Pullet very sick, vet appointment tomorrow. Should I try and keep it alive, or cull now?

[BastyPutt]

There are a couple of threads and articles devoted to living with Mareks in the flock which you might find useful going forward. I found this one especially good
https://www.backyardchickens.com/ar...-i-learned-to-deal-with-mareks-disease.76944/

This is an incredible article. I suggest that you don't click over this one, regardless of flock status.