When I took Cleo in recently for over grooming, it was fleas. I checked all my girls and never saw a single flea, but it just takes one, especially if your cat is highly sensitive to fleas like Cleo. My girls are inside cats, but there's a cat that lives next door who comes up on my porch and I've even brought them in before from my neighbor's cats living in my yard.
P.S. Dry food does not clean teeth any more than eating Fritos would clean ours. I know some vets still believe that, but cats either shatter the dry food on the ends of their teeth, or swallow it whole.
Info. & Links on why canned is better than all dry diet