Hamilton gets his Frontline+ on the 20th every month - so he's due in a few days here. We spend a lot of time outside, and at the park, and with other dogs. I was scratching his back by his tail this afternoon and found flea dirt. He hasn't been scratching at all, and I haven't found any live fleas. Can fleas bite, then poop (flea dirt) then die with the frontline? Or should I look into treating him more aggressively for fleas?