I have been dealing with Mareks for about 2 and a half weeks now. My black copper Marans ( cottage hill line) are the weakest. The brown or white leghorns have not been affected, not even the barn yard mixes have shown signs. I have lost over half of the BCM, and none of the other breeds. This is my first year for chickens, I purchased them from three different places all NPIP certified. I have fallen in love with the BCM, they are beautiful and sweet. however I am afraid that this line has had too much inbreeding to develop healthy birds. I am going to try the herbal treatment and see if I can save whats left.