5 out of 7 hens have lost their feathers on the belly area up to their vent. The exposed skin is very red. I know they are NOT molting. I'm assuming this is some mite or lice problem. I've not seen any bugs on the chickens. Where they still have feathers, where the feathers meet the skin, some of them look kind of brown or blackish, almost like they are rotting, but not sure they are actually rotting. I'd really appreciate any help here. Any suggestions how to treat mites? Thank you very much!!