Current Advice on Parasite Control:
Intestinal Parasites - Giardia
Video courtesy of Dr. Chris Adolph, Southpark Veterinary Hospital
- Trophozoite—motile stage in small intestine
- Cysts—resistant stage for environmental transmission
- Giardia duodenalis infection is common in dogs and cats.
- Regional differences in Giardia prevalence exist, but infections in dogs and cats with clinical signs averaged 15.6% in dogs and 10.3% in cats.
Host Associations and Transmission Between Hosts
- Giardia exists in different "assemblages," which vary in their infectivity for animals and humans. Dogs have mainly Assemblages C and D, cats have Assemblages A1 and F, humans are infected with Assemblages A2 and B. 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 occures upon ingestion of cysts shed by animals or humans.
- Cysts are acquired from fecal-contaminated water, food, or fomites or through self-grooming.
- Dog strains are not known to infect cats, and cat strains are not known to infect dogs.
- Human infections are primarily acquired from other humans; transmission from dogs and cats to humans appears to be rare.
Site of Infection and Pathogenesis
- Trophozoites attach to the surface of enterocytes in the small intestine, usually in the proximal portion.
- Attachment causes damage to enterocytes, resulting in functional changes and blunting of intestinal villi, which leads to maldigestion, malabsorption, and diarrhea.
- 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.
- 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).
- Dogs and cats may have subclinical infections and show no signs of disease.
- Various tests are used, including direct smear (with or without a fixative stain), fecal flotation via centrifugation, fecal ELISA, and direct fluorescent antibody assay.
- CAPC recommends testing symptomatic (intermittently or consistently diarrheic) dogs and cats with a combination of direct smear, fecal flotation with centrifugation, and a sensitive, specific fecal ELISA optimized for use in companion animals. Repeat testing performed over several (usually alternating) days may be necessary to identify infection.
- 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.
- Mix 1 to 5 g feces and 10 ml of flotation solution (ZnSO4 sp.gr. 1.18; sugar sp. gr. 1.25) and filter/strain into a 15-ml centrifuge tube. ZnSO4 is preferred, as sugar solution will collapse the Giardia cysts, albeit in a characteristic way.
- Top off with flotation solution to form a slightly positive meniscus, add coverslip, and centrifuge for 5 minutes at 1500 to 2000 rpm.
- If desired, a Lugol’s iodine stain may be added to aid identification at 40x.
- Currently, the SNAP® Giardia is the only commercially available Giardia ELISA assay approved for patient-side testing of giardiasis in dogs and cats [U.S. Vet License No. 313.]. Many laboratories use various ELISA plate 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.
- 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 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.
- No drugs are approved for treatment of giardiasis in dogs and cats in the United States.
- Metronidazole is the most commonly used extra-label therapy; however, efficacies as low as 50% to 60% are reported. Safety concerns also limit the use of metronidazole in dogs and cats.
- Albendazole is effective against Giardia but is not safe in dogs and cats and should not be used.
- Fenbendazole (50 mg/kg SID for 3 to 5 days) is effective in eliminating Giardia infection in dogs. Fenbendazole is approved for Giardia treatment in dogs in Europe, and available experimental evidence suggests that it is more effective than metronidazole in treating Giardia in dogs.
- A combination of febantel, pyrantel pamoate, and praziquantel (DrontalPlus) is effective in treating Giardia in dogs when administered daily for 3 days using the dose bands indicated on the DrontalPlus label.
CAPC recommendations for treatment of dogs
- Administer fenbendazole (50 mg/kg SID) for 5 days.
- Alternatively, fenbendazole (50 mg/kg SID) may be administered in combination with metronidazole (25 mg/kg BID) for 5 days. This combination therapy may result in better resolution of clinical disease and cyst shedding.
- If treatment combined with bathing (see Control and Prevention) does not eliminate infection (as evidenced by testing feces for persistence of cysts), treatment with either fenbendazole alone or in combination with metronidazole may be extended for another 10 days.
CAPC recommendations for treatment of cats
- Data on treatment of cats with Giardia are lacking. However, cats may be treated with either fenbendazole (50 mg/kg SID) for 5 days, metronidazole (25 mg/kg BID) for 5 days, or a combination of the two as described for dogs.
- There is anecdotal evidence that metronidazole benzoate is tolerated better in cats than metronidazole (USP).
Insufficient evidence is available for definitive recommendations in each clinical scenario; however, the majority opinion of the CAPC Board is that asymptomatic dogs and cats may not require treatment. If treatment is desired:
- A dog or cat without clinical signs that has been found to be infected with Giardia may be treated with a single course of anti-giardial therapy (see above).
- If other pets live with an infected dog or cat, all those of the same species may also be treated with a single course of anti-giardial therapy.
- Repeated courses of treatment are not indicated in dogs or cats without clinical signs.
- CAPC does not endorse routine vaccination of all pets for Giardia. However, preventive vaccination for Giardia may be useful in some specific control situations. Current data do not support the use of Giardia vaccines as part of a treatment protocol.
Control and Prevention
- Concomitant with treatment, animals should be bathed with shampoo to remove fecal debris and associated cysts.
- 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. Dogs and cats are not treated for the purpose of preventing zoonotic transmission.
- Canine and feline strains of G. duodenalis are not known to be infective to immunocompetent human hosts. 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 hands after using the toilet and before feeding or handling animals.
- Barr SC, Bowman DD, Heller RL, 1994. Efficacy of fenbendazole against giardiasis in dogs. Amer J Vet Res 55: 988-990.
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.
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.
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.
- 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.