I'm at a loss! Help?

[PeepShowEggs]

I have a mixed flock of approximately 58 chickens- EEs, Rocks, RIRs, NHRs, JGs, Welsummers, Marans and Salmon Favorelles. They range in age from 6 months to 1 year 8 months. My older birds (16 total) are showing some seriously scary stymptoms. Some are losing their feathers, on their backs just before their tails, at the wing joint nearest the back and on their tails. Some have some feather loss on their breast/crop area. Where the feathers are gone the skin is red and irritated. They have lost weight and I have gone from 15-20 eggs a day to 6! Some of the chickens legs are dry and splitting :/ They seem alert, they are eating and drinking fine. But they look pitiful! I have increased their feed, put Poultry Rx in their water. I thought about parasites too, so I have sanitized the coop once a week for three weeks now. I put down DE on the floor and sprayed the coop walls, roosts and nest boxes with Poultry Protector. I also put ot two dusting boxes with commercial poultry de-lousing powder (as much as I hate it, I'm desperate!)

Does anyone have ANY clue what could be going on? I am really quite worried and have a veterinary farm rotation in 2 weeks to check them out. I'd like to start treating them now!

My youngest birds are plump, fully feathered, eating and drinking well but aren't laying as well as I think they should be.

 

[Eggsoteric]

If you've ruled out lice/mites, could be they're molting. I have several that are in molt now and pitiful doesn't even describe how they look. I'd add a little extra protein to their diet.