Although it wouldn't be precisely definitive, if you cut open a deceased bird and look at the liver, lymphoid leucosis would present with an enlarged liver often covered in tiny white tumors. If you find a normal liver, then that would add weight to this being Marek's.
Thank you for being so willing to give detailed information. Many people will not, and it makes helping them tedious and frustrating. Knowing your location enables me to look up insects and other creatures that reside there that can be dangerous to chickens.
I was surprised you have a few...
This is disturbing to us here. Losing chicks like this and not knowing why, is scary as well as maddening. We need to know your location, not a map to your house, just state, province, and country. Location often provides clues.
What is your water source? Sometimes well water can be very toxic...
Mold is the only other thing that causes paralysis in young chickens.
Avian viruses start producing symptoms between eight weeks and five months.
There is always the truly freaky. Years ago, I had several baby chicks find a buck moth caterpillar and they pecked it. It had a powerful venom...
You are correct that these chicks are too young for this to be from Marek's.
I've had experience losing chicks to the symptoms you describe. One day, I was sitting in the run when an eight-week old chick came in and collapsed at my feet, literally its legs going out from under it. The chick...