Chickens can die suddenly, linger for days or weeks, or sometimes not get worse, possibly recovering from Mareks. Each case is different. But since you cannot know for sure what she has, and since it may affect your flock in the future, I would consider culling her and getting a necropsy by your state poultry lab. They can test for Mareks and also can diagnose the respiratory diseases she has from cultures done on sinuses, organs, or tumors found on nerves. This is the best method for a diagnosis. I am sorry that she has become worse, but I would spend the money to know how to proceed with your flock. What state are you located? Here is a list of poultry labs and state vets where you can call and ask questions:
http://www.metzerfarms.com/PoultryLabs.cfm
Here is an excellent article on Mareks disease:
https://www.merckvetmanual.com/poultry/neoplasms/marek’s-disease-in-poultry
http://www.metzerfarms.com/PoultryLabs.cfm
Here is an excellent article on Mareks disease:
https://www.merckvetmanual.com/poultry/neoplasms/marek’s-disease-in-poultry
