About six months ago, I agreed to adopt my son's pug, because his new wife didn't like her and was mean to her. Unfortunately, she brought some fleas with her. My 13 year old cat, Skippy, an indoor cat, had never had a problem with fleas, but we thought we would protect him, so my wife bought some Sergeant Flea application. We put it on his neck and his behavior immediately changed. Now whenever I pet him or touch his back, he aggressively starts chewing on his right front leg. He hasn't broken the skin or anything like that, but you can tell the hair is getting quite thin on that particular leg. It has been several months now, and he still has this issue. I brush him every day and use a flea comb on his back and sides, but never find any fleas. Any ideas on how to solve this problem?