Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066




Electron Micrograph of a Giardia trophozoite

Electron Micrograph of a Giardia trophozoite
(Photo Credit: CDC Public Health Image Library)


Giardia are single-celled organisms, infectious to many types of animals (including humans) all over the world. As one can see in the above image, Giardia organisms have little whip-like tentacles (called "flagella") which classify them as flagellates. They use their flagella to move around from place to place but when they find a spot where they wish to stay (like a cozy nook in the host's intestine), they use a suction cup-like structure (also visible in the image) to attach. Their presence in the host intestine can cause diarrhea, though some hosts are symptom-free carriers. Different types of Giardia infect different types of animals; it is rare for Giardia from a pet to transmit to a human; further, dog and cat Giardia species are separate and are unlikely to cross from dog to cat or vice versa.

Giardia have two forms: the trophozoite (image above) and the cyst. The trophozoite (or "troph" for short) is the form that lives within the host, swimming around and attaching with its suction cup. The cyst, however is the form that lives out in the environment. Trophs are passed in feces into the cold cruel world and must round up into cysts and form a shell if they are to withstand the temperature/moisture variability of the outside world. Cysts are the contagious stage. Trophs are the parasitic stage.


The Giardia trophozoite when stained appears to have a funny face.

The Giardia trophozoite when stained
appears to have a funny face.

(Photo Credit: Doc. RNDr. Josef Reischig, CSc.
via Wikimedia Commons)


As mentioned, trophozoites are passed in fresh feces but promptly round up into hard-shelled little cysts so as to withstand the conditions of the outside world. The cysts live in the environment (outside the host's body) potentially for months until they are consumed by a host. Inside the host, the cyst's shell is digested away releasing two trophozoites into the intestine and the cycle begins again. Contaminated water is the classical source of a Giardia infection.

When a fecal sample is analyzed, the appearance of the Giardia organism depends on whether the sample is freshly obtained or if it has been outside of the host's body for a while. Giardia organisms begin to round up into cysts in a matter of hours. The active trophozoites rather look like funny faces with the 2 nuclei forming the eyes and median bodies forming the mouth. Cysts look a bit more generic.

The Giardia cyst is basically a round egg-like structureThe Giardia cyst is basically
a round egg-like structure.

(Photocredit: Joel Mills via Wikimedia Commons)

In the environment, cysts survive in water and soil as long as it is relatively cool and wet. A host will accidentally swallow a cyst when drinking from a puddle, toilet, or when licking his or her fur. After the cyst has been swallowed, the cyst's shell is digested away freeing the two trophozoites who go forth and attach on the intestinal lining. The troph has a structure called a “ventral disc” which is sort of like a suction cup and is used to attach the organism's body to the intestine. If the troph wants to move to another spot, it lifts itself up and swims to a new spot via its flagella. Trophs tend to live in different intestinal areas in different host species but will move to other areas depending on the diet the host is eating. The troph may round itself up and form a cyst while still inside the host's body. If the host has diarrhea, both trophs and cysts may be shed in the diarrhea; either form can be found in fresh stool.

After infection, it takes 5-12 days in dogs or 5-16 days in cats for Giardia to be found in the host’s stool. Diarrhea can precede the shedding of the Giardia. Infection is more common in kennel situations where animals are housed in groups.



No one is completely sure but infection seems to cause problems with normal intestinal absorption of vitamins and other nutrients. Diarrhea is generally not bloody with a Giardia infection. Immune suppressive medications such as corticosteroids can re-activate an old Giardia infection. We do not know why some infected hosts get diarrhea while others never do.


A positive Giardia ELISA testA positive Giardia ELISA test
(The blue dot on the top is the control positive dot
for comparison. The blue dot on the right
is the host's sample showing positive.)

(original graphic by


In the past, diagnosis was very difficult. The stool sample being examined needed to be fresh plus Giardia rarely show up on the usual fecal testing methods used to detect other parasites. Several tricks have been developed to make Giardia easier to find (special stains, using special processing solutions etc.) but what has made the biggest difference in the diagnosis of Giardia is the ELISA test kit (similar in format to a home pregnancy test). This method has dramatically improved the ability to detect Giardia infections and the test can be completed in just a few minutes while the owner waits.

Giardia shed organisms intermittently and may be difficult to detect. Sometimes pets must be retested in order to find an infection and asymptomatic carrier animals are common.


A broad spectrum dewormer called fenbendazole (Panacur®) seems to be the most reliable treatment at this time. Metronidazole (Flagyl®) has been a classical treatment for Giardia but studies show it to only be effective in 67% of cases. For some resistant cases, both medications are used concurrently. Febantel is also commonly used for Giardia as it is converted to fenbendazole in the body.

Because cysts can stick to the fur of the infected patient and be a source for re-infection, the positive animal should receive a bath at least once in the course of treatment. At the very least, the patient should have a bath at the end of the treatment course plus it is especially important to promptly remove infected fecal matter to minimize environmental contamination.


The short answer is: only rarely, so concern is pretty low in general but maintain good hygiene practices such as regular hand-washing and remove fresh pet fecal matter promptly as mentioned.

That said, here is a more detailed answer:

Giardia duodenalis is a classified into several subcategories called "Assemblages" and designated A through G. Some Assemblages are very specific as to which host animals they can infect and other Assemblages are not so picky. Assemblage F, for example, only infects cats and Assemblages C and D only infect dogs but Assemblage A will infect dogs, cats, people, rodents, wild mammals and cattle. Common testing methods do not indicate what Assemblage has been detected so there is always a possibility of human transmission as long as the Assemblage is unknown. To play it safe, wear gloves to dispose of animal fecal matter and always thoroughly wash hands before eating.



Dog on outdoor patio(Photocredit:

Giardia cysts are killed in the environment by freezing temperatures and by direct sunlight. If neither of these are practical for the area to be disinfected, a chemical disinfectant will be needed. The most readily available effective disinfectant is probably bleach diluted 1:10 in water which required less than one minute of contact to kill Giardia cysts in one study. Organic matter such as dirt or stool is protective to the cyst so on a concrete surface basic cleaning should be effected prior to disinfection. Quaternary ammonia compounds can also be used to kill Giardia cysts.

Animals should be thoroughly bathed before being reintroduced into a “clean” area. A properly chlorinated swimming pool should not be able to become contaminated. As for areas with lawn or plants, decontamination will not be possible without killing the plants and allowing the area to dry out in direct sunlight.


A vaccine against Giardia was previously available, not to prevent infection in the vaccinated animal but to reduce the shedding of cysts by the vaccinated patient. In other words, the vaccine was designed to reduce the contamination of a kennel where Giardia was expected to be a problem. This would be helpful during an outbreak, in a shelter or rescue situation but is not particularly helpful to the average dog whose owner wants to simply prevent infection. Because of limited usefulness of the vaccine its manufacture was discontinued in 2009.


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Page last updated: 5/22/2021