 days using the dose bands indicated on the DrontalPlus label.
• Environmental considerations include bathing the pet and removing fecal matter and fecal contaminated materials should be included.

• Albendazole is effective against Giardia but is not safe in dogs and should not be used.
• A variety of other treatments, such as secnidazole and nitazoxanide, have been utilized in dogs. However, more research is needed to evaluate their safety and efficacy.

Follow-up Testing

• Follow up testing may be done 24-48 hours after the completion of therapy if clinical signs have not resolved. It is recommended to perform a fecal flotation with centrifugation primarily for detection of cysts in solid or semisolid stools.
• ELISA and PCR tests may remain positive even after treatment for variable periods of time and should not be used as a guide to determine reinfection or failure of treatment.

Refractory Treatment

• Treatment failures may result from: reinfection, inadequate drug levels, immunosuppression, drug resistance and Giardia sequestration in the gallbladder or pancreatic ducts. The presence of immunosuppression, reinfection, or sequestration can usually be determined in a clinical setting. Certain immunosuppressed patients are abnormally susceptible to giardiasis and their infections are often difficult to cure. Reinfection is common in endemic regions with high environmental contamination.

Control and Prevention

• Concomitant with treatment, animals should be bathed with shampoo to remove fecal debris and associated cysts on the last day of treatment (Payne et al., 2002).
• Remove feces daily and dispose of fecal material with municipal waste.
• Environmental areas (e.g., soil, grass, standing water) are difficult to decontaminate, but surfaces can be sanitized by steam-cleaning or use of commercially available disinfectants. Allow surfaces to dry thoroughly after cleaning.
• Post-treatment fecal examination by zinc sulfate centrifugation may be helpful in evaluating the success of therapy.

Public Health Considerations

• Human infection from a dog or cat source has not been conclusively demonstrated in North America. However, people with increased susceptibility to infection due to underlying disease should consider limiting their exposure to Giardia-infected pets.
• Advise clients to seek medical attention if they develop gastrointestinal symptoms following exposure to an infected pet.
• If both people and pets in the same household are infected, it does not necessarily imply zoonotic transmission.
• An infected person should wash their hands after using the toilet and before feeding or handling animals.

Selected References

• Barr SC, Bowman DD, Heller RL, 1994. Efficacy of fenbendazole against giardiasis in dogs. Amer J Vet Res 55: 988-990.
• Carlin EP, Bowman DD, Scarlett JM, Garrett J, Lorentzen, L, 2006. Prevalence of Giardia in symptomatic dogs and cats throughout the United States as determined by the IDEXX SNAP Giardia test. Vet Therapeutics 7(3):199-206.
• Cheung W, Russo R, Maher S, Malik R, Šlapeta J. 2019. Successful use of secnidazole to manage a giardiasis outbreak in a shelter. Vet Parasitol 274: Article 108911.
• Covacin, C.; Aucoin, D.P.; Elliot, A.; Thompson, R.C.A. Genotypic characterisation of Giardia from domestic dogs in the USA. Veterinary Parasitology, Volume 177, Issues 1-2, 19 April 2011: 28-32.
• Faure L, Fournel S, Nicolas C, Rigaut D. A field clinical study to confirm the efficacy and safety of a metronidazole-based oral suspension in dogs naturally infested by giardiasis: Comparison to fenbendazole. Inter J Appl Res Vet Med 2018: 16:110-116.
• Goldstein F, Thornton JJ, Szydlowski T. Biliary tract dysfunction in giardiasis. Am J Dig Dis 1978: 23:559-60.
• Nagamori Y, Warren Z, Houma M, Samarakoon N. 2025. Regional and seasonal variability in canine parasitism across the United States. Vet Parasitol 339: Article 110579.
• Nash TE, Ohl CA, Thomas E. Subramanian G, Keiser P, Moore TA. Treatment of Patients with Refractory Giardiasis. Clin Infect Dis. (2001)33(1): 22-28.
• Olson ME, Hannigan CJ, Gaviller PF, Fulton LA. (2001). The use of a Giardia vaccine as an immunotherapeutic agent in dogs. The Canadian Veterinary Journal, 42(11), 865-868.
• Payne PA, Ridley RK, Dryden MW, et al., 2002. Efficacy of a combination febantel-praziquantel-pyrantel product, with or without vaccination with a commercial Giardia vaccine, for treatment of dogs with naturally occurring giardiasis. J Amer Vet Med Assn 220:330-333.
• Romano F, Anete Lallo M. 2023. Efficacy of a single dose of nitazoxanide in dogs naturally infected with Giardia duodenalis. Res Vet Sci 159:252-256.
• Saleh MN, Gilley AD, Byrnes MK, Zajac AM. Development and evaluation of a protocol for control of Giardia duodenalis in a colony of group-housed dogs at a veterinary medical college. JAVMA. 2016;249(6):644 – 649.
• Scorza V. Giardiasis. Clinicians Brief. February 2013, 71-74. http://www.cliniciansbrief.com/sites/default/files/attachments/Giardiasis.pdf.
• Simpson KW, Rishniw M, Bellosa M, et al., 2009. Influence of Enterococcus faecium SF68 probiotic on giardiasis in dogs. J Vet Internal Med 23:476-481.
• Soto JM, Dreiling DA. A case presentation of chronic cholecystitis and duodenitis. Am J Gastroenterol1977:67:265-9.
• Tysnes KR, Skancke E, Robertson LJ. 2014. Subclinical Giardia in dogs: a veterinary conundrum relevant to human infection. Trends in Parasitol 30(11): 520-527.
• Giardiasis – The Center for Food Security and Public Health. www.cfsph.iastate.edu/Factsheets/pdfs/giardiasis.pdf.