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Giardia is a protozoan parasite infectious to both humans and pets all over the world.  Contaminated water is the classical source of a Giardia infection.  They live in the large intestine where it causes diarrhea.  After a short period of time outside the host’s intestine, the trophozoites (immature giardia) round up and form cysts which enable them to survive environmental conditions without a host to protect them.

After they have been swallowed, the cyst shell is digested away freeing the trophozoites who go and attach on the intestinal lining. The troph has a structure called a “ventral disc” which is sort of like a suction cup and this is used to stay attached to the intestine. If the host has diarrhea, trophs are shed in the diarrhea but Giardia may also form cysts within the host in preparation to be shed. 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. The disease produced is variable depending on the individual and age. Young puppies are most often affected. Signs usually occur 1-2 weeks after infection and often the disease goes unnoticed or is self limiting after a bout of diarrhea. It can produce severe diarrhea and fluid loss. Most cases that show signs are mild with minimal depression. The parasite is passed in the feces and is consumed directly by the next host. Giardia is very hardy and can remain in the environment for a number of months waiting for a suitable host.


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. Immune suppressive medications such as corticosteroids can re-activate an old Giardia infection.



Diagnosing giardia can be demanding and may require frequent microscopic fecal examinations. Giardia rarely show up on the usual fecal flotation testing methods used to detect other parasites. What has made Giardia testing infinitely easier is the development of a commercial ELISA test kit. A fecal sample is tested immunologically for Giardia proteins. 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.


Giardia responds very well to treatment. Metronidazole is the treatment of choice given daily. However, there is discussion of potential resistance. A broad spectrum dewormer called Fenbendazole (Panacur®) may be added into the treatment protocol if needed.  For some resistant cases, both medications are used concurrently. The ELISA test for Giardia should go negative within 2 weeks of treatment indicating success.

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.

Not all patients with Giardia actually have diarrhea but because Giardia is the most common intestinal parasite affecting humans in North America, treatment is generally recommended for the pet testing positive even if no symptoms are being shown. The idea is to reduce human exposure.

Environmental Decontamination

The most readily available effective disinfectant is probably bleach diluted 1:32 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. 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.