Twee Tea, that's a good reply. I agree that you should assume that every chicken has it. Marek's has so many symptoms aside from what's written, it's so hard to determine if its Marek's or not. A good flock history can help as well.
I've also had 2 adults about 2 years old die from it. Unusual, but it does happen. And they both lost the use of their legs, opaque eye, wasting, loss of wing function and neck function. And they both did the one leg forward one leg back. These are classic symptoms, there are many more symptoms that they can have.
Indeed, I read through your links
