Giardia Infections
World Small Animal Veterinary Association World Congress Proceedings, 2006
Michael R. Lappin, DVM, PhD, DACVIM
Professor, Department of Clinical Sciences, Colorado State University, Fort Collins, CO, USA

The genus Giardia contains multiple species of flagellated protozoans that are indistinguishable morphologically. Recent genetic analysis has revealed 2 major genotypes in people; assemblage A and assemblage B. These organisms are not usually found in dogs and cats in the United States. Dogs are most commonly infected with assemblages C and D and cats are most commonly infected with assemblage F.

Giardia spp. have specific antimicrobial sensitivity patterns like bacteria. However, some Giardia can be difficult to cultivate and so there is little in vitro susceptibility test results available. In addition, while there have been multiple drugs used for the treatment of giardiasis in dogs and cats, there are few studies that utilized dose titrations and evaluation of drugs in experimentally infected animals. In most studies, fecal samples were only assessed for short periods of time after treatment and immune suppression was not induced to evaluate whether infection was eliminated or merely suppressed to below detectable limits of assays used. Infection with Giardia does not appear to cause permanent immunity and so reinfection can occur, a finding that also hampers assessment of treatment studies. Treatment options currently available or used historically include the following.


Canine and feline dose

15-25 mg/kg, PO, q12-24 hr, for 5-7 days.


Feline dose
(primarily used for T. foetus)

30 mg/kg, PO, q12hr, for up to 14 days.


Canine dose

44 mg/kg, PO, q24hr for 3 days.


Canine dose

126 mg/liter of water, PO, ad libitum for 7 days.


Canine and feline dose

50 mg/kg, PO, daily for 3-5 days.


Canine dose

15 mg/kg, PO, q12hr for 2 days.


Canine dose

Label dose PO, daily for 3 days.

Feline dose

56 mg/kg, PO, daily for 5 days.


Canine dose

9 mg/kg, PO, q24hr, for 6 days.

Feline dose

11 mg/kg, PO, q24hr for 12 days.


Feline dose

4 mg/kg, PO, q12hr for 7-10 days.

The protozoal toxicity of metronidazole is from short lived intermediates or free radicals that produce damage by interacting with DNA and possibly other molecules. Metronidazole has effect against other GI protozoans, helps correct bacterial overgrowth, and may be an immune modulating agent and thus, has many potentially positive effects in animals with diarrhea and Giardia infection. However, efficacy is not 100% in most studies in dogs. We recently reported clinical resolution of diarrhea and cessation of cyst shedding in 26 of 26 cats with giardiasis when administered metronidazole benzoate at 25 mg/kg, PO, twice daily for 8 days. There was no recognized neurological or benzoate toxicity and the benzoate salt was well tolerated. Most cases of metronidazole neurotoxicity have occurred in animals on high or extended doses and so the maximal total daily dose should be < 50 mg/kg total daily dose. The related drugs tinidazole, ipronidazole, and ronidazole may also be effective for the treatment of giardiasis but there is limited information currently available.

Benzimidazoles may have anti-Giardia effect by interacting with the colchicines site in tubulin in the microtubules, resulting in the disruption of assembly and disassembly. Selective toxicity is achieved because the drug is minimally absorbed from the host intestine. Fenbendazole is very safe in dogs and cats and was effective (approximately 90%) for the treatment of giardiasis in at least 2 short term studies of experimentally infected dogs. However, cysts shedding was only eliminated in 4 of 8 cats with concurrent giardiasis and cryptosporidiosis when administered fenbendazole at 50 mg/kg, PO, daily for 5 days. Albendazole has been associated with hematologic toxicity in both dogs and cats. Since albendazole efficacy is unlikely to be greater than fenbendazole and the drug is apparently more toxic, it should not be used routinely. When given at the label dose for 3 days, a combination of pyrantel, praziquantel, and febantel was safe and effective in eliminating fecal cysts in 5 of 5 treated dogs. The canine product was used safely in cats but a higher dose was needed. Because febantel is metabolized in part to fenbendazole, use of this drug may be no more effective than using fenbendazole alone but it is possible pyrantel or praziquantel provide an additive effect even though they are not effective for giardiasis when used alone. Fenbendazole, albendazole, and the pyrantel, praziquantel, and febantel combination have the added benefit of having effect against other helminths and cestodes that may be coinfecting the animal. Resolution of clinical signs usually occurs within 7 days of instituting treatment.

Nitazoxanide and paromomycin are used in people with giardiasis and may prove to be effective treatments for giardiasis in dogs and cats. Paromomycin is an aminoglycoside and should not be used if there is blood in the stool because it could potentially be adsorbed and result in renal insufficiency.

There are a number of other things that may also aid in the treatment of giardiasis. The addition of fiber to the diet may help control clinical signs of giardiasis in some animals by helping with bacterial overgrowth or by inhibiting organism attachment to the microvillus. Immunotherapy with the Giardia vaccine has aided in the elimination of cyst shedding and diarrhea in some infected dogs. However, in a controlled study in 16 experimentally infected cats, vaccination as immunotherapy was ineffective with one strain of Giardia.

Because clinical signs induced by Giardia spp. can be intermittent and since some Giardia spp. may be zoonotic, treatment of subclinically infected animals should be considered. However, since natural infection does not result in protective immunity, reinfection from the same contaminated environment is likely. In resistant or recurrent cases, consider other underlying disorders such as inflammatory bowel diseases, bacterial overgrowth, exocrine pancreatic insufficiency, and immunodeficiency. Since cyst shedding can be intermittent, it is difficult to use fecal flotation results to predict a cure.


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9.  Zajac AM, et al. Efficacy of fenbedazole in the treatment of experimental Giardia infection in dogs. Am J Vet Res. 1998; 59: 61-63.

Speaker Information
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Michael R. Lappin, DVM, PhD, DACVIM
Department of Clinical Sciences
Colorado State University
Fort Collins, Colorado, USA

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