I've been taking care of an ear-tipped feral male cat. He's become very people-friendly and I've been putting Frontline on him every 4 weeks since July. He still is constantly nipping at himself. I took a flea comb through him and didn't find any fleas or flea dirt but he has little scabs all over him. These are what must be bothering him, but what could they be coming from if he has no fleas? Thank you!