Electron Micrograph of a Giardia trophozoite
WHAT ARE GIARDIA?
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.
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.
HOW DOES GIARDIA CAUSE DIARRHEA?
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 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.
CAN HUMANS BE INFECTED?
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.
A FOOTNOTE ON VACCINATION
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.
Page last updated: 5/22/2021