Chicken loses balance when running and losing weight

[cwaterbury]

One of my hens, about 4 years old, seems to stumble and lose her balance when she runs. She is also losing weight. At night, she doesn't get up on the roost and sleeps in the nest box. This has been going on for maybe a week or so. She has never really laid any eggs, except a few in the beginning and a few this year in February. I have just let her hang out and be a freeloader all this time. But I don't think the lack of eggs is related to her current problem, but added it in case it is.

 

[azygous]

Those symptoms could be from a long list of poultry disorders. But, they are common in Marek's and lymphoid leucosis.