Giardia duodenalis lives in the intestine and spreads through cysts in the stool. Young dogs in particular can get loose, foul-smelling or recurring diarrhoea, but infection can also be found in dogs that seem healthy. This means that a positive test does not automatically prove that Giardia is the cause of all of a dog's stomach problems.
The diagnosis is made with a stool test, and the test method used affects how often the infection is detected. During treatment, hygiene is important because cysts in the environment can contribute to reinfection. Wash your hands after contact with stool, keep the fur around the rear clean if there is diarrhoea and pick up stool promptly.
The Giardia types most common in dogs are often adapted to dogs, and the actual risk of infection passing from dog to human is considered uncertain and generally low. Good hygiene is still sensible, especially in households with people who have a weakened immune system.