diagnosis of sick birds - help!

[AmeraucanaLuver]

Last fall one of my ameraucana bantams became sick - started out droopy, lost appetite and weight, then became unsteady on her feet. After several weeks, she died. I tried electrolytes, terramycin, and wazine - finally tried pennicilin. Nothing seemed to give improvement. I did isolate her right away. She had been in a cage with 2 other birds. Neither show any signs of illness even now months later. However last week noticed another droopy bird. She is a large fowl Ameraucana and exhibits the same symptoms. She was never near the original sick bird. She was in a run with about 10 other hens - they all seem to be fine. The first symptom seems to be weight loss as the newly sick hen is very light. I have isolated her. I wormed her last week. I am startying terramycin today since I have that on hand.
Help! I have researched diseases but nothing quite fits. I have well over 100 birds. I keep them clean, warm, well fed adn watered. I want to be a good steward and stop this disease quickly.
I find no signs of parasites internal or external. They haven't laid eggs while sick.
Does anyone have any experience or advice?

 

[ranchhand]

I honestly have no clue what's happening to your birds. Any sniffles or runny noses? Is she getting her share of the feed?

 

[AmeraucanaLuver]

She has no respiratory symptoms at all. She has been in a large run with only 4 other birds, so I don't think she had any trouble with getting enough to eat. I have been doing some more research but not finding anything that I could say YES to. I find it most odd that if this is a contagious disease, it's either very slow to show symptoms or some birds are just resistant to it. Plus the birds that have shown symptoms were not raised together or near each other.

ANyone have any more ideas? I am wondering again about coccidiosis, but I just haven't seen any really odd stool.

Thanks for any help!