I have also had this problem recently. I'm wondering if my roosters aren't the problem. I'm getting ready to move them into a bigger space. Maybe that will help.
I wanted to speak to dipping them. I did a wash for mites one time that I found on here. Dawn soap mixture in one bucket, clean water in the next bucket then a vinegar wash in the last one. Repeat in 7 days. It was a pain, but it finally got rid of the mites. I haven't had a problem since.
Now, I have this vent problem. I haven't seen mites. I have a few who are getting plucked on their backs, so maybe that is what is happening around their vent also.. like everyone else, it's only a couple of them. However, we do also have mice/rats. They are terrible this year. I'm wondering if they carry some diseases that are affecting the birds. Hard to know.
I'm ramping up nutrition, moving to a larger pen, and dusting them. We shall see.