Hi, I am harness-training one of my four cats, and I'm worried about bringing fleas into the house. We used to let one of our cats out in the backyard when we were gardening, and the vet told us we needed to use Revolution on all four cats. That got to be really expensive!!! If you walk your cat, what do you do for flea control? Are there cheaper yet effective options to Revolution? And do you treat all the cats in the house or just the walker? Any advise from people who take their kitties out on walks would be helpful. Thanks!