What are giardia?
Giardia are one-celled protozoan parasites often confused with worms because they infect the gastrointestinal tract. These parasites can cause diarrhea, though most pets infected with giardia do not experience this or any other signs of illness. When giardia cysts are found in the stool of a pet without diarrhea, they are generally considered a transient, insignificant finding. However, in very young or very old pets, giardia may cause severe, watery diarrhea that can be fatal.
How did my pet get giardia?
Animals becomes infected with giardia when they swallow the cyst stage of the parasite (e.g., ingestion of fecally-contaminated food or water or chewing fecally-contaminated objects). Once inside the pet's intestine, the cyst goes through several stages of maturation. Eventually, the pet is able to pass infective cysts in the stool. These cysts can live in the environment and infect other animals, including humans. 
How is giardiasis diagnosed?
Giardiasis is diagnosed by performing a microscopic examination of a stool sample. The cysts are quite small and usually require a special floatation medium for detection, meaning they are not normally found on routine fecal examinations. Occasionally, the parasites may be seen on a direct smear of the feces.
A blood test is also available for detection of antigens, or cell proteins, of giardia in the blood. This test is typically more accurate than the stool exam, but it requires several days to get a result from the laboratory. 
How is giardiasis treated?
Giardiasis can be difficult to treat for two reasons: Giardia are often resistant to medication and can persist in the environment and lead to reinfection. The most common drug used to kill giardia is called Panacur (fenbendazole). Flagyl (metronidazole) is an antidiarrheal/antibiotic-type drug that may also be necessary to clear some giardia cases. Other drugs may be used if diarrhea and dehydration occur.
Can humans become infected with giardia?
Giardia can also cause diarrhea in humans; therefore, environmental disinfection is important. The use of chlorine bleach (one cup per gallon of water) is effective if the premises can be safely treated with it.
Follow these steps if your dog has been diagnosed with giardia:
  • Bathe your pet once a week for three weeks.
  • Disinfect the pet's environment.
  • Recheck the pet's fecal sample 14 days after the start of medication.
  • Pick up feces immediately.
  • Treat all animals in the household.