Boy, I WISH more cat owners would use those harness things!! It is my biggest pet peeve. My neighbors let their cats loose to roam through my yard. Of course they then kill the birds (I've found all too many decapitated cardinals), and poop in my yard. My little dog Pippin was just diagnosed with giardia (Casey was clear, but is getting treatment anyway). Vet said it can be transmitted from and to cats.
Another neighbor's cat was recently killed by a fox who sometimes wanders through our neighborhood. Even with that, my next door neighbors still let their cats loose!! I don't understand the mentality, at all. For me, my first and foremost responsibility as a pet owner is to keep my dogs safe; second is to keep them from doing harm to other animals or people. Cat owners don't seem to think like that, and I don't get it.