Having Marek's in my own flock, I can be very sure that I am seeing Marek's in the picture you posted of your bird. I am sorry, but having seen this so many times there can be no doubt. It is possible that your Vet did find a bacterial infection as well, but this is the classic, as I call it, roadrunner posture. From my own experiences with the virus, there is no coming back once they get to this point. Again, I am so very sorry for both you and your poor bird.
Having your birds vaccinated for Marek's is a good thing, but sadly if the vaccine didn't contain all 3 types of immunization meds, your bird could have been exposed to one not covered or even a mutation of the virus. Out of the 165 birds I bought last year, 120, to date, have been lost to this disease. the highest mortality, believe it or not, was with the birds given the 3 type vaccine. Some breeds were completely wiped out. I have one GL Wyandotte just like yours left out of 6 that I ordered. That breed, for some reason was hit pretty hard.
Since your other birds have alredy been exposed to her, it is now a waiting game to see if they will be affected like her. I don't know if you add anything like vitamins or probiotics to their feed or water, but I think I would start. Immunities start in the gut, if it's healthy it can negate or lessen attacks on the body. Especially as they were vaccinated for the Marek's, it can do nothing but help.
https://www.backyardchickens.com/t/776680/mareks-what-to-do/20 post # 29
Caco3 may be onto something that may help exposed birds fight this virus. I don't believe it will cure it, I don't think there will ever be a cure, but it may be an answer to protect what's left. I haven't finished researching everything as of yet, but I'd use this and in fact have bought some of the pills.
http://www.umm.edu/altmed/articles/lysine-000312.htm
After reading through the link above I would give my chickens 4 mgs of the Lysine to start. Lysine is part of their feed and also some vitamins so the levels would need to be checked for total consumption. I still think that adding in 4 mgs isn't too much and since it wouldn't be for long term, I don't think it would be a problem. I am still trying to find a good dietary guideline for poultry I can trust to figure how much would be theraputic as opposed to regular daily intake or overdosing. Your B vitamins are also going to be a good friend to your birds. I use 1/2 a Super B Complex vitamin tablet, crushed and disolved in their water, as well as a light dosage of poultry vitamins and full dose of the electrolites and probiotics.
One thing I can tell you is do not despair. There is life after Marek's and your others may never become symptomatic at all. Just watch and question everything you see in your birds. If you see something a little off, give them the Super B in their water for a few days. Even if one does show some symptoms, it doesn't mean they cannot recover to their normal. I have Marek's survivors who are 1 year and older now and in fact have hatched babies from their eggs and then babies from their babies.
