Mareks is curable with Colloidal Silver which acts as an anti-viral. I've cured two chickens with it. Here's where I posted their two stories.
https://www.backyardchickens.com/t/566250/more-on-colloidal-silver-and-mareks
It's important to keep Colloidal Silver in your medical kit. It is an amazing product.
How do you know that they had Marek's?
-Kathy