After following up on the info on Denagard that
@Allsfairinloveandbugs gave me, I am having a moment of enlightenment. Reading up on the ingredients in Denagard and what they treat, one of them is Mycoplasma arthritis.
Su-su has been lame for years, and it resembles arthritis. What if she's got Mycoplasma synoviae and that's where these pullets got it? They've been snuggled up with her since they were just a few weeks old.
Years ago, I had several hens go lame, and it was severe enough where they couldn't get around at all and I euthanized them. Su-su was the only one during that period that survived well enough to still function.
I tried to order the Denagard online from Valley Vet but couldn't complete the order, so will call and order it first thing in the morning. This has the potential of helping Su-su, too. I'm excited about that.
But, the latest pullet to come down with this was worse this evening. I'm becoming more certain that it's most likely MS I'm dealing with. Unfortunately, getting the Denagard anytime before Christmas is unlikely with shipping clogged with holiday packages.