Puzzled. Flea allergy???

I noticed yesterday that one of the lodgers has a bald patch and some scabs on her back, and found some on her neck this morning. My immediate thought was a flea allergy, and was going to ask her owner if she has had this before, but she is away for the weekend, so wasn't in work. I checked all 6 cats and none of them have any sign of fleas, so could it be an allergy? I was going to de-flea the lodgers and ask the woman to give me the money next week, but now I can't see any, I don't know if I should wait till I can talk to her on Mon in case this has happened before, and she forgot about it.

maybe got in a fight with another cat? My one cat, Orion gets that from the other cats beating on him.

Yeh, my immediate thought is fighting. Or maybe excessive licking. Probably not an allergy as they dont usually cause scabs.

It could be ringworm, sometimes ringworm patches can look like scabs

Didn't think of fighting. Her and her sister do play rather rough at times, so maybe that could be it. I don't think it could be ringworm, as these cats haven't gone outside in 2 months. Will just leave it till I talk to her owner then, was dreading saying I had had to treat her cats for fleas, but seeing as I haven't found any I don't have to!!