While the potential health risks to people exposed to dogs with Giardia have received a lot of attention (and concern), there’s more and more evidence indicating that dogs pose a very limited risk to people. Giardia is a protozoal parasite that can cause diarrhea in numerous animal species, including people and dogs. It can also be found in healthy individuals, with several reports describing evidence of Giardia in the feces of approximately 7% of healthy dogs.
In the recent past, as easier-to-perform tests for Giardia became available, people sometimes tested healthy animals for the parasite, and then had to figure out what to do with positive but healthy pets. It has not been unusual for such dogs to be treated because of concerns about the potential for infecting people. However, it’s becoming increasing clear that this risk is minimal.
The reason the risk is so low is due to the fact that not all Giardia are created alike. There are different assemblages (types) of Giardia, and not all types can infect all species. Knowing the types found in dogs and the types that cause illness in people can tell a lot about the potential human health risk. One more recent study has indicated that risk of animal-to-human transmission is likely very small.
In that study (Itoh et al, Veterinary Parasitology 2010), researchers tested fecal samples from 1794 puppies in pet stores in Japan. Giardia was detected in 23% of puppies. This is a pretty high number but it’s not really surprising, as this highly mixed population of young, stressed animals would be considered high-risk compared to the average pet dog. All Giardia were assemblages C or D, which are dog-specific. Therefore, despite 23% of the puppies shedding Giardia, none posed a risk for human infection.
Yes, Giardia is a zoonotic infection, but the risk of dogs infecting people appears to be very low because dogs so rarely carry the types able to infect people. Beyond that, this is a "don’t eat poop" disease, whereby the use of good hygiene practices to prevent inadvertent ingestion of Giardia from dog feces can reduce the risks even further.