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Summer 2021, Parasitology

Giardiasis in Cats

Limiting access to standing water can help cat owners prevent giardiasis.

Ann WortingerBIS, LVT, VTS (ECC, SAIM, Nutrition)

Ann is a 1983 graduate of Michigan State University and got her specialty certification in Emergency/ Critical Care in 2000, in Small Animal Internal Medicine in 2008 and in Nutrition in 2013. In 2017 she attained her Fear Free Level on certification, and has since moved into level 2.

She has worked in general, emergency, specialty practice, education and management. Ann is active in her state, national and specialty organizations, and served on the organizing committees for Internal Medicine and Nutrition. She has mentored over 20 fellow VTSs and has worked on a variety of committees and positions. She is currently an instructor and Academic Advisor for Ashworth College’s Veterinary Technology Program, as well as an active speaker and writer.

Ann has over 50 published articles in various professional magazines as well as book chapters and a book, Nutrition and Disease Management for Veterinary Technicians and Nurses in its second edition in 2016 coauthored with Kara Burns. Ann received the 2009 Service Award for her state association (MAVT), the 2010 Achievement Award for the Academy of Internal Medicine for Veterinary Technicians (AIMVT), and in 2012 received the Jack L. Mara Memorial Lecture Award presented at NAVC.

Her fur/feather/fin family consists of 4 cats, 2 domestic geese, 14 chickens and a pond full of goldfish.

Giardiasis in Cats
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Giardiasis in cats is an infection caused by the single-celled protozoal parasites Giardia duodenalis (also known as G intestinalis and G lamblia) or G cati.1 Giardia parasites were found in cats in 1925 and even before that were found in humans in 1865.2,3

The Giardia genus is divided into 8 major assemblages, which vary in their ability to infect different animals. Polymerase chain reaction (PCR) testing is available at commercial laboratories to identify the specific assemblage responsible for causing an infection.1 For cats, infections are caused by assemblages A1 or F. Assemblage F, in which the cat is the definitive host, is found in G cati; assemblage A1 is found in G duodenalis.1,2 

The parasite has undergone numerous name changes based on changes in scientific classification and species in which researchers found the various assemblages.2 Although Giardia can pose a zoonotic risk to humans, the primary assemblages of A1 and F of cats have not been shown to conclusively infect humans, who are most susceptible to assemblages A2 and B. Treatment of cats is directed at preventing disease in cats, not for the purpose of preventing transmission to humans.1


Giardia parasites are distributed worldwide and survive best in cool, moist environments, especially those with standing water. The cysts are resistant to freezing and municipal water chlorination but can be killed by boiling contaminated water, filtration, and disinfection.2


The Giardia parasite is transmitted through the fecal–oral route, by contaminated water, fomites (e.g., fur and contaminated housing), and the environment.1,4 Most Giardia infections are subclinical;1 however, cats with subclinical infection can be a source of infection for other cats.5

The Companion Animal Parasite Council describes the infection rate of Giardia in cats with clinical signs as 10.3%, and in western Canada, Giardia is found in 9.9% of samples tested.2 The overall infection rate among cats in North America is as high as 4%.6 Kittens younger than 1 year are at increased risk for infection, as are cats housed in shelters and other crowded situations.1,5,6 Infections do not cause permanent immunity, and cats can easily become reinfected.7


Giardia can be found in 2 forms, cysts and trophozoites.1 Each cyst contains 2 trophozoites. After a cat ingests a cyst, gastric acid and pancreatic enzymes dissolve the cyst wall, releasing the 2 trophozoites. These trophozoites quickly mature, and within 5 to 16 days signs appear in the host cat. 

Trophozoites are the active, motile form of Giardia. Trophozoites are 12 to 18 µm × 10 to 12 µm, motile, flagellated parasites that are shaped like a teardrop or a pear. They are bilaterally symmetrical, with a large ventral adhesive disc, 2 visible nuclei, and a large endosome, which contribute to the typical “face” seen in the trophozoites.2 Mature trophozoites travel down the small intestine to their preferred sites, the jejunum and ileum, where they use their adhesive disc to attach to the brush border of the intestines. This attachment can cause sloughing of the intestinal epithelial cells and villous blunting. After the trophozoites attach to the intestinal wall, the flagellae help create a vacuum for the adhesive disc, preventing accidental loss of adhesion.3 These intestinal changes reduce the jejunum’s absorptive surface, causing maldigestion and malabsorption of nutrients, ultimately leading to diarrhea.1,4,5 Giardia has no intracellular phase and does not cause infections outside of the intestinal tract unless the intestines have been perforated by another cause.2 Giardia trophozoites multiply by simple binary fission, in which the organism splits into 2 similar-sized individuals.1,4 As the trophozoites travel down the digestive tract reaching the colon, they may encyst. Encysted immature trophozoites can survive for several months outside the host in cool, wet conditions. Unencysted trophozoites are sometimes found in fresh diarrhea but not usually in normally formed feces.1

Cysts each contain 2 immature trophozoites.1 Cysts are elliptical, nonmotile, and measure 8 to 12 µm × 7 to 10 µm. They have 2 nuclei and a thick refractile wall.2 As the cysts leave the infected animal in the feces, they are immediately infective. When picked up by another animal, their life cycle continues.4,8 If Giardia cysts are not picked up by another animal, their survival in the environment varies according to conditions. At soil temperatures of 39 °F/4 °C, a cyst can remain infective for 7 weeks; however, at 77 °F/25 °C, they remain infective for only 1 week. When exposed to a dry environment with direct sunlight, cysts will remain infective for a few days. When in water at 50 °F/10 °C, such as a pond or persistent puddle, a cyst can remain infective for 1 to 3 months.8

Most Giardia infections are self-limiting; cyst shedding usually ends within 27 to 35 days. However, some hosts cannot eliminate the Giardia parasite on their own, and cyst shedding can last for several months or until treatment is successful.1


In cats with subclinical infections, no signs are seen. For kittens and stressed adults, clinical signs are typically acute, watery, pale, and malodorous diarrhea with mucus.1,4,6 The signs may be short-lived, intermittent, or chronic. Steatorrhea and weight loss are seen in cats with chronic cases. Less commonly seen signs include emesis, fever, and anorexia.4 The diarrhea is caused primarily by malabsorption resulting from disruption of the small intestine lining and hypersecretion.1,4 If blood is seen in the feces, the cat should be evaluated for a secondary infection with Ancylostoma species, Isospora species, or Tritrichomonas species parasites or with Clostridium species bacteria.1


Fecal Tests

Fecal examination techniques include wet mount, fixed fecal smears, and fecal flotation.

Wet Mount

The most effective way to detect Giardia cysts or trophozoites is by examination of fresh (<30 minutes old), unrefrigerated feces.1,2 Place a small amount of fresh diarrhea/feces mixed with 2 to 3 drops of saline on a clean microscope slide with a cover slip. Use a darker field (lower the condenser to prevent burning through fragile objects on the slide), and view the slide at 40× and 100×. Motile trophozoites can occasionally be seen, but cysts are more commonly seen. Identification can be enhanced by mixing the sample with Lugol’s iodine.2 Presence of cysts or trophozoites indicates a definitive diagnosis of giardiasis, but their absence does not indicate a negative diagnosis.4

Fixed Fecal Smear

Another technique is a fixed fecal smear, also made with fresh feces. Use a wooden applicator stick to collect a small amount of diarrhea/feces. Spread it thinly on a glass slide by rolling the stick across the slide. Allow the slide to air dry, and fix and stain by using a commercial Romanowsky stain such as Diff-Quik. View under oil magnification (100×). Preparing fixed fecal smears enables you to keep a permanent record of the infection and preserves a teaching slide for future use.3

Fecal Flotation

Fecal flotation is a concentration technique with the potential to yield a larger number of cysts. Trophozoites will not be seen on fecal flotation. The preferred flotation solution is zinc sulfate because it produces the fewest changes to the size and structure of the cysts. The solution should have a specific gravity of 1.18. All samples should be processed by centrifugation, not by using gravity methods.2 Because cysts are small and translucent, view them with dark-field microscopy. 

Enzyme-linked Immunosorbent Assay

Enzyme-linked immunosorbent assays (ELISAs) are available for veterinary use. Most diagnostic laboratories use ELISA well-testing methods, and in-house testing is available via the SNAP test (IDEXX Laboratories, idexx.com).2 Sensitivity and specificity of the ELISA are similar to those of fecal flotation testing (85.3%),6 and when combined, the sensitivity increases to 97.8%.1 Unfortunately, however, ELISAs detect cyst wall protein6 and cannot determine which assemblage the infection belongs to.1 The benefits are that the test is easy to perform and the results are easy to interpret. Because the antigen can persist for up to 4 weeks after cysts are detected, ELISA testing is not recommended for the purpose of evaluating treatment effectiveness.4,6 

Blood Analysis

Complete blood counts and blood chemistry tests seldom show any significant changes, except for kittens with severe diarrhea, in which total protein plasma levels are low, secondary to maldigestion and malabsorption.1


Changes on abdominal radiographs are nonspecific and may or may not show signs of diffuse enteritis.1

Duodenal Aspiration

Duodenal aspirates are unlikely to yield results because Giardia infections are usually located in the jejunum and ileum.1

Other Diagnostic Tests 

Additional testing done at diagnostic laboratories includes direct immunofluorescence assays (require fluorescent microscopy) and PCRs (can determine to which assemblage an infection belongs).1 A positive direct immunofluorescence result is easy to interpret because the organism fluoresces (glows) under the microscope and morphology can be confirmed.1 PCR consistency can vary with the laboratory conducting the test. False-positive and false-negative results are possible; thus, PCR testing should not be used as the sole testing method.1


Gross examination of intestines reveals mild intestinal thickening with mucus but usually no blood.1 Histopathologic examination reveals diffuse loss of the intestinal brush border with shortened villi. Increased intraepithelial lymphocytes and mast cell hyperplasia are seen. Histopathology findings may be unremarkable for some patients, even if they had diarrhea.1


In the United States, there are no Giardia treatments approved for veterinary use. All medications available for cats have been extrapolated from human use6 and are used off-label.2 

The primary goal of any treatment is to resolve the clinical signs, specifically to stop diarrhea.7 The antimicrobial sensitivity of Giardia is similar to that of bacteria, and there is no way to predict which medication will work for a specific infection.7 Because Giardia protozoa do not grow in culture, sensitivity testing cannot be used to determine drug effectiveness.7 Repeated courses of the same medication are not indicated.6

The drug most commonly used off-label to treat Giardia in cats is metronidazole. When administered at 25 mg/kg q12h for 5 days, it is 50% to 60% effective in most cats. There are safety concerns with metronidazole use in both dogs and cats. High doses (>60 mg/kg/day) can cause lethargy, central nervous system depression, ataxia, tremors, seizures, vomiting, and weakness.9 Use is contraindicated with pregnancy, lactation, and liver disease.2,6 Fenbendazole at 50 mg/kg q24h for 5 days is ~50% effective, but combination with metronidazole increases effectiveness.2,6 Albendazole is relatively effective when dosed at 25 mg/kg q12h for 5 days, but its use has been associated with pancytopenia and is teratogenic in cats.2,6 Although albendazole is effective, the Companion Animal Parasite Council does not recommend its use in cats due to safety concerns.1

Treatment failures can be associated with reinfection, inadequate drug levels or duration of therapy, immunosuppression, drug resistance, and sequestration of the parasite in the gallbladder or pancreatic duct.2

Adding moderately fermentable fiber to the diet can also help control diarrhea and has the added benefit of helping to modulate small intestinal bacterial overgrowth, another contributor to diarrhea. Feeding lower-fat diets can also be helpful by reducing the work needed to digest the food.7 Veterinary therapeutic intestinal diets contain both of these modifications.7 


Therapy effectiveness is indicated by resolution of clinical signs and absence of cysts on centrifuged fecal flotations.7 ELISAs are not effective for monitoring because the antigen they detect can persist longer than cysts. Retesting and retreatment are not indicated if the cat appears to be healthy, clinical signs have resolved, and fecal flotation is negative for cysts.7


Although a Giardia species–specific vaccine was developed, it was not shown to prevent disease7 and has been removed from the market.1 Preventing initial environmental infections requires boiling or filtering contaminated water before cats are allowed to drink, using correct disinfection products, and decreasing environmental exposure in crowded housing conditions.1 Preventing reinfections requires bathing the animal and properly decontaminating the environment to remove infective cysts.6 

For hard surfaces, clean until all signs of feces are gone, then disinfect with quaternary ammonium products or bleach (1:32 dilution). For carpets and other soft surfaces, remove all signs of feces and steam clean for a minimum of 1 minute. For items that can be washed in a dishwasher, such as toys or food dishes, a regular full cycle is sufficient. For items that can be laundered and dried in machines, ensure that the minimum drying cycle is set on high heat for 30 minutes.8 Because the outdoor environment is challenging to disinfect, limit access and remove any sources of standing water. Bleach and quaternary ammonium disinfectants are not effective outside.8 When using any disinfectant, ensure that the correct dilution is used and that adequate contact times are followed. This information can be found on the individual product labels.2


Although Giardia species can be challenging to identify because of the small size of the cysts and the fact that they are shed intermittently, use of a multimodal approach (e.g., direct fecal smears, fecal flotation, and ELISA testing) will detect most symptomatic infections. Treatment also requires a multimodal approach of treating only symptomatic cats, cleaning the environment and the cat(s), disinfecting the environment, and decreasing crowding and stress in multicat situations. These tasks are all within the scope of care and responsibilities for veterinary nurses. 


1. Lappin MR. Giardiasis. In: Sykes JE, ed. Canine and Feline Infectious Diseases. St. Louis, MO: Elsevier; 2014:771–777.

2. Companion Animal Parasite Council. Cat Giardia. capcvet.org/guidelines/giardia. Accessed December 2020.

3. Bowman A. Giardia felis. American Association of Veterinary Parasitologists. aavp.org/wiki/catprotozoa/coccidia-apicomplexan/sarcomastigophora/mucosoflagellates/giardia-felis. Accessed December 2020.

4. Conboy GA. Giardiasis. In: Cohn LA, Côté E, eds. Clinical Veterinary Advisor Dogs and Cats. 4th ed. St. Louis, MO: Elsevier; 2020:386–387.

5. Cornell Feline Health Center. Gastrointestinal parasites in cats. vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/gastrointestinal-parasites-cats. Accessed December 2020.

6. Marks S. Rational approach to diagnosing and managing infectious causes of diarrhea in kittens. In: Little SE, ed. August’s Consultation in Feline Internal Medicine. Vol 7. St. Louis, MO: Elsevier; 2016:10–11.

7. Lappin MR. How I treat Giardia infections. Veterinary Information Network. vin.com/apputil/content/defaultadv1.aspx?id=5124224&pid=11343. Accessed December 2020.

8. Centers for Disease Control and Prevention. Giardia and pets.
cdc.gov/parasites/giardia/prevention-control-pets.html. Accessed December 2020.

9. Papich MG. Metronidazole. In: Papich MG, ed. Saunders Handbook of Veterinary Drugs. 2nd ed. St. Louis, MO: Saunders; 2007:433.