Sick chicken for 3 months might need to cull :(

[Kijkej]

Sorry about your pullet. It does sound like she had Mareks disease. California has one of the most easy and inexpensive labs for doing necropsies and testing. I would close your flock from any birds leaving or being rehomed until all birds are gone. If you see any problems with others in the future, describe symptoms and start another thread. Many on this forum deal with Mareks in their flocks, and you may find many of their threads here. Here are some articles to read:
https://vetmedbiosci.colostate.edu/vdl/colorado-avian-health-program/mareks-disease/

https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/

It has been 24 hours since she’s been culled. Is it too late to send her? I can’t find any info on that.

 

[azygous]

Has the body been refrigerated? If not, it's probably degraded by now to the point where it would be difficult for the lab to get any accurate pathology results.

But this doesn't mean you can't do-it-yourself. Certain changes to the organs can be visible to the naked eye, and they can point to either Marek's or leucosis.

In both, you might see hundreds of tiny tumors on the organs. In leucosis, you might see a liver grossly enlarged. A chicken of mine who had leucosis had a two pound liver. Your results wouldn't be conclusive, but it would be enough to clue you that a virus may be in your flock, and the next time this happens to a chicken, you would then be prepared to refrigerate the body while you make arrangements with the lab.