I still have (4) adult cats that stay outside all the time. I can handle (3) of them but the other one won't even let me get close to her. They're scratching quite often so I'm thinking that it's most likely fleas, although when i examine the hair/skin of the (3) I can handle, I really don't see any. Anyway I thought perhaps if I bought some flea collars and put them on the (3) it would give me a good indication if it's a flea problem or not. But do they work? Especially on cats that stay outside all the time? Thanks.