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Giardia spp are protozoal parasites that infect many different animals via the fecal–oral route. After an animal ingests an environmentally resistant cyst, excystation occurs and the pathogenic trophozoites are released. The trophozoites attach to intestinal villi, causing damage and potentially resulting in the most common clinical sign, small-bowel diarrhea.1


Dogs, cats: 50 mg/kg PO q24h for 3 to 5 days1-3 

Dose is extra-label and based on FDA-approved use for helminth infections.

Fenbendazole is a broad-spectrum parasiticide that primarily has activity against nematodes, along with some tapeworms.4 Like other benzimidazole anthelmintics, the mechanism of action for fenbendazole occurs via binding β-tubulin of the cytoskeleton.5 

  • Fenbendazole is the treatment of choice for Giardia spp infections. Longer treatment times of 5 to 7 days may be required if the animal is still positive after the initial course of treatment. 
  • No adverse effects have been noted at usual doses. 

Febantel–Pyrantel Pamoate–Praziquantel

Dogs: 37.8 mg/kg, 7.56 mg/kg, 7.56 mg/kg, respectively, PO q24h for 3 to 5 days6,7 

Cats: 37.8 mg/kg, 7.56 mg/kg, 7.56 mg/kg, respectively, PO q24h for 5 days8 

Doses are extra-label and based on FDA-approved use for helminth infections. Longer treatment times of 5 to 7 days may be required if the animal is still positive after the initial course of treatment. 

Febantel is metabolized by the host to oxfendazole and fenbendazole, the latter of which has efficacy against Giardia spp, as previously stated.9 Pyrantel and praziquantel have efficacy against certain nematodes and cestodes, respectively.

  • No adverse effects have been noted at usual doses. 


Dogs, cats: 15-25 mg/kg PO q12-24h for 5 to 7 days10 (extra-label)

Metronidazole benzoate possibly better tolerated by cats11-13

Metronidazole is a drug in the nitroimidazole class of antibiotics. Its mechanism of action against Giardia spp is hypothesized to be caused by DNA damage, specifically by interfering with helical structure.1,14

  • If metronidazole is used at doses higher than those suggested here, there is a possibility of adverse neurologic effects (eg, mydriasis, ataxia, tremors, seizures).1,13 At the recommended dosage, these signs may also develop with chronic use (ie, dose schedules longer than those recommended here). Therefore, longer treatment times are not recommended.
  • Nausea, vomiting, and lethargy may also be noted.14 
  • Diazepam has been used to treat metronidazole toxicosis in dogs.15

Environmental Management

Because Giardia spp transmission results from ingestion of environmental cysts, cysts should be removed or destroyed. Methods include1:

  • Daily removal of feces from litter boxes and common areas inhabited by animals
  • Steam cleaning contaminated floor surfaces and furniture
  • Regularly washing feeding bowls and toys with hot, soapy water
  • Bathing the infected animal to remove fecal matter from the hair. If bathing is not possible, wiping fecal-contaminated areas after each bowel movement is an alternative. 

Chronic Giardiasis

Some dogs may continue to shed cysts despite repeated treatment. In these cases, the animal’s entire environment should be examined, including areas that are considered less frequented, and repeat treatment is warranted.

References and author information Show
  1. Payne PA, Artzer M. The biology and control of Giardia spp and Tritrichomonas foetus. Vet Clin North Am Small Anim Pract. 2009;39(6):993-1007.
  2. Barr SC, Bowman DD, Heller RL. Efficacy of fenbendazole against giardiasis in dogs. Am J Vet Res. 1994;55(7):988-990.
  3. Zajac AM, LaBranche TP, Donoghue AR, Chu TC. Efficacy of fenbendazole in the treatment of experimental Giardia infection in dogs. Am J Vet Res. 1998;59(1):61-63.
  4. Roberson EL, Burke TM. Evaluation of granulated fenbendazole as a treatment for helminth infections in dogs. J Am Vet Med Assoc. 1982;180(1):53-55.
  5. MacDonald LM, Armson A, Thompson AR, Reynoldson JA. Characterisation of benzimidazole binding with recombinant tubulin from Giardia duodenalis, Encephalitozoon intestinalis, and Cryptosporidium parvum. Mol Biochem Parasitol. 2004;138(1):89-96.
  6. Barr SC, Bowman DD, Frongillo MF, Joseph SL. Efficacy of a drug combination of praziquantel, pyrantel pamoate, and febantel against giardiasis in dogs. Am J Vet Res. 1998;59(9):1134-1136.
  7. Payne PA, Ridley RK, Dryden MW, Bathgate C, Milliken GA, Stewart PW. 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 Am Vet Med Assoc. 2002;220(3):330-333.
  8. Scorza AV, Radecki SV, Lappin MR. Efficacy of a combination of febantel, pyrantel, and praziquantel for the treatment of kittens experimentally infected with Giardia species. J Feline Med Surg. 2006;8(1):7-13. 
  9. McKellar QA, Scott EW. The benzimidazole anthelmintic agents—a review. J Vet Pharmacol Ther. 1990;13(3):223-247.
  10. Tangtrongsup S, Scorza V. Update on the diagnosis and management of Giardia spp infections in dogs and cats. Top Companion Anim Med. 2010;25(3):155-162. 
  11. Patton S. Overview of giardiasis. Merck Veterinary Manual website. Accessed June 28, 2017.
  12. Scorza V. Giardiasis. Clinician’s Brief. 2013;11(2):71-74.
  13. Plumb DC. Veterinary Drug Handbook. 3rd ed. Ames, IA: Iowa State University Press; 1999:492-494.
  14. Uzlikova M, Nohynkova E. The effect of metronidazole on the cell cycle and DNA in metronidazole-susceptible and -resistant Giardia cell lines. Mol Biochem Parasitol. 2014;198(2):75-81. 
  15. Evans J, Levesque D, Knowles K, Longshore R, Plummer S. Diazepam as a treatment for metronidazole toxicosis in dogs: a retrospective study of 21 cases. J Vet Intern Med. 2003;17(3):304-310.


Suggested Reading

Tysnes KR, Skancke E, Robertson LJ. Subclinical Giardia in dogs: a veterinary conundrum relevant to human infection. Trends Parasitol. 2014;30(11):520-527.


Andrew R. Moorhead

DVM, MS, PhD, DACVM (Parasitology) University of Georgia

Andrew R. Moorhead, DVM, MS, PhD, DACVM (Parasitology), is an associate research scientist and parasitologist in the department of infectious diseases at University of Georgia. His research interests include clinical management of canine heartworm, along with researching initial events in establishment of the parasite/host niche by filarial worms. Dr. Moorhead is the director of the Filariasis Research Reagent Resource Center, an NIH-funded resource providing filarial reagents to researchers across North America. He earned his DVM from North Carolina State University, MS from Purdue University, and PhD from Cornell University.

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