Giardia is a protozoal parasite that can cause diarrhea in multiple animal species. This microscopic parasite is a zoonotic pathogen that can be transmitted between animals and humans, and there are conerns about the role of pets in human disease. Various studies have evaluated the presence of Giardia in healthy dogs and, to a lesser degree, cats. Typically these studies report that about 7% of healthy dogs are shedding Giardia in their stool, but all Giardia are not the same in terms of the risk of transmission from dogs to humans. There are various types of Giardia, and some only infect specific animal species and not people. In dogs, assemblages (types) C and D are most commonly reported. These are considered canine-specific types and are therefore not a concern for transmission to humans. Assemblage A is an important zoonotic type which can infect dogs and humans, and this type can certainly be found in healthy dogs, but it seems to be relatively uncommon.
Emerging information about Giardia typing and zoonotic disease risks shows that this is a more complex issue than previously thought. Studies that determine the prevalence of Giardia shedding in dogs and cats are useful, but they only tell part of the story. Comments about the human health implications of Giardia shedding in pets can only be made when information about the Giardia assemblages found in these animals is also reported.
More information about Giardia can be found on the Worms & Germs Resources page.