When we got our mutt pup from the rescue, he had giardia (and hook and whip). It took 6 months before he had 3 clean tests in a row. My greyhound never got it. I kept the yard very, very clean. And perhaps it helped that Sam and Maggie had limited exposure to each other till he was 25# due to her high prey drive. My vet feels Sam had a very difficult time with giardia because he probably got it from his momma dog. Adult dogs don't usually have so hard a time with it, in the vet's opinion. Good luck.