I am a caretaker for 7 feral cats. I am able to use Advantage II for 5 of them but the other 2 adults would hiss and won't let me come near. I have heard about an oral once a month Flea medication Comfortis but this requires a prescription from the vet. I have asked if the Humane Society can authorize this, they said they don't fulfill prescription. So, if you are taking care of feral cats in your backyard, do you use flea prevention on the ferals, if so, what do you use for the ones you can't get near to apply on their backs?