Of are three dogs, only Spot's sample ever showed worms, and that was a tape worm when we first got him from the shelter. Our vet wormed Sadie and Cincy as puppies anyway, and we wormed them again when we got Spot. Even though he showed tape worm we didn't know what else he might have, so again, it was a precaution. We also give all three Heartguard Plus every month, so they should be worm free