Hen can't standup; waddles

[EJonFire]

This hen is 1.5 years old Bovans Brown, about 6 months ago she developed an irratated red patch on her underside with no feathers. About 3 months ago she was placed in isolation because we suspected it was caused by the other hens picking on her, although the skin never appeared broken. Also started dusting the birds and their area. Today, it's red with just stubs of feathers sprouting, and a week ago she stopped being able standup, and now waddles around without getting her body off the ground. Her tail also appears to be drooping. She is still laying eggs on a daily basis, and all appears normal with them. Examining her, unable to find any other issues that appear to cause it. I'm guessing Marek's disease; any thoughts? See the photos and video below and let me know what you think.

 

[Nutcase]

Poor thing. It looks like mites or lice. You should be very wary of the other chickens pecking and drawing blood from the bare skin. One of my hens had bald patches on her shoulders for a long time while the others were fully feathered. Then without warning, she grew them back within two or three weeks. I have not been able to work out what caused this.

She appears to be in a lot of pain. Check her over for any other symptoms - eg pale comb, matted bottom etc. It crossed my mind that the baldness might have been caused by the waddling/rubbing against the ground.