Very sorry you are facing this. :(
Immediate removal is the best option unfortunately. Are you in California also?
It should NOT impact your "older" birds. The best information available is all in that link in post #2 but I will provide it again..
Marek's Virus FAQ
Also.. keep nutrition...
Hi there, welcome to BYC! :frow
Very sorry for your loss and experience. :hugs
You've already gotten lots of great information. I want to say thank you so much for taking the time to try and get answers! I hope your remaining hens will thrive and they usually do once they get past they get...