Fortunately, even though the infection rate is high in cats and dogs, clinical disease is less common. Infection is relatively rare in healthy cats. It is more common in densely-populated groups of animals, such as in a cattery, pet store, or animal shelter. Also, kittens have been shown to shed more Giardia cysts in their feces than older cats. The stool may range from soft to watery, and occasionally contains blood.
Infected cats tend to have excess mucus in the feces.
Sometimes, vomiting can occur. Ingestion of the cyst stage of the parasite leads to infection. Once inside the cat's intestine, the cyst goes through several stages of maturation. Eventually, the cat is able to pass infective cysts in the stool, where they can contaminate the environment and infect other cats. Infection can also occur from drinking water which has been contaminated with the cysts.