How I Treat Giardiasis
World Small Animal Veterinary Association Congress Proceedings, 2018
Susan E. Little, DVM, PhD, DACVM (Parasit)
Regents Professor and Krull-Ewing Chair in Veterinary Parasitology, Veterinary Pathobiology, Center for Veterinary Health Sciences, Oklahoma State University, Stillwater, Oklahoma, USA

Giardia duodenalis is one of the most commonly identified gastrointestinal protozoal parasites in dogs, cats, and people worldwide. Despite the common nature of this infection, much confusion remains about the clinical significance of Giardia infections, the zoonotic nature of this parasite, and the diagnostic, treatment, and management strategies that are best suited to control the infection. One reason for the confusion is the fact that Giardia populations exist in different assemblages which vary in their infectivity for animals and humans. The total number of Giardia strains and the full range of host infectivity is unknown, but some patterns have been recognized.1 For example, dog strains are not known to infect cats, and cat strains are not known to infect dogs. Similarly, human infections appear to be primarily acquired from other humans, and transmission from dogs or cats to humans, while still of great research interest, appears to be relatively uncommon.

For all Giardia isolates, cysts which are immediately infective when shed are passed into the environment in feces. Transmission to the next host occurs upon ingestion of cysts from fecal-contaminated water, food, fomites or through self-grooming. Trophozoites may also be passed in feces but do not establish an infection in the next host. In dogs and cats, the greatest risk for infection leading to clinical disease appears to be in young animals—puppies and kittens—that have not yet acquired immunity. Dogs and cats frequently have subclinical Giardia infections but show no signs of disease; many parasitologists and gastroenterologists do not recommend treatment of these infected yet clinically normal patients unless there is specific concern about environmental contamination with cysts creating a risk to other animals in the facility.

When disease does develop, diarrhea is the most common clinical sign and is thought to occur due to trophozoites attaching to enterocytes in the small intestine leading to malabsorption and maldigestion. Diagnosis of Giardia may be achieved by identifying trophozoites on a direct smear or wet mount of diarrheic feces, or by identifying cysts in a fecal smear using a fluorescent antibody assay or on fecal flotation with centrifugation. Zinc sulfate (S.G. 1.18) is preferred for fecal flotation as it is less likely to collapse the cysts and prevent their recovery. Fecal antigen tests are also widely available and detect cyst antigen in fecal samples. Cysts can be shed intermittently, and repeat testing performed over several days may be necessary to identify infection even when all available methods are used. Advisory groups like the Companion Animal Parasite Council (CAPC) recommend only testing symptomatic dogs and cats with the fecal ELISA.2 Many clinically normal animals will test positive for cyst antigen but are unlikely to need or benefit from treatment.

In many areas of the world, label-approved drugs to treat Giardia in dogs and cats may not be available. However, several effective treatments have been reported. Effective treatments for canine and feline giardiasis include fenbendazole (50 mg/kg PO q 24 h x 5–7 days), febantel (30 mg/kg x 3 days) (usually combined with praziquantel and pyrantel pamoate), and ronidazole (30–50 mg/kg PO q 12 h x 7 days).3-6 Metronidazole was used historically to treat giardiasis but has poor efficacy; metronidazole (25 mg/kg PO q 12 h x 5–7 days) in combination with fenbendazole is considered more likely to be effective.2 Safety concerns exist with both ronidazole and metronidazole. Regardless of the treatment selected, intensive hygiene management including disinfection of kennels and frequent bathing is often necessary to prevent reinfection and continued cyst shedding.3

When an initial course of fenbendazole (5 days) fails to eliminate clinical disease, a second, longer course (10–14 days) should be implemented and combined with strict attention to hygiene to prevent continued re-infection. Bathing, with particular attention paid to the hind quarters to remove any fecal debris and associated cysts, is important. If acceptable to the owner, clipping the hair in the perianal area and around the tail and legs may facilitate keeping the area clean and thus prevent reinfections from self-grooming. Discussion with the owner about pet lifestyle habits can also be helpful; dogs that often visit dog parks or swim in areas frequented by other dogs are at particular risk for re-infection.


1.  Ballweber LR, Xiao L, Bowman DD, Kahn G, Cama VA. Giardiasis in dogs and cats: update on epidemiology and public health significance. Trends Parasitol. 2010 Apr;26(4):180–9.

2.  Companion Animal Parasite Council. Accessed 01 May 2018.

3.  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. J Am Vet Med Assoc. 2016 Sep 15;249(6):644–9.

4.  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 Sep;59(9):1134–6.

5.  Barr SC, Bowman DD, Heller RL. Efficacy of fenbendazole against giardiasis in dogs. Am J Vet Res. 1994 Jul;55(7):988–90.

6.  Fiechter R, Deplazes P, Schnyder M. Control of Giardia infections with ronidazole and intensive hygiene management in a dog kennel. Vet Parasitol. 2012 Jun 8;187(1–2):93–8.


Speaker Information
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Susan E. Little, DVM, PhD, DACVM (Parasit)
Veterinary Pathobiology
Center for Veterinary Health Sciences
Oklahoma State University
Stillwater, OK, USA