Giardia is a protozoan parasite that can cause intestinal infection in cats, dogs, and humans. The organism has two main forms: a cyst form and a swimming trophozoite form. The cysts actually contain two trophozoites, protecting them from the environment.
The cysts, rather than the trophozoites, cause cats, dogs, and humans to contract Giardia. Cysts are shed or passed from the infected animal through its feces. If another animal ingests the fecal material with the cysts in it or consumes contaminated water or food, the cysts progress through the digestive tract into the small intestines. Upon reaching the small intestines, the trophozoites exit the cysts, mature, divide, and eventually go back into cyst form. Then, they will be passed back into the feces and out into the environment.
Some trophozoites do not go into the cyst form but get passed into the feces. However, trophozoites cannot survive for long in the environment and are not able to cause infection if ingested by another animal. Cysts, on the other hand, can survive for weeks in the soil before being ingested and infecting another animal.
Healthy cats often can tolerate Giardia without becoming ill. Giardia is most likely to cause symptoms in cats that have a poorly functioning immune system, in young kittens, and in catteries or group-housed cats, where the parasites may exist in extremely high numbers. Giardia is uncommon in healthy, older cats that receive good care.
