If it is Marek's, there is nothing that can be done. I don't want to take away hope so, I will give you what I try each and every time...
.5 mls poly vi sol, without iron, once daily
400 ius vitamin E twice daily
5 drops of 30 units of Hypericum dissolved in distilled water, twice daily. Each time I make fresh.
I have never been able to save a chicken that has Marek's. I have tried everything. I have lost 40+ chickens in a little over a year. It is a horrible disease. That being said, I would not take the work of the vet. There is just no way she can make that diagnosis without a necropsy. There are other diseases that can cause the same thing.
Where would I start with treatment? I would treat for coccidiosis first. Use Corid and treat for 5 - 7 days. If no improvement, switch to the vitamin therapy. If there is no improvement within 2 week or, she gets worse...you will then know that you are dealing with something far worse then coccidiosis or a vitamin deficiency. At that point, I would suggest euthanizing and having a necropsy performed. Contact your local government extension office. They will be able to guide you to where there is a state lab that will do necropsies for little/no cost. Gather this information now.
In the meantime, mix crumbles, water and yogurt together. Make a mash with this mixture. Feed her this..most chickens just love it! If this doesn't work try scrambled egg, mealworms, hotdogs, etc to get some protein into her. The best part about the mash is that no other water is necessary. They will get all the moisture that they need from the mixture. With the yogurt, it will also provide protein and probiotics.
Good luck with your hen. Where did you get her? How long have you had her? Any new chickens? A good flock history goes a long way towards knowing if Marek's is even a possibility.
