More cats, for sure! I could say a lot about this, but let me just emphasize that large cat populations really do lead to a low flea population.
Fleas are the main 'transmission vector' for boubonic plague, and a several other bad afflictions. The big base population of fleas live on rats, mice, moles, squirrels, et cetera. Vermin. Cats despise vermin in love to destroy them. Waterbugs, moths, anything. They sometimes get a lizard or some kind of bird or nice butterfly, but the big trend is to kill vermin.
Most cats kill any fleas that get on them, provided the base flea population is reasonably low, as it will be without vermin around to nourish them.
Any outside cat, especially one who is a prolific mouser, should be treated regularly for worms. A small price to pay.
LOL forget the cats send in the real experts in the field. No cat can touch this fine dogs kill ratio. The Rat Terrier or Feist is the master of the art. They make very fine pets also and are great with kids :>}