I was asking about hatching eggs because Marek's and leucosis transmit differently as far as eggs. While leucosis is passed from an infected hen to her eggs and infecting the embyos, Marek's does not. Both are passed directly from chicken to chicken. If one chicken has it, they all carry it. Therefore, if all of these chickens were hatched from eggs you bought and incubated yourself, this didn't come from the eggs if it's Marek's. It could be leucosis, however.
I have sixteen years experience with leucosis as I have it in my flock. I've hatched within my flock as well as obtained hatchery chicks. There is almost no chance of hatchery chicks coming to you carrying leucosis. But if you bought older chickens from the breeder, they may have flocks that are carrying this virus. The most recent of my chickens to become symptomatic and die all were hatchery chicks brooded by a symptomatic hen. They were infected directly and all became partially lame at between five and eight months. The chickens hatched from my flock have become symptomatic at around one year. All died except for two that are still alive at seven years.
The only other thing that could produce these symptoms is gradual toxin exposure, usually petroleum distillates - everything from pant thinner to transmission fluid. From what I'm already picking up from you, you are not the sort to be careless with those substances in proximity to your chickens.
If her fate is the same as the others, she also will die. You might be fortunate to have an agricultural college or other animal lab that can do a necropsy. Then you will have confirmation of an avian virus or what ever else may have killed these chickens.
Please continue this thread and document what happens. It's a big help to others who may be in similar circumstances.
