If she has Mareks, the others may also test positive, but may not have the symptoms. Mareks is spread through the dust and dander of feathers. Here is some reading about it:
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/
She sure looks very disabled in the video. Would you consider contacting this lab below to find out how to get Mareks testing on her blood for about $20? Here is the link to contact them:
http://www.vetdna.com/application/forms/aviansubmittalform.pdf
If you decide to cull her, I would...
Many people let the chickens sleep on the floor at night so they can stretch and move. It probably depends on the chicken’s condition and problems. Does her lower leg at the ankle appear swollen to you? You can check it in the morning.
Check out some chicken slings in the thread below. They can help get a chicken upright, keep them cleaner, and in front of food and water for periods through the day. Some chickens do not tolerate them and may try to escape them. You can raise them off the floor or put them touching the floor...
Well, physical injury should always be the first consideration. I always hard to even mention Mareks as a possibility, but it can be a possibility. Usually if a riboflavin deficiency, which normally affects both legs, is treated soon with extra b vitamins, one should see signs of improvement...
It is always best to continue an ongoing thread so we can keep up on what has taken place. Walking on a hock can be from a hock tendon injury, B2 deficiency, or sometimes Mareks disease. Where did the 3 new birds come from in July? Sometimes a person might have Mareks in their flock, and if a...