I do not think Mareks is a reportable disease, but I could be wrong. You could check with your local state poultry vet to find out, though. I doubt that your vet could help much if it is Mareks. Many vets do not know much about chickens. You may want to save money or invest in getting a necropsy by the state vet. Not all chickens will get Mareks when exposed, but keep in mind that they will always be carriers, so don't give them away or sell them. If getting new chicks, I would have them vaccinated at the hatchery, and kept away from your present flock and coop for 2-3 weeks to develop full immunity. They will be exposed to it, but hopefully, will not develop the disease. Mareks can remain in the environment for years. Here is a list of state poultry vets and labs with contact info:
http://www.metzerfarms.com/PoultryLabs.cfm
http://www.metzerfarms.com/PoultryLabs.cfm