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You can incinerate a carcass, which is probably the best thing when an infectious disease is suspected. I have had one die during a snowstorm, and just placed the body far away from my chicken area, and the turkey buzzards took care of the remains. When you speak to the state vet, you might say that you wonder if it could be avian flu, and in some states where it is a problem, they will sometimes check for free. Coccidiosis can cause symptoms sometimes that look like Mareks. I hope she responds to the Corid. It should be mixed daily in plain water. Dosage is 2 tsp of the liquid Corid, or 1.5 tsp of the powder per gallon of water for 5 days.
That's the good thing about getting a necropsy, it can save a lot of worry if it's not Mareks. If she has Mareks, then they have been exposed, but may or may not get the disease. Hopefully, it is not that. I would give some concentrated Corid to her in case she hasn't taken enough, and continue the medicine in the water for 5 days. Sorry that this is happening.