Giardia are single-celled parasitic organisms that infect many types of animals around the world. Humans can become infected as well, but it is rare for Giardia from dogs and cats to transmit to people, and the same is true for transmission between dogs and kitties.
Organisms are passed into the environment in feces and once outside the body, they turn into hardy tiny cysts that can survive for months. Once they are ingested by a new host (for example from contaminated water, or a dog licking her feet after walking in the grass) the shell dissolves and every cyst releases two infectious organisms. Once free from the cyst, Giardia "swim" around inside the host's intestines until they find a good spot to feed and become attached to that area. Once there, they can move around in different parts of the intestines looking for different nutrients.
It takes about five days to two weeks for the Giardia to be passed out into the stool of an infected pet and diarrhea can precede the shedding. Infection is more prevalent in places with high dog density, such as dog parks, kennels and day care facilities. Not all dog become ill from Giardia, and therefore treating all pets in a household is not always recommended. However, many cats and dogs will develop diarrhea (sometimes bloody), vomiting and loss of appetite.
Testing has become more reliable efficient with the development of a newer test called a Elisa SNAP test. This test takes minutes and can be done while you are at the clinic for the appointment. Because of the inconsistent shedding habits of the organism, repeat testing is sometimes necessary to detect it.
Several medications are available to treat Giardia, such as Metronidazole and Fenbendazole, often used together. A prescription intestinal diet, gentle on the intestinal tract is often used, and so are canine and feline specific probiotics.
As with prevention of all parasites, cleaning your yard of stools daily helps greatly in reducing contamination and re-infection. Freezing temperature and direct sunlight kill the cysts, and so does diluted bleach solutions and other chemicals.
Giardia cysts can attach to the fur, so bathing your pet is also recommended.