I'm so sorry you had to make that decision for her. I know how hard it is, but it was the kindest thing to do.
Have you given her a vitamin supplement to boost her immune system?
What type of "weird" breathing? Have you considered treating her with Tylan 50? I'm not a fan of using antibiotics willy nilly but if you have Marek's she will need as much help as possible to fight a respiratory infection. Unfortunately Marek's can cause respiratory problems either as a direct result of the virus (tumours on the lungs) or through secondary infection due to it's immune suppression.I do have one hen who was breathing kind of weird, but I’ve had her quarantined for a few days and she’s eating and drinking.. any suggestions? Is she safe to go back with the others? I’m not sure what to do, if the rest survive, will I not be able to get any more chickens until I no longer have them?
Have you given her a vitamin supplement to boost her immune system?