Testing by fecal floatation does not always detect parasites.
AS a rule deworm your pet on a regular basis,indoor twice a year and outdoor 3 x's a year is basically a rule of thumb ....but as you deworm, deworm them at the same time.
Cat litter boxes are very dangerous indeed, wear a mask to clean it out.
If canine d.or other worms (sesame seeds in your stools etc) and are a continuous problem..it just might be best to not have a pet.
All animals carry worms..even pet snakes!
One cannot rely on all vets or testing procedures or even kill some of these parasites with even medication like prazinquantel for tapes and roundworms and others, they like humans have resistant strains of worms! FACT!